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docs-rst: convert usb docbooks to ReST 

As we're moving out of DocBook, let's convert the remaining
USB docbooks to ReST.

The transformation itself on this patch is a no-brainer
conversion using pandoc via this script:
	Documentation/sphinx/tmplcvt

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mauro Carvalho Chehab 2017-04-05 10:22:57 -03:00 committed by Jonathan Corbet
parent d76a085bc8
commit 4ad4b21b1b
10 changed files with 1719 additions and 2167 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
Documentation/DocBook/Makefile
View File

@ -8,12 +8,11 @@
DOCBOOKS := z8530book.xml \
kernel-hacking.xml kernel-locking.xml \
writing_usb_driver.xml networking.xml \
networking.xml \
filesystems.xml lsm.xml kgdb.xml \
gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \
libata.xml mtdnand.xml librs.xml rapidio.xml \
s390-drivers.xml scsi.xml \
sh.xml w1.xml \
writing_musb_glue_layer.xml
sh.xml w1.xml
ifeq ($(DOCBOOKS),)

793
Documentation/DocBook/gadget.tmpl
View File

@ -1,793 +0,0 @@
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
<book id="USB-Gadget-API">
<bookinfo>
<title>USB Gadget API for Linux</title>
<date>20 August 2004</date>
<edition>20 August 2004</edition>
<legalnotice>
<para>
This documentation is free software; you can redistribute
it and/or modify it under the terms of the GNU General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later
version.
</para>
<para>
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
</para>
<para>
You should have received a copy of the GNU General Public
License along with this program; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA
</para>
<para>
For more details see the file COPYING in the source
distribution of Linux.
</para>
</legalnotice>
<copyright>
<year>2003-2004</year>
<holder>David Brownell</holder>
</copyright>
<author>
<firstname>David</firstname>
<surname>Brownell</surname>
<affiliation>
<address><email>dbrownell@users.sourceforge.net</email></address>
</affiliation>
</author>
</bookinfo>
<toc></toc>
<chapter id="intro"><title>Introduction</title>
<para>This document presents a Linux-USB "Gadget"
kernel mode
API, for use within peripherals and other USB devices
that embed Linux.
It provides an overview of the API structure,
and shows how that fits into a system development project.
This is the first such API released on Linux to address
a number of important problems, including: </para>
<itemizedlist>
<listitem><para>Supports USB 2.0, for high speed devices which
can stream data at several dozen megabytes per second.
</para></listitem>
<listitem><para>Handles devices with dozens of endpoints just as
well as ones with just two fixed-function ones. Gadget drivers
can be written so they're easy to port to new hardware.
</para></listitem>
<listitem><para>Flexible enough to expose more complex USB device
capabilities such as multiple configurations, multiple interfaces,
composite devices,
and alternate interface settings.
</para></listitem>
<listitem><para>USB "On-The-Go" (OTG) support, in conjunction
with updates to the Linux-USB host side.
</para></listitem>
<listitem><para>Sharing data structures and API models with the
Linux-USB host side API. This helps the OTG support, and
looks forward to more-symmetric frameworks (where the same
I/O model is used by both host and device side drivers).
</para></listitem>
<listitem><para>Minimalist, so it's easier to support new device
controller hardware. I/O processing doesn't imply large
demands for memory or CPU resources.
</para></listitem>
</itemizedlist>
<para>Most Linux developers will not be able to use this API, since they
have USB "host" hardware in a PC, workstation, or server.
Linux users with embedded systems are more likely to
have USB peripheral hardware.
To distinguish drivers running inside such hardware from the
more familiar Linux "USB device drivers",
which are host side proxies for the real USB devices,
a different term is used:
the drivers inside the peripherals are "USB gadget drivers".
In USB protocol interactions, the device driver is the master
(or "client driver")
and the gadget driver is the slave (or "function driver").
</para>
<para>The gadget API resembles the host side Linux-USB API in that both
use queues of request objects to package I/O buffers, and those requests
may be submitted or canceled.
They share common definitions for the standard USB
<emphasis>Chapter 9</emphasis> messages, structures, and constants.
Also, both APIs bind and unbind drivers to devices.
The APIs differ in detail, since the host side's current
URB framework exposes a number of implementation details
and assumptions that are inappropriate for a gadget API.
While the model for control transfers and configuration
management is necessarily different (one side is a hardware-neutral master,
the other is a hardware-aware slave), the endpoint I/0 API used here
should also be usable for an overhead-reduced host side API.
</para>
</chapter>
<chapter id="structure"><title>Structure of Gadget Drivers</title>
<para>A system running inside a USB peripheral
normally has at least three layers inside the kernel to handle
USB protocol processing, and may have additional layers in
user space code.
The "gadget" API is used by the middle layer to interact
with the lowest level (which directly handles hardware).
</para>
<para>In Linux, from the bottom up, these layers are:
</para>
<variablelist>
<varlistentry>
<term><emphasis>USB Controller Driver</emphasis></term>
<listitem>
<para>This is the lowest software level.
It is the only layer that talks to hardware,
through registers, fifos, dma, irqs, and the like.
The <filename>&lt;linux/usb/gadget.h&gt;</filename> API abstracts
the peripheral controller endpoint hardware.
That hardware is exposed through endpoint objects, which accept
streams of IN/OUT buffers, and through callbacks that interact
with gadget drivers.
Since normal USB devices only have one upstream
port, they only have one of these drivers.
The controller driver can support any number of different
gadget drivers, but only one of them can be used at a time.
</para>
<para>Examples of such controller hardware include
the PCI-based NetChip 2280 USB 2.0 high speed controller,
the SA-11x0 or PXA-25x UDC (found within many PDAs),
and a variety of other products.
</para>
</listitem></varlistentry>
<varlistentry>
<term><emphasis>Gadget Driver</emphasis></term>
<listitem>
<para>The lower boundary of this driver implements hardware-neutral
USB functions, using calls to the controller driver.
Because such hardware varies widely in capabilities and restrictions,
and is used in embedded environments where space is at a premium,
the gadget driver is often configured at compile time
to work with endpoints supported by one particular controller.
Gadget drivers may be portable to several different controllers,
using conditional compilation.
(Recent kernels substantially simplify the work involved in
supporting new hardware, by <emphasis>autoconfiguring</emphasis>
endpoints automatically for many bulk-oriented drivers.)
Gadget driver responsibilities include:
</para>
<itemizedlist>
<listitem><para>handling setup requests (ep0 protocol responses)
possibly including class-specific functionality
</para></listitem>
<listitem><para>returning configuration and string descriptors
</para></listitem>
<listitem><para>(re)setting configurations and interface
altsettings, including enabling and configuring endpoints
</para></listitem>
<listitem><para>handling life cycle events, such as managing
bindings to hardware,
USB suspend/resume, remote wakeup,
and disconnection from the USB host.
</para></listitem>
<listitem><para>managing IN and OUT transfers on all currently
enabled endpoints
</para></listitem>
</itemizedlist>
<para>
Such drivers may be modules of proprietary code, although
that approach is discouraged in the Linux community.
</para>
</listitem></varlistentry>
<varlistentry>
<term><emphasis>Upper Level</emphasis></term>
<listitem>
<para>Most gadget drivers have an upper boundary that connects
to some Linux driver or framework in Linux.
Through that boundary flows the data which the gadget driver
produces and/or consumes through protocol transfers over USB.
Examples include:
</para>
<itemizedlist>
<listitem><para>user mode code, using generic (gadgetfs)
or application specific files in
<filename>/dev</filename>
</para></listitem>
<listitem><para>networking subsystem (for network gadgets,
like the CDC Ethernet Model gadget driver)
</para></listitem>
<listitem><para>data capture drivers, perhaps video4Linux or
a scanner driver; or test and measurement hardware.
</para></listitem>
<listitem><para>input subsystem (for HID gadgets)
</para></listitem>
<listitem><para>sound subsystem (for audio gadgets)
</para></listitem>
<listitem><para>file system (for PTP gadgets)
</para></listitem>
<listitem><para>block i/o subsystem (for usb-storage gadgets)
</para></listitem>
<listitem><para>... and more </para></listitem>
</itemizedlist>
</listitem></varlistentry>
<varlistentry>
<term><emphasis>Additional Layers</emphasis></term>
<listitem>
<para>Other layers may exist.
These could include kernel layers, such as network protocol stacks,
as well as user mode applications building on standard POSIX
system call APIs such as
<emphasis>open()</emphasis>, <emphasis>close()</emphasis>,
<emphasis>read()</emphasis> and <emphasis>write()</emphasis>.
On newer systems, POSIX Async I/O calls may be an option.
Such user mode code will not necessarily be subject to
the GNU General Public License (GPL).
</para>
</listitem></varlistentry>
</variablelist>
<para>OTG-capable systems will also need to include a standard Linux-USB
host side stack,
with <emphasis>usbcore</emphasis>,
one or more <emphasis>Host Controller Drivers</emphasis> (HCDs),
<emphasis>USB Device Drivers</emphasis> to support
the OTG "Targeted Peripheral List",
and so forth.
There will also be an <emphasis>OTG Controller Driver</emphasis>,
which is visible to gadget and device driver developers only indirectly.
That helps the host and device side USB controllers implement the
two new OTG protocols (HNP and SRP).
Roles switch (host to peripheral, or vice versa) using HNP
during USB suspend processing, and SRP can be viewed as a
more battery-friendly kind of device wakeup protocol.
</para>
<para>Over time, reusable utilities are evolving to help make some
gadget driver tasks simpler.
For example, building configuration descriptors from vectors of
descriptors for the configurations interfaces and endpoints is
now automated, and many drivers now use autoconfiguration to
choose hardware endpoints and initialize their descriptors.
A potential example of particular interest
is code implementing standard USB-IF protocols for
HID, networking, storage, or audio classes.
Some developers are interested in KDB or KGDB hooks, to let
target hardware be remotely debugged.
Most such USB protocol code doesn't need to be hardware-specific,
any more than network protocols like X11, HTTP, or NFS are.
Such gadget-side interface drivers should eventually be combined,
to implement composite devices.
</para>
</chapter>
<chapter id="api"><title>Kernel Mode Gadget API</title>
<para>Gadget drivers declare themselves through a
<emphasis>struct usb_gadget_driver</emphasis>, which is responsible for
most parts of enumeration for a <emphasis>struct usb_gadget</emphasis>.
The response to a set_configuration usually involves
enabling one or more of the <emphasis>struct usb_ep</emphasis> objects
exposed by the gadget, and submitting one or more
<emphasis>struct usb_request</emphasis> buffers to transfer data.
Understand those four data types, and their operations, and
you will understand how this API works.
</para>
<note><title>Incomplete Data Type Descriptions</title>
<para>This documentation was prepared using the standard Linux
kernel <filename>docproc</filename> tool, which turns text
and in-code comments into SGML DocBook and then into usable
formats such as HTML or PDF.
Other than the "Chapter 9" data types, most of the significant
data types and functions are described here.
</para>
<para>However, docproc does not understand all the C constructs
that are used, so some relevant information is likely omitted from
what you are reading.
One example of such information is endpoint autoconfiguration.
You'll have to read the header file, and use example source
code (such as that for "Gadget Zero"), to fully understand the API.
</para>
<para>The part of the API implementing some basic
driver capabilities is specific to the version of the
Linux kernel that's in use.
The 2.6 kernel includes a <emphasis>driver model</emphasis>
framework that has no analogue on earlier kernels;
so those parts of the gadget API are not fully portable.
(They are implemented on 2.4 kernels, but in a different way.)
The driver model state is another part of this API that is
ignored by the kerneldoc tools.
</para>
</note>
<para>The core API does not expose
every possible hardware feature, only the most widely available ones.
There are significant hardware features, such as device-to-device DMA
(without temporary storage in a memory buffer)
that would be added using hardware-specific APIs.
</para>
<para>This API allows drivers to use conditional compilation to handle
endpoint capabilities of different hardware, but doesn't require that.
Hardware tends to have arbitrary restrictions, relating to
transfer types, addressing, packet sizes, buffering, and availability.
As a rule, such differences only matter for "endpoint zero" logic
that handles device configuration and management.
The API supports limited run-time
detection of capabilities, through naming conventions for endpoints.
Many drivers will be able to at least partially autoconfigure
themselves.
In particular, driver init sections will often have endpoint
autoconfiguration logic that scans the hardware's list of endpoints
to find ones matching the driver requirements
(relying on those conventions), to eliminate some of the most
common reasons for conditional compilation.
</para>
<para>Like the Linux-USB host side API, this API exposes
the "chunky" nature of USB messages: I/O requests are in terms
of one or more "packets", and packet boundaries are visible to drivers.
Compared to RS-232 serial protocols, USB resembles
synchronous protocols like HDLC
(N bytes per frame, multipoint addressing, host as the primary
station and devices as secondary stations)
more than asynchronous ones
(tty style: 8 data bits per frame, no parity, one stop bit).
So for example the controller drivers won't buffer
two single byte writes into a single two-byte USB IN packet,
although gadget drivers may do so when they implement
protocols where packet boundaries (and "short packets")
are not significant.
</para>
<sect1 id="lifecycle"><title>Driver Life Cycle</title>
<para>Gadget drivers make endpoint I/O requests to hardware without
needing to know many details of the hardware, but driver
setup/configuration code needs to handle some differences.
Use the API like this:
</para>
<orderedlist numeration='arabic'>
<listitem><para>Register a driver for the particular device side
usb controller hardware,
such as the net2280 on PCI (USB 2.0),
sa11x0 or pxa25x as found in Linux PDAs,
and so on.
At this point the device is logically in the USB ch9 initial state
("attached"), drawing no power and not usable
(since it does not yet support enumeration).
Any host should not see the device, since it's not
activated the data line pullup used by the host to
detect a device, even if VBUS power is available.
</para></listitem>
<listitem><para>Register a gadget driver that implements some higher level
device function. That will then bind() to a usb_gadget, which
activates the data line pullup sometime after detecting VBUS.
</para></listitem>
<listitem><para>The hardware driver can now start enumerating.
The steps it handles are to accept USB power and set_address requests.
Other steps are handled by the gadget driver.
If the gadget driver module is unloaded before the host starts to
enumerate, steps before step 7 are skipped.
</para></listitem>
<listitem><para>The gadget driver's setup() call returns usb descriptors,
based both on what the bus interface hardware provides and on the
functionality being implemented.
That can involve alternate settings or configurations,
unless the hardware prevents such operation.
For OTG devices, each configuration descriptor includes
an OTG descriptor.
</para></listitem>
<listitem><para>The gadget driver handles the last step of enumeration,
when the USB host issues a set_configuration call.
It enables all endpoints used in that configuration,
with all interfaces in their default settings.
That involves using a list of the hardware's endpoints, enabling each
endpoint according to its descriptor.
It may also involve using <function>usb_gadget_vbus_draw</function>
to let more power be drawn from VBUS, as allowed by that configuration.
For OTG devices, setting a configuration may also involve reporting
HNP capabilities through a user interface.
</para></listitem>
<listitem><para>Do real work and perform data transfers, possibly involving
changes to interface settings or switching to new configurations, until the
device is disconnect()ed from the host.
Queue any number of transfer requests to each endpoint.
It may be suspended and resumed several times before being disconnected.
On disconnect, the drivers go back to step 3 (above).
</para></listitem>
<listitem><para>When the gadget driver module is being unloaded,
the driver unbind() callback is issued. That lets the controller
driver be unloaded.
</para></listitem>
</orderedlist>
<para>Drivers will normally be arranged so that just loading the
gadget driver module (or statically linking it into a Linux kernel)
allows the peripheral device to be enumerated, but some drivers
will defer enumeration until some higher level component (like
a user mode daemon) enables it.
Note that at this lowest level there are no policies about how
ep0 configuration logic is implemented,
except that it should obey USB specifications.
Such issues are in the domain of gadget drivers,
including knowing about implementation constraints
imposed by some USB controllers
or understanding that composite devices might happen to
be built by integrating reusable components.
</para>
<para>Note that the lifecycle above can be slightly different
for OTG devices.
Other than providing an additional OTG descriptor in each
configuration, only the HNP-related differences are particularly
visible to driver code.
They involve reporting requirements during the SET_CONFIGURATION
request, and the option to invoke HNP during some suspend callbacks.
Also, SRP changes the semantics of
<function>usb_gadget_wakeup</function>
slightly.
</para>
</sect1>
<sect1 id="ch9"><title>USB 2.0 Chapter 9 Types and Constants</title>
<para>Gadget drivers
rely on common USB structures and constants
defined in the
<filename>&lt;linux/usb/ch9.h&gt;</filename>
header file, which is standard in Linux 2.6 kernels.
These are the same types and constants used by host
side drivers (and usbcore).
</para>
!Iinclude/linux/usb/ch9.h
</sect1>
<sect1 id="core"><title>Core Objects and Methods</title>
<para>These are declared in
<filename>&lt;linux/usb/gadget.h&gt;</filename>,
and are used by gadget drivers to interact with
USB peripheral controller drivers.
</para>
<!-- yeech, this is ugly in nsgmls PDF output.
the PDF bookmark and refentry output nesting is wrong,
and the member/argument documentation indents ugly.
plus something (docproc?) adds whitespace before the
descriptive paragraph text, so it can't line up right
unless the explanations are trivial.
-->
!Iinclude/linux/usb/gadget.h
</sect1>
<sect1 id="utils"><title>Optional Utilities</title>
<para>The core API is sufficient for writing a USB Gadget Driver,
but some optional utilities are provided to simplify common tasks.
These utilities include endpoint autoconfiguration.
</para>
!Edrivers/usb/gadget/usbstring.c
!Edrivers/usb/gadget/config.c
<!-- !Edrivers/usb/gadget/epautoconf.c -->
</sect1>
<sect1 id="composite"><title>Composite Device Framework</title>
<para>The core API is sufficient for writing drivers for composite
USB devices (with more than one function in a given configuration),
and also multi-configuration devices (also more than one function,
but not necessarily sharing a given configuration).
There is however an optional framework which makes it easier to
reuse and combine functions.
</para>
<para>Devices using this framework provide a <emphasis>struct
usb_composite_driver</emphasis>, which in turn provides one or
more <emphasis>struct usb_configuration</emphasis> instances.
Each such configuration includes at least one
<emphasis>struct usb_function</emphasis>, which packages a user
visible role such as "network link" or "mass storage device".
Management functions may also exist, such as "Device Firmware
Upgrade".
</para>
!Iinclude/linux/usb/composite.h
!Edrivers/usb/gadget/composite.c
</sect1>
<sect1 id="functions"><title>Composite Device Functions</title>
<para>At this writing, a few of the current gadget drivers have
been converted to this framework.
Near-term plans include converting all of them, except for "gadgetfs".
</para>
!Edrivers/usb/gadget/function/f_acm.c
!Edrivers/usb/gadget/function/f_ecm.c
!Edrivers/usb/gadget/function/f_subset.c
!Edrivers/usb/gadget/function/f_obex.c
!Edrivers/usb/gadget/function/f_serial.c
</sect1>
</chapter>
<chapter id="controllers"><title>Peripheral Controller Drivers</title>
<para>The first hardware supporting this API was the NetChip 2280
controller, which supports USB 2.0 high speed and is based on PCI.
This is the <filename>net2280</filename> driver module.
The driver supports Linux kernel versions 2.4 and 2.6;
contact NetChip Technologies for development boards and product
information.
</para>
<para>Other hardware working in the "gadget" framework includes:
Intel's PXA 25x and IXP42x series processors
(<filename>pxa2xx_udc</filename>),
Toshiba TC86c001 "Goku-S" (<filename>goku_udc</filename>),
Renesas SH7705/7727 (<filename>sh_udc</filename>),
MediaQ 11xx (<filename>mq11xx_udc</filename>),
Hynix HMS30C7202 (<filename>h7202_udc</filename>),
National 9303/4 (<filename>n9604_udc</filename>),
Texas Instruments OMAP (<filename>omap_udc</filename>),
Sharp LH7A40x (<filename>lh7a40x_udc</filename>),
and more.
Most of those are full speed controllers.
</para>
<para>At this writing, there are people at work on drivers in
this framework for several other USB device controllers,
with plans to make many of them be widely available.
</para>
<!-- !Edrivers/usb/gadget/net2280.c -->
<para>A partial USB simulator,
the <filename>dummy_hcd</filename> driver, is available.
It can act like a net2280, a pxa25x, or an sa11x0 in terms
of available endpoints and device speeds; and it simulates
control, bulk, and to some extent interrupt transfers.
That lets you develop some parts of a gadget driver on a normal PC,
without any special hardware, and perhaps with the assistance
of tools such as GDB running with User Mode Linux.
At least one person has expressed interest in adapting that
approach, hooking it up to a simulator for a microcontroller.
Such simulators can help debug subsystems where the runtime hardware
is unfriendly to software development, or is not yet available.
</para>
<para>Support for other controllers is expected to be developed
and contributed
over time, as this driver framework evolves.
</para>
</chapter>
<chapter id="gadget"><title>Gadget Drivers</title>
<para>In addition to <emphasis>Gadget Zero</emphasis>
(used primarily for testing and development with drivers
for usb controller hardware), other gadget drivers exist.
</para>
<para>There's an <emphasis>ethernet</emphasis> gadget
driver, which implements one of the most useful
<emphasis>Communications Device Class</emphasis> (CDC) models.
One of the standards for cable modem interoperability even
specifies the use of this ethernet model as one of two
mandatory options.
Gadgets using this code look to a USB host as if they're
an Ethernet adapter.
It provides access to a network where the gadget's CPU is one host,
which could easily be bridging, routing, or firewalling
access to other networks.
Since some hardware can't fully implement the CDC Ethernet
requirements, this driver also implements a "good parts only"
subset of CDC Ethernet.
(That subset doesn't advertise itself as CDC Ethernet,
to avoid creating problems.)
</para>
<para>Support for Microsoft's <emphasis>RNDIS</emphasis>
protocol has been contributed by Pengutronix and Auerswald GmbH.
This is like CDC Ethernet, but it runs on more slightly USB hardware
(but less than the CDC subset).
However, its main claim to fame is being able to connect directly to
recent versions of Windows, using drivers that Microsoft bundles
and supports, making it much simpler to network with Windows.
</para>
<para>There is also support for user mode gadget drivers,
using <emphasis>gadgetfs</emphasis>.
This provides a <emphasis>User Mode API</emphasis> that presents
each endpoint as a single file descriptor. I/O is done using
normal <emphasis>read()</emphasis> and <emphasis>read()</emphasis> calls.
Familiar tools like GDB and pthreads can be used to
develop and debug user mode drivers, so that once a robust
controller driver is available many applications for it
won't require new kernel mode software.
Linux 2.6 <emphasis>Async I/O (AIO)</emphasis>
support is available, so that user mode software
can stream data with only slightly more overhead
than a kernel driver.
</para>
<para>There's a USB Mass Storage class driver, which provides
a different solution for interoperability with systems such
as MS-Windows and MacOS.
That <emphasis>Mass Storage</emphasis> driver uses a
file or block device as backing store for a drive,
like the <filename>loop</filename> driver.
The USB host uses the BBB, CB, or CBI versions of the mass
storage class specification, using transparent SCSI commands
to access the data from the backing store.
</para>
<para>There's a "serial line" driver, useful for TTY style
operation over USB.
The latest version of that driver supports CDC ACM style
operation, like a USB modem, and so on most hardware it can
interoperate easily with MS-Windows.
One interesting use of that driver is in boot firmware (like a BIOS),
which can sometimes use that model with very small systems without
real serial lines.
</para>
<para>Support for other kinds of gadget is expected to
be developed and contributed
over time, as this driver framework evolves.
</para>
</chapter>
<chapter id="otg"><title>USB On-The-GO (OTG)</title>
<para>USB OTG support on Linux 2.6 was initially developed
by Texas Instruments for
<ulink url="http://www.omap.com">OMAP</ulink> 16xx and 17xx
series processors.
Other OTG systems should work in similar ways, but the
hardware level details could be very different.
</para>
<para>Systems need specialized hardware support to implement OTG,
notably including a special <emphasis>Mini-AB</emphasis> jack
and associated transceiver to support <emphasis>Dual-Role</emphasis>
operation:
they can act either as a host, using the standard
Linux-USB host side driver stack,
or as a peripheral, using this "gadget" framework.
To do that, the system software relies on small additions
to those programming interfaces,
and on a new internal component (here called an "OTG Controller")
affecting which driver stack connects to the OTG port.
In each role, the system can re-use the existing pool of
hardware-neutral drivers, layered on top of the controller
driver interfaces (<emphasis>usb_bus</emphasis> or
<emphasis>usb_gadget</emphasis>).
Such drivers need at most minor changes, and most of the calls
added to support OTG can also benefit non-OTG products.
</para>
<itemizedlist>
<listitem><para>Gadget drivers test the <emphasis>is_otg</emphasis>
flag, and use it to determine whether or not to include
an OTG descriptor in each of their configurations.
</para></listitem>
<listitem><para>Gadget drivers may need changes to support the
two new OTG protocols, exposed in new gadget attributes
such as <emphasis>b_hnp_enable</emphasis> flag.
HNP support should be reported through a user interface
(two LEDs could suffice), and is triggered in some cases
when the host suspends the peripheral.
SRP support can be user-initiated just like remote wakeup,
probably by pressing the same button.
</para></listitem>
<listitem><para>On the host side, USB device drivers need
to be taught to trigger HNP at appropriate moments, using
<function>usb_suspend_device()</function>.
That also conserves battery power, which is useful even
for non-OTG configurations.
</para></listitem>
<listitem><para>Also on the host side, a driver must support the
OTG "Targeted Peripheral List". That's just a whitelist,
used to reject peripherals not supported with a given
Linux OTG host.
<emphasis>This whitelist is product-specific;
each product must modify <filename>otg_whitelist.h</filename>
to match its interoperability specification.
</emphasis>
</para>
<para>Non-OTG Linux hosts, like PCs and workstations,
normally have some solution for adding drivers, so that
peripherals that aren't recognized can eventually be supported.
That approach is unreasonable for consumer products that may
never have their firmware upgraded, and where it's usually
unrealistic to expect traditional PC/workstation/server kinds
of support model to work.
For example, it's often impractical to change device firmware
once the product has been distributed, so driver bugs can't
normally be fixed if they're found after shipment.
</para></listitem>
</itemizedlist>
<para>
Additional changes are needed below those hardware-neutral
<emphasis>usb_bus</emphasis> and <emphasis>usb_gadget</emphasis>
driver interfaces; those aren't discussed here in any detail.
Those affect the hardware-specific code for each USB Host or Peripheral
controller, and how the HCD initializes (since OTG can be active only
on a single port).
They also involve what may be called an <emphasis>OTG Controller
Driver</emphasis>, managing the OTG transceiver and the OTG state
machine logic as well as much of the root hub behavior for the
OTG port.
The OTG controller driver needs to activate and deactivate USB
controllers depending on the relevant device role.
Some related changes were needed inside usbcore, so that it
can identify OTG-capable devices and respond appropriately
to HNP or SRP protocols.
</para>
</chapter>
</book>
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
<book id="Writing-MUSB-Glue-Layer">
<bookinfo>
<title>Writing an MUSB Glue Layer</title>
<authorgroup>
<author>
<firstname>Apelete</firstname>
<surname>Seketeli</surname>
<affiliation>
<address>
<email>apelete at seketeli.net</email>
</address>
</affiliation>
</author>
</authorgroup>
<copyright>
<year>2014</year>
<holder>Apelete Seketeli</holder>
</copyright>
<legalnotice>
<para>
This documentation is free software; you can redistribute it
and/or modify it under the terms of the GNU General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.
</para>
<para>
This documentation is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
</para>
<para>
You should have received a copy of the GNU General Public License
along with this documentation; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA
</para>
<para>
For more details see the file COPYING in the Linux kernel source
tree.
</para>
</legalnotice>
</bookinfo>
<toc></toc>
<chapter id="introduction">
<title>Introduction</title>
<para>
The Linux MUSB subsystem is part of the larger Linux USB
subsystem. It provides support for embedded USB Device Controllers
(UDC) that do not use Universal Host Controller Interface (UHCI)
or Open Host Controller Interface (OHCI).
</para>
<para>
Instead, these embedded UDC rely on the USB On-the-Go (OTG)
specification which they implement at least partially. The silicon
reference design used in most cases is the Multipoint USB
Highspeed Dual-Role Controller (MUSB HDRC) found in the Mentor
Graphics Inventra™ design.
</para>
<para>
As a self-taught exercise I have written an MUSB glue layer for
the Ingenic JZ4740 SoC, modelled after the many MUSB glue layers
in the kernel source tree. This layer can be found at
drivers/usb/musb/jz4740.c. In this documentation I will walk
through the basics of the jz4740.c glue layer, explaining the
different pieces and what needs to be done in order to write your
own device glue layer.
</para>
</chapter>
<chapter id="linux-musb-basics">
<title>Linux MUSB Basics</title>
<para>
To get started on the topic, please read USB On-the-Go Basics (see
Resources) which provides an introduction of USB OTG operation at
the hardware level. A couple of wiki pages by Texas Instruments
and Analog Devices also provide an overview of the Linux kernel
MUSB configuration, albeit focused on some specific devices
provided by these companies. Finally, getting acquainted with the
USB specification at USB home page may come in handy, with
practical instance provided through the Writing USB Device Drivers
documentation (again, see Resources).
</para>
<para>
Linux USB stack is a layered architecture in which the MUSB
controller hardware sits at the lowest. The MUSB controller driver
abstract the MUSB controller hardware to the Linux USB stack.
</para>
<programlisting>
------------------------
| | &lt;------- drivers/usb/gadget
| Linux USB Core Stack | &lt;------- drivers/usb/host
| | &lt;------- drivers/usb/core
------------------------
⬍
--------------------------
| | &lt;------ drivers/usb/musb/musb_gadget.c
| MUSB Controller driver | &lt;------ drivers/usb/musb/musb_host.c
| | &lt;------ drivers/usb/musb/musb_core.c
--------------------------
⬍
---------------------------------
| MUSB Platform Specific Driver |
| | &lt;-- drivers/usb/musb/jz4740.c
| aka &quot;Glue Layer&quot; |
---------------------------------
⬍
---------------------------------
| MUSB Controller Hardware |
---------------------------------
</programlisting>
<para>
As outlined above, the glue layer is actually the platform
specific code sitting in between the controller driver and the
controller hardware.
</para>
<para>
Just like a Linux USB driver needs to register itself with the
Linux USB subsystem, the MUSB glue layer needs first to register
itself with the MUSB controller driver. This will allow the
controller driver to know about which device the glue layer
supports and which functions to call when a supported device is
detected or released; remember we are talking about an embedded
controller chip here, so no insertion or removal at run-time.
</para>
<para>
All of this information is passed to the MUSB controller driver
through a platform_driver structure defined in the glue layer as:
</para>
<programlisting linenumbering="numbered">
static struct platform_driver jz4740_driver = {
.probe = jz4740_probe,
.remove = jz4740_remove,
.driver = {
.name = "musb-jz4740",
},
};
</programlisting>
<para>
The probe and remove function pointers are called when a matching
device is detected and, respectively, released. The name string
describes the device supported by this glue layer. In the current
case it matches a platform_device structure declared in
arch/mips/jz4740/platform.c. Note that we are not using device
tree bindings here.
</para>
<para>
In order to register itself to the controller driver, the glue
layer goes through a few steps, basically allocating the
controller hardware resources and initialising a couple of
circuits. To do so, it needs to keep track of the information used
throughout these steps. This is done by defining a private
jz4740_glue structure:
</para>
<programlisting linenumbering="numbered">
struct jz4740_glue {
struct device *dev;
struct platform_device *musb;
struct clk *clk;
};
</programlisting>
<para>
The dev and musb members are both device structure variables. The
first one holds generic information about the device, since it's
the basic device structure, and the latter holds information more
closely related to the subsystem the device is registered to. The
clk variable keeps information related to the device clock
operation.
</para>
<para>
Let's go through the steps of the probe function that leads the
glue layer to register itself to the controller driver.
</para>
<para>
N.B.: For the sake of readability each function will be split in
logical parts, each part being shown as if it was independent from
the others.
</para>
<programlisting linenumbering="numbered">
static int jz4740_probe(struct platform_device *pdev)
{
struct platform_device *musb;
struct jz4740_glue *glue;
struct clk *clk;
int ret;
glue = devm_kzalloc(&amp;pdev->dev, sizeof(*glue), GFP_KERNEL);
if (!glue)
return -ENOMEM;
musb = platform_device_alloc("musb-hdrc", PLATFORM_DEVID_AUTO);
if (!musb) {
dev_err(&amp;pdev->dev, "failed to allocate musb device\n");
return -ENOMEM;
}
clk = devm_clk_get(&amp;pdev->dev, "udc");
if (IS_ERR(clk)) {
dev_err(&amp;pdev->dev, "failed to get clock\n");
ret = PTR_ERR(clk);
goto err_platform_device_put;
}
ret = clk_prepare_enable(clk);
if (ret) {
dev_err(&amp;pdev->dev, "failed to enable clock\n");
goto err_platform_device_put;
}
musb->dev.parent = &amp;pdev->dev;
glue->dev = &amp;pdev->dev;
glue->musb = musb;
glue->clk = clk;
return 0;
err_platform_device_put:
platform_device_put(musb);
return ret;
}
</programlisting>
<para>
The first few lines of the probe function allocate and assign the
glue, musb and clk variables. The GFP_KERNEL flag (line 8) allows
the allocation process to sleep and wait for memory, thus being
usable in a blocking situation. The PLATFORM_DEVID_AUTO flag (line
12) allows automatic allocation and management of device IDs in
order to avoid device namespace collisions with explicit IDs. With
devm_clk_get() (line 18) the glue layer allocates the clock -- the
<literal>devm_</literal> prefix indicates that clk_get() is
managed: it automatically frees the allocated clock resource data
when the device is released -- and enable it.
</para>
<para>
Then comes the registration steps:
</para>
<programlisting linenumbering="numbered">
static int jz4740_probe(struct platform_device *pdev)
{
struct musb_hdrc_platform_data *pdata = &amp;jz4740_musb_platform_data;
pdata->platform_ops = &amp;jz4740_musb_ops;
platform_set_drvdata(pdev, glue);
ret = platform_device_add_resources(musb, pdev->resource,
pdev->num_resources);
if (ret) {
dev_err(&amp;pdev->dev, "failed to add resources\n");
goto err_clk_disable;
}
ret = platform_device_add_data(musb, pdata, sizeof(*pdata));
if (ret) {
dev_err(&amp;pdev->dev, "failed to add platform_data\n");
goto err_clk_disable;
}
return 0;
err_clk_disable:
clk_disable_unprepare(clk);
err_platform_device_put:
platform_device_put(musb);
return ret;
}
</programlisting>
<para>
The first step is to pass the device data privately held by the
glue layer on to the controller driver through
platform_set_drvdata() (line 7). Next is passing on the device
resources information, also privately held at that point, through
platform_device_add_resources() (line 9).
</para>
<para>
Finally comes passing on the platform specific data to the
controller driver (line 16). Platform data will be discussed in
<link linkend="device-platform-data">Chapter 4</link>, but here
we are looking at the platform_ops function pointer (line 5) in
musb_hdrc_platform_data structure (line 3). This function
pointer allows the MUSB controller driver to know which function
to call for device operation:
</para>
<programlisting linenumbering="numbered">
static const struct musb_platform_ops jz4740_musb_ops = {
.init = jz4740_musb_init,
.exit = jz4740_musb_exit,
};
</programlisting>
<para>
Here we have the minimal case where only init and exit functions
are called by the controller driver when needed. Fact is the
JZ4740 MUSB controller is a basic controller, lacking some
features found in other controllers, otherwise we may also have
pointers to a few other functions like a power management function
or a function to switch between OTG and non-OTG modes, for
instance.
</para>
<para>
At that point of the registration process, the controller driver
actually calls the init function:
</para>
<programlisting linenumbering="numbered">
static int jz4740_musb_init(struct musb *musb)
{
musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
if (!musb->xceiv) {
pr_err("HS UDC: no transceiver configured\n");
return -ENODEV;
}
/* Silicon does not implement ConfigData register.
* Set dyn_fifo to avoid reading EP config from hardware.
*/
musb->dyn_fifo = true;
musb->isr = jz4740_musb_interrupt;
return 0;
}
</programlisting>
<para>
The goal of jz4740_musb_init() is to get hold of the transceiver
driver data of the MUSB controller hardware and pass it on to the
MUSB controller driver, as usual. The transceiver is the circuitry
inside the controller hardware responsible for sending/receiving
the USB data. Since it is an implementation of the physical layer
of the OSI model, the transceiver is also referred to as PHY.
</para>
<para>
Getting hold of the MUSB PHY driver data is done with
usb_get_phy() which returns a pointer to the structure
containing the driver instance data. The next couple of
instructions (line 12 and 14) are used as a quirk and to setup
IRQ handling respectively. Quirks and IRQ handling will be
discussed later in <link linkend="device-quirks">Chapter
5</link> and <link linkend="handling-irqs">Chapter 3</link>.
</para>
<programlisting linenumbering="numbered">
static int jz4740_musb_exit(struct musb *musb)
{
usb_put_phy(musb->xceiv);
return 0;
}
</programlisting>
<para>
Acting as the counterpart of init, the exit function releases the
MUSB PHY driver when the controller hardware itself is about to be
released.
</para>
<para>
Again, note that init and exit are fairly simple in this case due
to the basic set of features of the JZ4740 controller hardware.
When writing an musb glue layer for a more complex controller
hardware, you might need to take care of more processing in those
two functions.
</para>
<para>
Returning from the init function, the MUSB controller driver jumps
back into the probe function:
</para>
<programlisting linenumbering="numbered">
static int jz4740_probe(struct platform_device *pdev)
{
ret = platform_device_add(musb);
if (ret) {
dev_err(&amp;pdev->dev, "failed to register musb device\n");
goto err_clk_disable;
}
return 0;
err_clk_disable:
clk_disable_unprepare(clk);
err_platform_device_put:
platform_device_put(musb);
return ret;
}
</programlisting>
<para>
This is the last part of the device registration process where the
glue layer adds the controller hardware device to Linux kernel
device hierarchy: at this stage, all known information about the
device is passed on to the Linux USB core stack.
</para>
<programlisting linenumbering="numbered">
static int jz4740_remove(struct platform_device *pdev)
{
struct jz4740_glue *glue = platform_get_drvdata(pdev);
platform_device_unregister(glue->musb);
clk_disable_unprepare(glue->clk);
return 0;
}
</programlisting>
<para>
Acting as the counterpart of probe, the remove function unregister
the MUSB controller hardware (line 5) and disable the clock (line
6), allowing it to be gated.
</para>
</chapter>
<chapter id="handling-irqs">
<title>Handling IRQs</title>
<para>
Additionally to the MUSB controller hardware basic setup and
registration, the glue layer is also responsible for handling the
IRQs:
</para>
<programlisting linenumbering="numbered">
static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
{
unsigned long flags;
irqreturn_t retval = IRQ_NONE;
struct musb *musb = __hci;
spin_lock_irqsave(&amp;musb->lock, flags);
musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
/*
* The controller is gadget only, the state of the host mode IRQ bits is
* undefined. Mask them to make sure that the musb driver core will
* never see them set
*/
musb->int_usb &amp;= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
MUSB_INTR_RESET | MUSB_INTR_SOF;
if (musb->int_usb || musb->int_tx || musb->int_rx)
retval = musb_interrupt(musb);
spin_unlock_irqrestore(&amp;musb->lock, flags);
return retval;
}
</programlisting>
<para>
Here the glue layer mostly has to read the relevant hardware
registers and pass their values on to the controller driver which
will handle the actual event that triggered the IRQ.
</para>
<para>
The interrupt handler critical section is protected by the
spin_lock_irqsave() and counterpart spin_unlock_irqrestore()
functions (line 7 and 24 respectively), which prevent the
interrupt handler code to be run by two different threads at the
same time.
</para>
<para>
Then the relevant interrupt registers are read (line 9 to 11):
</para>
<itemizedlist>
<listitem>
<para>
MUSB_INTRUSB: indicates which USB interrupts are currently
active,
</para>
</listitem>
<listitem>
<para>
MUSB_INTRTX: indicates which of the interrupts for TX
endpoints are currently active,
</para>
</listitem>
<listitem>
<para>
MUSB_INTRRX: indicates which of the interrupts for TX
endpoints are currently active.
</para>
</listitem>
</itemizedlist>
<para>
Note that musb_readb() is used to read 8-bit registers at most,
while musb_readw() allows us to read at most 16-bit registers.
There are other functions that can be used depending on the size
of your device registers. See musb_io.h for more information.
</para>
<para>
Instruction on line 18 is another quirk specific to the JZ4740
USB device controller, which will be discussed later in <link
linkend="device-quirks">Chapter 5</link>.
</para>
<para>
The glue layer still needs to register the IRQ handler though.
Remember the instruction on line 14 of the init function:
</para>
<programlisting linenumbering="numbered">
static int jz4740_musb_init(struct musb *musb)
{
musb->isr = jz4740_musb_interrupt;
return 0;
}
</programlisting>
<para>
This instruction sets a pointer to the glue layer IRQ handler
function, in order for the controller hardware to call the handler
back when an IRQ comes from the controller hardware. The interrupt
handler is now implemented and registered.
</para>
</chapter>
<chapter id="device-platform-data">
<title>Device Platform Data</title>
<para>
In order to write an MUSB glue layer, you need to have some data
describing the hardware capabilities of your controller hardware,
which is called the platform data.
</para>
<para>
Platform data is specific to your hardware, though it may cover a
broad range of devices, and is generally found somewhere in the
arch/ directory, depending on your device architecture.
</para>
<para>
For instance, platform data for the JZ4740 SoC is found in
arch/mips/jz4740/platform.c. In the platform.c file each device of
the JZ4740 SoC is described through a set of structures.
</para>
<para>
Here is the part of arch/mips/jz4740/platform.c that covers the
USB Device Controller (UDC):
</para>
<programlisting linenumbering="numbered">
/* USB Device Controller */
struct platform_device jz4740_udc_xceiv_device = {
.name = "usb_phy_gen_xceiv",
.id = 0,
};
static struct resource jz4740_udc_resources[] = {
[0] = {
.start = JZ4740_UDC_BASE_ADDR,
.end = JZ4740_UDC_BASE_ADDR + 0x10000 - 1,
.flags = IORESOURCE_MEM,
},
[1] = {
.start = JZ4740_IRQ_UDC,
.end = JZ4740_IRQ_UDC,
.flags = IORESOURCE_IRQ,
.name = "mc",
},
};
struct platform_device jz4740_udc_device = {
.name = "musb-jz4740",
.id = -1,
.dev = {
.dma_mask = &amp;jz4740_udc_device.dev.coherent_dma_mask,
.coherent_dma_mask = DMA_BIT_MASK(32),
},
.num_resources = ARRAY_SIZE(jz4740_udc_resources),
.resource = jz4740_udc_resources,
};
</programlisting>
<para>
The jz4740_udc_xceiv_device platform device structure (line 2)
describes the UDC transceiver with a name and id number.
</para>
<para>
At the time of this writing, note that
&quot;usb_phy_gen_xceiv&quot; is the specific name to be used for
all transceivers that are either built-in with reference USB IP or
autonomous and doesn't require any PHY programming. You will need
to set CONFIG_NOP_USB_XCEIV=y in the kernel configuration to make
use of the corresponding transceiver driver. The id field could be
set to -1 (equivalent to PLATFORM_DEVID_NONE), -2 (equivalent to
PLATFORM_DEVID_AUTO) or start with 0 for the first device of this
kind if we want a specific id number.
</para>
<para>
The jz4740_udc_resources resource structure (line 7) defines the
UDC registers base addresses.
</para>
<para>
The first array (line 9 to 11) defines the UDC registers base
memory addresses: start points to the first register memory
address, end points to the last register memory address and the
flags member defines the type of resource we are dealing with. So
IORESOURCE_MEM is used to define the registers memory addresses.
The second array (line 14 to 17) defines the UDC IRQ registers
addresses. Since there is only one IRQ register available for the
JZ4740 UDC, start and end point at the same address. The
IORESOURCE_IRQ flag tells that we are dealing with IRQ resources,
and the name &quot;mc&quot; is in fact hard-coded in the MUSB core
in order for the controller driver to retrieve this IRQ resource
by querying it by its name.
</para>
<para>
Finally, the jz4740_udc_device platform device structure (line 21)
describes the UDC itself.
</para>
<para>
The &quot;musb-jz4740&quot; name (line 22) defines the MUSB
driver that is used for this device; remember this is in fact
the name that we used in the jz4740_driver platform driver
structure in <link linkend="linux-musb-basics">Chapter
2</link>. The id field (line 23) is set to -1 (equivalent to
PLATFORM_DEVID_NONE) since we do not need an id for the device:
the MUSB controller driver was already set to allocate an
automatic id in <link linkend="linux-musb-basics">Chapter
2</link>. In the dev field we care for DMA related information
here. The dma_mask field (line 25) defines the width of the DMA
mask that is going to be used, and coherent_dma_mask (line 26)
has the same purpose but for the alloc_coherent DMA mappings: in
both cases we are using a 32 bits mask. Then the resource field
(line 29) is simply a pointer to the resource structure defined
before, while the num_resources field (line 28) keeps track of
the number of arrays defined in the resource structure (in this
case there were two resource arrays defined before).
</para>
<para>
With this quick overview of the UDC platform data at the arch/
level now done, let's get back to the MUSB glue layer specific
platform data in drivers/usb/musb/jz4740.c:
</para>
<programlisting linenumbering="numbered">
static struct musb_hdrc_config jz4740_musb_config = {
/* Silicon does not implement USB OTG. */
.multipoint = 0,
/* Max EPs scanned, driver will decide which EP can be used. */
.num_eps = 4,
/* RAMbits needed to configure EPs from table */
.ram_bits = 9,
.fifo_cfg = jz4740_musb_fifo_cfg,
.fifo_cfg_size = ARRAY_SIZE(jz4740_musb_fifo_cfg),
};
static struct musb_hdrc_platform_data jz4740_musb_platform_data = {
.mode = MUSB_PERIPHERAL,
.config = &amp;jz4740_musb_config,
};
</programlisting>
<para>
First the glue layer configures some aspects of the controller
driver operation related to the controller hardware specifics.
This is done through the jz4740_musb_config musb_hdrc_config
structure.
</para>
<para>
Defining the OTG capability of the controller hardware, the
multipoint member (line 3) is set to 0 (equivalent to false)
since the JZ4740 UDC is not OTG compatible. Then num_eps (line
5) defines the number of USB endpoints of the controller
hardware, including endpoint 0: here we have 3 endpoints +
endpoint 0. Next is ram_bits (line 7) which is the width of the
RAM address bus for the MUSB controller hardware. This
information is needed when the controller driver cannot
automatically configure endpoints by reading the relevant
controller hardware registers. This issue will be discussed when
we get to device quirks in <link linkend="device-quirks">Chapter
5</link>. Last two fields (line 8 and 9) are also about device
quirks: fifo_cfg points to the USB endpoints configuration table
and fifo_cfg_size keeps track of the size of the number of
entries in that configuration table. More on that later in <link
linkend="device-quirks">Chapter 5</link>.
</para>
<para>
Then this configuration is embedded inside
jz4740_musb_platform_data musb_hdrc_platform_data structure (line
11): config is a pointer to the configuration structure itself,
and mode tells the controller driver if the controller hardware
may be used as MUSB_HOST only, MUSB_PERIPHERAL only or MUSB_OTG
which is a dual mode.
</para>
<para>
Remember that jz4740_musb_platform_data is then used to convey
platform data information as we have seen in the probe function
in <link linkend="linux-musb-basics">Chapter 2</link>
</para>
</chapter>
<chapter id="device-quirks">
<title>Device Quirks</title>
<para>
Completing the platform data specific to your device, you may also
need to write some code in the glue layer to work around some
device specific limitations. These quirks may be due to some
hardware bugs, or simply be the result of an incomplete
implementation of the USB On-the-Go specification.
</para>
<para>
The JZ4740 UDC exhibits such quirks, some of which we will discuss
here for the sake of insight even though these might not be found
in the controller hardware you are working on.
</para>
<para>
Let's get back to the init function first:
</para>
<programlisting linenumbering="numbered">
static int jz4740_musb_init(struct musb *musb)
{
musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
if (!musb->xceiv) {
pr_err("HS UDC: no transceiver configured\n");
return -ENODEV;
}
/* Silicon does not implement ConfigData register.
* Set dyn_fifo to avoid reading EP config from hardware.
*/
musb->dyn_fifo = true;
musb->isr = jz4740_musb_interrupt;
return 0;
}
</programlisting>
<para>
Instruction on line 12 helps the MUSB controller driver to work
around the fact that the controller hardware is missing registers
that are used for USB endpoints configuration.
</para>
<para>
Without these registers, the controller driver is unable to read
the endpoints configuration from the hardware, so we use line 12
instruction to bypass reading the configuration from silicon, and
rely on a hard-coded table that describes the endpoints
configuration instead:
</para>
<programlisting linenumbering="numbered">
static struct musb_fifo_cfg jz4740_musb_fifo_cfg[] = {
{ .hw_ep_num = 1, .style = FIFO_TX, .maxpacket = 512, },
{ .hw_ep_num = 1, .style = FIFO_RX, .maxpacket = 512, },
{ .hw_ep_num = 2, .style = FIFO_TX, .maxpacket = 64, },
};
</programlisting>
<para>
Looking at the configuration table above, we see that each
endpoints is described by three fields: hw_ep_num is the endpoint
number, style is its direction (either FIFO_TX for the controller
driver to send packets in the controller hardware, or FIFO_RX to
receive packets from hardware), and maxpacket defines the maximum
size of each data packet that can be transmitted over that
endpoint. Reading from the table, the controller driver knows that
endpoint 1 can be used to send and receive USB data packets of 512
bytes at once (this is in fact a bulk in/out endpoint), and
endpoint 2 can be used to send data packets of 64 bytes at once
(this is in fact an interrupt endpoint).
</para>
<para>
Note that there is no information about endpoint 0 here: that one
is implemented by default in every silicon design, with a
predefined configuration according to the USB specification. For
more examples of endpoint configuration tables, see musb_core.c.
</para>
<para>
Let's now get back to the interrupt handler function:
</para>
<programlisting linenumbering="numbered">
static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
{
unsigned long flags;
irqreturn_t retval = IRQ_NONE;
struct musb *musb = __hci;
spin_lock_irqsave(&amp;musb->lock, flags);
musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
/*
* The controller is gadget only, the state of the host mode IRQ bits is
* undefined. Mask them to make sure that the musb driver core will
* never see them set
*/
musb->int_usb &amp;= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
MUSB_INTR_RESET | MUSB_INTR_SOF;
if (musb->int_usb || musb->int_tx || musb->int_rx)
retval = musb_interrupt(musb);
spin_unlock_irqrestore(&amp;musb->lock, flags);
return retval;
}
</programlisting>
<para>
Instruction on line 18 above is a way for the controller driver to
work around the fact that some interrupt bits used for USB host
mode operation are missing in the MUSB_INTRUSB register, thus left
in an undefined hardware state, since this MUSB controller
hardware is used in peripheral mode only. As a consequence, the
glue layer masks these missing bits out to avoid parasite
interrupts by doing a logical AND operation between the value read
from MUSB_INTRUSB and the bits that are actually implemented in
the register.
</para>
<para>
These are only a couple of the quirks found in the JZ4740 USB
device controller. Some others were directly addressed in the MUSB
core since the fixes were generic enough to provide a better
handling of the issues for others controller hardware eventually.
</para>
</chapter>
<chapter id="conclusion">
<title>Conclusion</title>
<para>
Writing a Linux MUSB glue layer should be a more accessible task,
as this documentation tries to show the ins and outs of this
exercise.
</para>
<para>
The JZ4740 USB device controller being fairly simple, I hope its
glue layer serves as a good example for the curious mind. Used
with the current MUSB glue layers, this documentation should
provide enough guidance to get started; should anything gets out
of hand, the linux-usb mailing list archive is another helpful
resource to browse through.
</para>
</chapter>
<chapter id="acknowledgements">
<title>Acknowledgements</title>
<para>
Many thanks to Lars-Peter Clausen and Maarten ter Huurne for
answering my questions while I was writing the JZ4740 glue layer
and for helping me out getting the code in good shape.
</para>
<para>
I would also like to thank the Qi-Hardware community at large for
its cheerful guidance and support.
</para>
</chapter>
<chapter id="resources">
<title>Resources</title>
<para>
USB Home Page:
<ulink url="http://www.usb.org">http://www.usb.org</ulink>
</para>
<para>
linux-usb Mailing List Archives:
<ulink url="http://marc.info/?l=linux-usb">http://marc.info/?l=linux-usb</ulink>
</para>
<para>
USB On-the-Go Basics:
<ulink url="http://www.maximintegrated.com/app-notes/index.mvp/id/1822">http://www.maximintegrated.com/app-notes/index.mvp/id/1822</ulink>
</para>
<para>
Writing USB Device Drivers:
<ulink url="https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html">https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html</ulink>
</para>
<para>
Texas Instruments USB Configuration Wiki Page:
<ulink url="http://processors.wiki.ti.com/index.php/Usbgeneralpage">http://processors.wiki.ti.com/index.php/Usbgeneralpage</ulink>
</para>
<para>
Analog Devices Blackfin MUSB Configuration:
<ulink url="http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb">http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb</ulink>
</para>
</chapter>
</book>

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@ -1,412 +0,0 @@
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
<book id="USBDeviceDriver">
<bookinfo>
<title>Writing USB Device Drivers</title>
<authorgroup>
<author>
<firstname>Greg</firstname>
<surname>Kroah-Hartman</surname>
<affiliation>
<address>
<email>greg@kroah.com</email>
</address>
</affiliation>
</author>
</authorgroup>
<copyright>
<year>2001-2002</year>
<holder>Greg Kroah-Hartman</holder>
</copyright>
<legalnotice>
<para>
This documentation is free software; you can redistribute
it and/or modify it under the terms of the GNU General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later
version.
</para>
<para>
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
</para>
<para>
You should have received a copy of the GNU General Public
License along with this program; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA
</para>
<para>
For more details see the file COPYING in the source
distribution of Linux.
</para>
<para>
This documentation is based on an article published in
Linux Journal Magazine, October 2001, Issue 90.
</para>
</legalnotice>
</bookinfo>
<toc></toc>
<chapter id="intro">
<title>Introduction</title>
<para>
The Linux USB subsystem has grown from supporting only two different
types of devices in the 2.2.7 kernel (mice and keyboards), to over 20
different types of devices in the 2.4 kernel. Linux currently supports
almost all USB class devices (standard types of devices like keyboards,
mice, modems, printers and speakers) and an ever-growing number of
vendor-specific devices (such as USB to serial converters, digital
cameras, Ethernet devices and MP3 players). For a full list of the
different USB devices currently supported, see Resources.
</para>
<para>
The remaining kinds of USB devices that do not have support on Linux are
almost all vendor-specific devices. Each vendor decides to implement a
custom protocol to talk to their device, so a custom driver usually needs
to be created. Some vendors are open with their USB protocols and help
with the creation of Linux drivers, while others do not publish them, and
developers are forced to reverse-engineer. See Resources for some links
to handy reverse-engineering tools.
</para>
<para>
Because each different protocol causes a new driver to be created, I have
written a generic USB driver skeleton, modelled after the pci-skeleton.c
file in the kernel source tree upon which many PCI network drivers have
been based. This USB skeleton can be found at drivers/usb/usb-skeleton.c
in the kernel source tree. In this article I will walk through the basics
of the skeleton driver, explaining the different pieces and what needs to
be done to customize it to your specific device.
</para>
</chapter>
<chapter id="basics">
<title>Linux USB Basics</title>
<para>
If you are going to write a Linux USB driver, please become familiar with
the USB protocol specification. It can be found, along with many other
useful documents, at the USB home page (see Resources). An excellent
introduction to the Linux USB subsystem can be found at the USB Working
Devices List (see Resources). It explains how the Linux USB subsystem is
structured and introduces the reader to the concept of USB urbs
(USB Request Blocks), which are essential to USB drivers.
</para>
<para>
The first thing a Linux USB driver needs to do is register itself with
the Linux USB subsystem, giving it some information about which devices
the driver supports and which functions to call when a device supported
by the driver is inserted or removed from the system. All of this
information is passed to the USB subsystem in the usb_driver structure.
The skeleton driver declares a usb_driver as:
</para>
<programlisting>
static struct usb_driver skel_driver = {
.name = "skeleton",
.probe = skel_probe,
.disconnect = skel_disconnect,
.fops = &amp;skel_fops,
.minor = USB_SKEL_MINOR_BASE,
.id_table = skel_table,
};
</programlisting>
<para>
The variable name is a string that describes the driver. It is used in
informational messages printed to the system log. The probe and
disconnect function pointers are called when a device that matches the
information provided in the id_table variable is either seen or removed.
</para>
<para>
The fops and minor variables are optional. Most USB drivers hook into
another kernel subsystem, such as the SCSI, network or TTY subsystem.
These types of drivers register themselves with the other kernel
subsystem, and any user-space interactions are provided through that
interface. But for drivers that do not have a matching kernel subsystem,
such as MP3 players or scanners, a method of interacting with user space
is needed. The USB subsystem provides a way to register a minor device
number and a set of file_operations function pointers that enable this
user-space interaction. The skeleton driver needs this kind of interface,
so it provides a minor starting number and a pointer to its
file_operations functions.
</para>
<para>
The USB driver is then registered with a call to usb_register, usually in
the driver's init function, as shown here:
</para>
<programlisting>
static int __init usb_skel_init(void)
{
int result;
/* register this driver with the USB subsystem */
result = usb_register(&amp;skel_driver);
if (result &lt; 0) {
err(&quot;usb_register failed for the &quot;__FILE__ &quot;driver.&quot;
&quot;Error number %d&quot;, result);
return -1;
}
return 0;
}
module_init(usb_skel_init);
</programlisting>
<para>
When the driver is unloaded from the system, it needs to deregister
itself with the USB subsystem. This is done with the usb_deregister
function:
</para>
<programlisting>
static void __exit usb_skel_exit(void)
{
/* deregister this driver with the USB subsystem */
usb_deregister(&amp;skel_driver);
}
module_exit(usb_skel_exit);
</programlisting>
<para>
To enable the linux-hotplug system to load the driver automatically when
the device is plugged in, you need to create a MODULE_DEVICE_TABLE. The
following code tells the hotplug scripts that this module supports a
single device with a specific vendor and product ID:
</para>
<programlisting>
/* table of devices that work with this driver */
static struct usb_device_id skel_table [] = {
{ USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) },
{ } /* Terminating entry */
};
MODULE_DEVICE_TABLE (usb, skel_table);
</programlisting>
<para>
There are other macros that can be used in describing a usb_device_id for
drivers that support a whole class of USB drivers. See usb.h for more
information on this.
</para>
</chapter>
<chapter id="device">
<title>Device operation</title>
<para>
When a device is plugged into the USB bus that matches the device ID
pattern that your driver registered with the USB core, the probe function
is called. The usb_device structure, interface number and the interface ID
are passed to the function:
</para>
<programlisting>
static int skel_probe(struct usb_interface *interface,
const struct usb_device_id *id)
</programlisting>
<para>
The driver now needs to verify that this device is actually one that it
can accept. If so, it returns 0.
If not, or if any error occurs during initialization, an errorcode
(such as <literal>-ENOMEM</literal> or <literal>-ENODEV</literal>)
is returned from the probe function.
</para>
<para>
In the skeleton driver, we determine what end points are marked as bulk-in
and bulk-out. We create buffers to hold the data that will be sent and
received from the device, and a USB urb to write data to the device is
initialized.
</para>
<para>
Conversely, when the device is removed from the USB bus, the disconnect
function is called with the device pointer. The driver needs to clean any
private data that has been allocated at this time and to shut down any
pending urbs that are in the USB system.
</para>
<para>
Now that the device is plugged into the system and the driver is bound to
the device, any of the functions in the file_operations structure that
were passed to the USB subsystem will be called from a user program trying
to talk to the device. The first function called will be open, as the
program tries to open the device for I/O. We increment our private usage
count and save a pointer to our internal structure in the file
structure. This is done so that future calls to file operations will
enable the driver to determine which device the user is addressing. All
of this is done with the following code:
</para>
<programlisting>
/* increment our usage count for the module */
++skel->open_count;
/* save our object in the file's private structure */
file->private_data = dev;
</programlisting>
<para>
After the open function is called, the read and write functions are called
to receive and send data to the device. In the skel_write function, we
receive a pointer to some data that the user wants to send to the device
and the size of the data. The function determines how much data it can
send to the device based on the size of the write urb it has created (this
size depends on the size of the bulk out end point that the device has).
Then it copies the data from user space to kernel space, points the urb to
the data and submits the urb to the USB subsystem. This can be seen in
the following code:
</para>
<programlisting>
/* we can only write as much as 1 urb will hold */
bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count;
/* copy the data from user space into our urb */
copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written);
/* set up our urb */
usb_fill_bulk_urb(skel->write_urb,
skel->dev,
usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr),
skel->write_urb->transfer_buffer,
bytes_written,
skel_write_bulk_callback,
skel);
/* send the data out the bulk port */
result = usb_submit_urb(skel->write_urb);
if (result) {
err(&quot;Failed submitting write urb, error %d&quot;, result);
}
</programlisting>
<para>
When the write urb is filled up with the proper information using the
usb_fill_bulk_urb function, we point the urb's completion callback to call our
own skel_write_bulk_callback function. This function is called when the
urb is finished by the USB subsystem. The callback function is called in
interrupt context, so caution must be taken not to do very much processing
at that time. Our implementation of skel_write_bulk_callback merely
reports if the urb was completed successfully or not and then returns.
</para>
<para>
The read function works a bit differently from the write function in that
we do not use an urb to transfer data from the device to the driver.
Instead we call the usb_bulk_msg function, which can be used to send or
receive data from a device without having to create urbs and handle
urb completion callback functions. We call the usb_bulk_msg function,
giving it a buffer into which to place any data received from the device
and a timeout value. If the timeout period expires without receiving any
data from the device, the function will fail and return an error message.
This can be shown with the following code:
</para>
<programlisting>
/* do an immediate bulk read to get data from the device */
retval = usb_bulk_msg (skel->dev,
usb_rcvbulkpipe (skel->dev,
skel->bulk_in_endpointAddr),
skel->bulk_in_buffer,
skel->bulk_in_size,
&amp;count, HZ*10);
/* if the read was successful, copy the data to user space */
if (!retval) {
if (copy_to_user (buffer, skel->bulk_in_buffer, count))
retval = -EFAULT;
else
retval = count;
}
</programlisting>
<para>
The usb_bulk_msg function can be very useful for doing single reads or
writes to a device; however, if you need to read or write constantly to a
device, it is recommended to set up your own urbs and submit them to the
USB subsystem.
</para>
<para>
When the user program releases the file handle that it has been using to
talk to the device, the release function in the driver is called. In this
function we decrement our private usage count and wait for possible
pending writes:
</para>
<programlisting>
/* decrement our usage count for the device */
--skel->open_count;
</programlisting>
<para>
One of the more difficult problems that USB drivers must be able to handle
smoothly is the fact that the USB device may be removed from the system at
any point in time, even if a program is currently talking to it. It needs
to be able to shut down any current reads and writes and notify the
user-space programs that the device is no longer there. The following
code (function <function>skel_delete</function>)
is an example of how to do this: </para>
<programlisting>
static inline void skel_delete (struct usb_skel *dev)
{
kfree (dev->bulk_in_buffer);
if (dev->bulk_out_buffer != NULL)
usb_free_coherent (dev->udev, dev->bulk_out_size,
dev->bulk_out_buffer,
dev->write_urb->transfer_dma);
usb_free_urb (dev->write_urb);
kfree (dev);
}
</programlisting>
<para>
If a program currently has an open handle to the device, we reset the flag
<literal>device_present</literal>. For
every read, write, release and other functions that expect a device to be
present, the driver first checks this flag to see if the device is
still present. If not, it releases that the device has disappeared, and a
-ENODEV error is returned to the user-space program. When the release
function is eventually called, it determines if there is no device
and if not, it does the cleanup that the skel_disconnect
function normally does if there are no open files on the device (see
Listing 5).
</para>
</chapter>
<chapter id="iso">
<title>Isochronous Data</title>
<para>
This usb-skeleton driver does not have any examples of interrupt or
isochronous data being sent to or from the device. Interrupt data is sent
almost exactly as bulk data is, with a few minor exceptions. Isochronous
data works differently with continuous streams of data being sent to or
from the device. The audio and video camera drivers are very good examples
of drivers that handle isochronous data and will be useful if you also
need to do this.
</para>
</chapter>
<chapter id="Conclusion">
<title>Conclusion</title>
<para>
Writing Linux USB device drivers is not a difficult task as the
usb-skeleton driver shows. This driver, combined with the other current
USB drivers, should provide enough examples to help a beginning author
create a working driver in a minimal amount of time. The linux-usb-devel
mailing list archives also contain a lot of helpful information.
</para>
</chapter>
<chapter id="resources">
<title>Resources</title>
<para>
The Linux USB Project: <ulink url="http://www.linux-usb.org">http://www.linux-usb.org/</ulink>
</para>
<para>
Linux Hotplug Project: <ulink url="http://linux-hotplug.sourceforge.net">http://linux-hotplug.sourceforge.net/</ulink>
</para>
<para>
Linux USB Working Devices List: <ulink url="http://www.qbik.ch/usb/devices">http://www.qbik.ch/usb/devices/</ulink>
</para>
<para>
linux-usb-devel Mailing List Archives: <ulink url="http://marc.theaimsgroup.com/?l=linux-usb-devel">http://marc.theaimsgroup.com/?l=linux-usb-devel</ulink>
</para>
<para>
Programming Guide for Linux USB Device Drivers: <ulink url="http://usb.cs.tum.edu/usbdoc">http://usb.cs.tum.edu/usbdoc</ulink>
</para>
<para>
USB Home Page: <ulink url="http://www.usb.org">http://www.usb.org</ulink>
</para>
</chapter>
</book>

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@ -26,7 +26,7 @@ available subsections can be seen below.
regulator
iio/index
input
usb
usb/index
pci
spi
i2c

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========================
USB Gadget API for Linux
========================
:Author: David Brownell
:Date: 20 August 2004
Introduction
============
This document presents a Linux-USB "Gadget" kernel mode API, for use
within peripherals and other USB devices that embed Linux. It provides
an overview of the API structure, and shows how that fits into a system
development project. This is the first such API released on Linux to
address a number of important problems, including:
- Supports USB 2.0, for high speed devices which can stream data at
several dozen megabytes per second.
- Handles devices with dozens of endpoints just as well as ones with
just two fixed-function ones. Gadget drivers can be written so
they're easy to port to new hardware.
- Flexible enough to expose more complex USB device capabilities such
as multiple configurations, multiple interfaces, composite devices,
and alternate interface settings.
- USB "On-The-Go" (OTG) support, in conjunction with updates to the
Linux-USB host side.
- Sharing data structures and API models with the Linux-USB host side
API. This helps the OTG support, and looks forward to more-symmetric
frameworks (where the same I/O model is used by both host and device
side drivers).
- Minimalist, so it's easier to support new device controller hardware.
I/O processing doesn't imply large demands for memory or CPU
resources.
Most Linux developers will not be able to use this API, since they have
USB "host" hardware in a PC, workstation, or server. Linux users with
embedded systems are more likely to have USB peripheral hardware. To
distinguish drivers running inside such hardware from the more familiar
Linux "USB device drivers", which are host side proxies for the real USB
devices, a different term is used: the drivers inside the peripherals
are "USB gadget drivers". In USB protocol interactions, the device
driver is the master (or "client driver") and the gadget driver is the
slave (or "function driver").
The gadget API resembles the host side Linux-USB API in that both use
queues of request objects to package I/O buffers, and those requests may
be submitted or canceled. They share common definitions for the standard
USB *Chapter 9* messages, structures, and constants. Also, both APIs
bind and unbind drivers to devices. The APIs differ in detail, since the
host side's current URB framework exposes a number of implementation
details and assumptions that are inappropriate for a gadget API. While
the model for control transfers and configuration management is
necessarily different (one side is a hardware-neutral master, the other
is a hardware-aware slave), the endpoint I/0 API used here should also
be usable for an overhead-reduced host side API.
Structure of Gadget Drivers
===========================
A system running inside a USB peripheral normally has at least three
layers inside the kernel to handle USB protocol processing, and may have
additional layers in user space code. The "gadget" API is used by the
middle layer to interact with the lowest level (which directly handles
hardware).
In Linux, from the bottom up, these layers are:
*USB Controller Driver*
This is the lowest software level. It is the only layer that talks
to hardware, through registers, fifos, dma, irqs, and the like. The
``<linux/usb/gadget.h>`` API abstracts the peripheral controller
endpoint hardware. That hardware is exposed through endpoint
objects, which accept streams of IN/OUT buffers, and through
callbacks that interact with gadget drivers. Since normal USB
devices only have one upstream port, they only have one of these
drivers. The controller driver can support any number of different
gadget drivers, but only one of them can be used at a time.
Examples of such controller hardware include the PCI-based NetChip
2280 USB 2.0 high speed controller, the SA-11x0 or PXA-25x UDC
(found within many PDAs), and a variety of other products.
*Gadget Driver*
The lower boundary of this driver implements hardware-neutral USB
functions, using calls to the controller driver. Because such
hardware varies widely in capabilities and restrictions, and is used
in embedded environments where space is at a premium, the gadget
driver is often configured at compile time to work with endpoints
supported by one particular controller. Gadget drivers may be
portable to several different controllers, using conditional
compilation. (Recent kernels substantially simplify the work
involved in supporting new hardware, by *autoconfiguring* endpoints
automatically for many bulk-oriented drivers.) Gadget driver
responsibilities include:
- handling setup requests (ep0 protocol responses) possibly
including class-specific functionality
- returning configuration and string descriptors
- (re)setting configurations and interface altsettings, including
enabling and configuring endpoints
- handling life cycle events, such as managing bindings to
hardware, USB suspend/resume, remote wakeup, and disconnection
from the USB host.
- managing IN and OUT transfers on all currently enabled endpoints
Such drivers may be modules of proprietary code, although that
approach is discouraged in the Linux community.
*Upper Level*
Most gadget drivers have an upper boundary that connects to some
Linux driver or framework in Linux. Through that boundary flows the
data which the gadget driver produces and/or consumes through
protocol transfers over USB. Examples include:
- user mode code, using generic (gadgetfs) or application specific
files in ``/dev``
- networking subsystem (for network gadgets, like the CDC Ethernet
Model gadget driver)
- data capture drivers, perhaps video4Linux or a scanner driver; or
test and measurement hardware.
- input subsystem (for HID gadgets)
- sound subsystem (for audio gadgets)
- file system (for PTP gadgets)
- block i/o subsystem (for usb-storage gadgets)
- ... and more
*Additional Layers*
Other layers may exist. These could include kernel layers, such as
network protocol stacks, as well as user mode applications building
on standard POSIX system call APIs such as *open()*, *close()*,
*read()* and *write()*. On newer systems, POSIX Async I/O calls may
be an option. Such user mode code will not necessarily be subject to
the GNU General Public License (GPL).
OTG-capable systems will also need to include a standard Linux-USB host
side stack, with *usbcore*, one or more *Host Controller Drivers*
(HCDs), *USB Device Drivers* to support the OTG "Targeted Peripheral
List", and so forth. There will also be an *OTG Controller Driver*,
which is visible to gadget and device driver developers only indirectly.
That helps the host and device side USB controllers implement the two
new OTG protocols (HNP and SRP). Roles switch (host to peripheral, or
vice versa) using HNP during USB suspend processing, and SRP can be
viewed as a more battery-friendly kind of device wakeup protocol.
Over time, reusable utilities are evolving to help make some gadget
driver tasks simpler. For example, building configuration descriptors
from vectors of descriptors for the configurations interfaces and
endpoints is now automated, and many drivers now use autoconfiguration
to choose hardware endpoints and initialize their descriptors. A
potential example of particular interest is code implementing standard
USB-IF protocols for HID, networking, storage, or audio classes. Some
developers are interested in KDB or KGDB hooks, to let target hardware
be remotely debugged. Most such USB protocol code doesn't need to be
hardware-specific, any more than network protocols like X11, HTTP, or
NFS are. Such gadget-side interface drivers should eventually be
combined, to implement composite devices.
Kernel Mode Gadget API
======================
Gadget drivers declare themselves through a *struct
usb_gadget_driver*, which is responsible for most parts of enumeration
for a *struct usb_gadget*. The response to a set_configuration usually
involves enabling one or more of the *struct usb_ep* objects exposed by
the gadget, and submitting one or more *struct usb_request* buffers to
transfer data. Understand those four data types, and their operations,
and you will understand how this API works.
**Note**
This documentation was prepared using the standard Linux kernel
``docproc`` tool, which turns text and in-code comments into SGML
DocBook and then into usable formats such as HTML or PDF. Other than
the "Chapter 9" data types, most of the significant data types and
functions are described here.
However, docproc does not understand all the C constructs that are
used, so some relevant information is likely omitted from what you
are reading. One example of such information is endpoint
autoconfiguration. You'll have to read the header file, and use
example source code (such as that for "Gadget Zero"), to fully
understand the API.
The part of the API implementing some basic driver capabilities is
specific to the version of the Linux kernel that's in use. The 2.6
kernel includes a *driver model* framework that has no analogue on
earlier kernels; so those parts of the gadget API are not fully
portable. (They are implemented on 2.4 kernels, but in a different
way.) The driver model state is another part of this API that is
ignored by the kerneldoc tools.
The core API does not expose every possible hardware feature, only the
most widely available ones. There are significant hardware features,
such as device-to-device DMA (without temporary storage in a memory
buffer) that would be added using hardware-specific APIs.
This API allows drivers to use conditional compilation to handle
endpoint capabilities of different hardware, but doesn't require that.
Hardware tends to have arbitrary restrictions, relating to transfer
types, addressing, packet sizes, buffering, and availability. As a rule,
such differences only matter for "endpoint zero" logic that handles
device configuration and management. The API supports limited run-time
detection of capabilities, through naming conventions for endpoints.
Many drivers will be able to at least partially autoconfigure
themselves. In particular, driver init sections will often have endpoint
autoconfiguration logic that scans the hardware's list of endpoints to
find ones matching the driver requirements (relying on those
conventions), to eliminate some of the most common reasons for
conditional compilation.
Like the Linux-USB host side API, this API exposes the "chunky" nature
of USB messages: I/O requests are in terms of one or more "packets", and
packet boundaries are visible to drivers. Compared to RS-232 serial
protocols, USB resembles synchronous protocols like HDLC (N bytes per
frame, multipoint addressing, host as the primary station and devices as
secondary stations) more than asynchronous ones (tty style: 8 data bits
per frame, no parity, one stop bit). So for example the controller
drivers won't buffer two single byte writes into a single two-byte USB
IN packet, although gadget drivers may do so when they implement
protocols where packet boundaries (and "short packets") are not
significant.
Driver Life Cycle
-----------------
Gadget drivers make endpoint I/O requests to hardware without needing to
know many details of the hardware, but driver setup/configuration code
needs to handle some differences. Use the API like this:
1. Register a driver for the particular device side usb controller
hardware, such as the net2280 on PCI (USB 2.0), sa11x0 or pxa25x as
found in Linux PDAs, and so on. At this point the device is logically
in the USB ch9 initial state ("attached"), drawing no power and not
usable (since it does not yet support enumeration). Any host should
not see the device, since it's not activated the data line pullup
used by the host to detect a device, even if VBUS power is available.
2. Register a gadget driver that implements some higher level device
function. That will then bind() to a usb_gadget, which activates the
data line pullup sometime after detecting VBUS.
3. The hardware driver can now start enumerating. The steps it handles
are to accept USB power and set_address requests. Other steps are
handled by the gadget driver. If the gadget driver module is unloaded
before the host starts to enumerate, steps before step 7 are skipped.
4. The gadget driver's setup() call returns usb descriptors, based both
on what the bus interface hardware provides and on the functionality
being implemented. That can involve alternate settings or
configurations, unless the hardware prevents such operation. For OTG
devices, each configuration descriptor includes an OTG descriptor.
5. The gadget driver handles the last step of enumeration, when the USB
host issues a set_configuration call. It enables all endpoints used
in that configuration, with all interfaces in their default settings.
That involves using a list of the hardware's endpoints, enabling each
endpoint according to its descriptor. It may also involve using
:c:func:`usb_gadget_vbus_draw()` to let more power be drawn
from VBUS, as allowed by that configuration. For OTG devices, setting
a configuration may also involve reporting HNP capabilities through a
user interface.
6. Do real work and perform data transfers, possibly involving changes
to interface settings or switching to new configurations, until the
device is disconnect()ed from the host. Queue any number of transfer
requests to each endpoint. It may be suspended and resumed several
times before being disconnected. On disconnect, the drivers go back
to step 3 (above).
7. When the gadget driver module is being unloaded, the driver unbind()
callback is issued. That lets the controller driver be unloaded.
Drivers will normally be arranged so that just loading the gadget driver
module (or statically linking it into a Linux kernel) allows the
peripheral device to be enumerated, but some drivers will defer
enumeration until some higher level component (like a user mode daemon)
enables it. Note that at this lowest level there are no policies about
how ep0 configuration logic is implemented, except that it should obey
USB specifications. Such issues are in the domain of gadget drivers,
including knowing about implementation constraints imposed by some USB
controllers or understanding that composite devices might happen to be
built by integrating reusable components.
Note that the lifecycle above can be slightly different for OTG devices.
Other than providing an additional OTG descriptor in each configuration,
only the HNP-related differences are particularly visible to driver
code. They involve reporting requirements during the SET_CONFIGURATION
request, and the option to invoke HNP during some suspend callbacks.
Also, SRP changes the semantics of :c:func:`usb_gadget_wakeup()`
slightly.
USB 2.0 Chapter 9 Types and Constants
-------------------------------------
Gadget drivers rely on common USB structures and constants defined in
the ``<linux/usb/ch9.h>`` header file, which is standard in Linux 2.6
kernels. These are the same types and constants used by host side
drivers (and usbcore).
.. kernel-doc:: include/linux/usb/ch9.h
:internal:
Core Objects and Methods
------------------------
These are declared in ``<linux/usb/gadget.h>``, and are used by gadget
drivers to interact with USB peripheral controller drivers.
.. kernel-doc:: include/linux/usb/gadget.h
:internal:
Optional Utilities
------------------
The core API is sufficient for writing a USB Gadget Driver, but some
optional utilities are provided to simplify common tasks. These
utilities include endpoint autoconfiguration.
.. kernel-doc:: drivers/usb/gadget/usbstring.c
:export:
.. kernel-doc:: drivers/usb/gadget/config.c
:export:
Composite Device Framework
--------------------------
The core API is sufficient for writing drivers for composite USB devices
(with more than one function in a given configuration), and also
multi-configuration devices (also more than one function, but not
necessarily sharing a given configuration). There is however an optional
framework which makes it easier to reuse and combine functions.
Devices using this framework provide a *struct usb_composite_driver*,
which in turn provides one or more *struct usb_configuration*
instances. Each such configuration includes at least one *struct
usb_function*, which packages a user visible role such as "network
link" or "mass storage device". Management functions may also exist,
such as "Device Firmware Upgrade".
.. kernel-doc:: include/linux/usb/composite.h
:internal:
.. kernel-doc:: drivers/usb/gadget/composite.c
:export:
Composite Device Functions
--------------------------
At this writing, a few of the current gadget drivers have been converted
to this framework. Near-term plans include converting all of them,
except for "gadgetfs".
.. kernel-doc:: drivers/usb/gadget/function/f_acm.c
:export:
.. kernel-doc:: drivers/usb/gadget/function/f_ecm.c
:export:
.. kernel-doc:: drivers/usb/gadget/function/f_subset.c
:export:
.. kernel-doc:: drivers/usb/gadget/function/f_obex.c
:export:
.. kernel-doc:: drivers/usb/gadget/function/f_serial.c
:export:
Peripheral Controller Drivers
=============================
The first hardware supporting this API was the NetChip 2280 controller,
which supports USB 2.0 high speed and is based on PCI. This is the
``net2280`` driver module. The driver supports Linux kernel versions 2.4
and 2.6; contact NetChip Technologies for development boards and product
information.
Other hardware working in the "gadget" framework includes: Intel's PXA
25x and IXP42x series processors (``pxa2xx_udc``), Toshiba TC86c001
"Goku-S" (``goku_udc``), Renesas SH7705/7727 (``sh_udc``), MediaQ 11xx
(``mq11xx_udc``), Hynix HMS30C7202 (``h7202_udc``), National 9303/4
(``n9604_udc``), Texas Instruments OMAP (``omap_udc``), Sharp LH7A40x
(``lh7a40x_udc``), and more. Most of those are full speed controllers.
At this writing, there are people at work on drivers in this framework
for several other USB device controllers, with plans to make many of
them be widely available.
A partial USB simulator, the ``dummy_hcd`` driver, is available. It can
act like a net2280, a pxa25x, or an sa11x0 in terms of available
endpoints and device speeds; and it simulates control, bulk, and to some
extent interrupt transfers. That lets you develop some parts of a gadget
driver on a normal PC, without any special hardware, and perhaps with
the assistance of tools such as GDB running with User Mode Linux. At
least one person has expressed interest in adapting that approach,
hooking it up to a simulator for a microcontroller. Such simulators can
help debug subsystems where the runtime hardware is unfriendly to
software development, or is not yet available.
Support for other controllers is expected to be developed and
contributed over time, as this driver framework evolves.
Gadget Drivers
==============
In addition to *Gadget Zero* (used primarily for testing and development
with drivers for usb controller hardware), other gadget drivers exist.
There's an *ethernet* gadget driver, which implements one of the most
useful *Communications Device Class* (CDC) models. One of the standards
for cable modem interoperability even specifies the use of this ethernet
model as one of two mandatory options. Gadgets using this code look to a
USB host as if they're an Ethernet adapter. It provides access to a
network where the gadget's CPU is one host, which could easily be
bridging, routing, or firewalling access to other networks. Since some
hardware can't fully implement the CDC Ethernet requirements, this
driver also implements a "good parts only" subset of CDC Ethernet. (That
subset doesn't advertise itself as CDC Ethernet, to avoid creating
problems.)
Support for Microsoft's *RNDIS* protocol has been contributed by
Pengutronix and Auerswald GmbH. This is like CDC Ethernet, but it runs
on more slightly USB hardware (but less than the CDC subset). However,
its main claim to fame is being able to connect directly to recent
versions of Windows, using drivers that Microsoft bundles and supports,
making it much simpler to network with Windows.
There is also support for user mode gadget drivers, using *gadgetfs*.
This provides a *User Mode API* that presents each endpoint as a single
file descriptor. I/O is done using normal *read()* and *read()* calls.
Familiar tools like GDB and pthreads can be used to develop and debug
user mode drivers, so that once a robust controller driver is available
many applications for it won't require new kernel mode software. Linux
2.6 *Async I/O (AIO)* support is available, so that user mode software
can stream data with only slightly more overhead than a kernel driver.
There's a USB Mass Storage class driver, which provides a different
solution for interoperability with systems such as MS-Windows and MacOS.
That *Mass Storage* driver uses a file or block device as backing store
for a drive, like the ``loop`` driver. The USB host uses the BBB, CB, or
CBI versions of the mass storage class specification, using transparent
SCSI commands to access the data from the backing store.
There's a "serial line" driver, useful for TTY style operation over USB.
The latest version of that driver supports CDC ACM style operation, like
a USB modem, and so on most hardware it can interoperate easily with
MS-Windows. One interesting use of that driver is in boot firmware (like
a BIOS), which can sometimes use that model with very small systems
without real serial lines.
Support for other kinds of gadget is expected to be developed and
contributed over time, as this driver framework evolves.
USB On-The-GO (OTG)
===================
USB OTG support on Linux 2.6 was initially developed by Texas
Instruments for `OMAP <http://www.omap.com>`__ 16xx and 17xx series
processors. Other OTG systems should work in similar ways, but the
hardware level details could be very different.
Systems need specialized hardware support to implement OTG, notably
including a special *Mini-AB* jack and associated transceiver to support
*Dual-Role* operation: they can act either as a host, using the standard
Linux-USB host side driver stack, or as a peripheral, using this
"gadget" framework. To do that, the system software relies on small
additions to those programming interfaces, and on a new internal
component (here called an "OTG Controller") affecting which driver stack
connects to the OTG port. In each role, the system can re-use the
existing pool of hardware-neutral drivers, layered on top of the
controller driver interfaces (*usb_bus* or *usb_gadget*). Such drivers
need at most minor changes, and most of the calls added to support OTG
can also benefit non-OTG products.
- Gadget drivers test the *is_otg* flag, and use it to determine
whether or not to include an OTG descriptor in each of their
configurations.
- Gadget drivers may need changes to support the two new OTG protocols,
exposed in new gadget attributes such as *b_hnp_enable* flag. HNP
support should be reported through a user interface (two LEDs could
suffice), and is triggered in some cases when the host suspends the
peripheral. SRP support can be user-initiated just like remote
wakeup, probably by pressing the same button.
- On the host side, USB device drivers need to be taught to trigger HNP
at appropriate moments, using :c:func:`usb_suspend_device()`.
That also conserves battery power, which is useful even for non-OTG
configurations.
- Also on the host side, a driver must support the OTG "Targeted
Peripheral List". That's just a whitelist, used to reject peripherals
not supported with a given Linux OTG host. *This whitelist is
product-specific; each product must modify ``otg_whitelist.h`` to
match its interoperability specification.*
Non-OTG Linux hosts, like PCs and workstations, normally have some
solution for adding drivers, so that peripherals that aren't
recognized can eventually be supported. That approach is unreasonable
for consumer products that may never have their firmware upgraded,
and where it's usually unrealistic to expect traditional
PC/workstation/server kinds of support model to work. For example,
it's often impractical to change device firmware once the product has
been distributed, so driver bugs can't normally be fixed if they're
found after shipment.
Additional changes are needed below those hardware-neutral *usb_bus*
and *usb_gadget* driver interfaces; those aren't discussed here in any
detail. Those affect the hardware-specific code for each USB Host or
Peripheral controller, and how the HCD initializes (since OTG can be
active only on a single port). They also involve what may be called an
*OTG Controller Driver*, managing the OTG transceiver and the OTG state
machine logic as well as much of the root hub behavior for the OTG port.
The OTG controller driver needs to activate and deactivate USB
controllers depending on the relevant device role. Some related changes
were needed inside usbcore, so that it can identify OTG-capable devices
and respond appropriately to HNP or SRP protocols.

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=============
Linux USB API
=============
.. toctree::
usb
gadget
writing_usb_driver
writing_musb_glue_layer
.. only:: subproject and html
Indices
=======
* :ref:`genindex`

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==========================
Writing an MUSB Glue Layer
==========================
:Author: Apelete Seketeli
Introduction
============
The Linux MUSB subsystem is part of the larger Linux USB subsystem. It
provides support for embedded USB Device Controllers (UDC) that do not
use Universal Host Controller Interface (UHCI) or Open Host Controller
Interface (OHCI).
Instead, these embedded UDC rely on the USB On-the-Go (OTG)
specification which they implement at least partially. The silicon
reference design used in most cases is the Multipoint USB Highspeed
Dual-Role Controller (MUSB HDRC) found in the Mentor Graphics Inventra™
design.
As a self-taught exercise I have written an MUSB glue layer for the
Ingenic JZ4740 SoC, modelled after the many MUSB glue layers in the
kernel source tree. This layer can be found at
drivers/usb/musb/jz4740.c. In this documentation I will walk through the
basics of the jz4740.c glue layer, explaining the different pieces and
what needs to be done in order to write your own device glue layer.
Linux MUSB Basics
=================
To get started on the topic, please read USB On-the-Go Basics (see
Resources) which provides an introduction of USB OTG operation at the
hardware level. A couple of wiki pages by Texas Instruments and Analog
Devices also provide an overview of the Linux kernel MUSB configuration,
albeit focused on some specific devices provided by these companies.
Finally, getting acquainted with the USB specification at USB home page
may come in handy, with practical instance provided through the Writing
USB Device Drivers documentation (again, see Resources).
Linux USB stack is a layered architecture in which the MUSB controller
hardware sits at the lowest. The MUSB controller driver abstract the
MUSB controller hardware to the Linux USB stack.
::
------------------------
| | <------- drivers/usb/gadget
| Linux USB Core Stack | <------- drivers/usb/host
| | <------- drivers/usb/core
------------------------
⬍
--------------------------
| | <------ drivers/usb/musb/musb_gadget.c
| MUSB Controller driver | <------ drivers/usb/musb/musb_host.c
| | <------ drivers/usb/musb/musb_core.c
--------------------------
⬍
---------------------------------
| MUSB Platform Specific Driver |
| | <-- drivers/usb/musb/jz4740.c
| aka "Glue Layer" |
---------------------------------
⬍
---------------------------------
| MUSB Controller Hardware |
---------------------------------
As outlined above, the glue layer is actually the platform specific code
sitting in between the controller driver and the controller hardware.
Just like a Linux USB driver needs to register itself with the Linux USB
subsystem, the MUSB glue layer needs first to register itself with the
MUSB controller driver. This will allow the controller driver to know
about which device the glue layer supports and which functions to call
when a supported device is detected or released; remember we are talking
about an embedded controller chip here, so no insertion or removal at
run-time.
All of this information is passed to the MUSB controller driver through
a platform_driver structure defined in the glue layer as:
::
static struct platform_driver jz4740_driver = {
.probe = jz4740_probe,
.remove = jz4740_remove,
.driver = {
.name = "musb-jz4740",
},
};
The probe and remove function pointers are called when a matching device
is detected and, respectively, released. The name string describes the
device supported by this glue layer. In the current case it matches a
platform_device structure declared in arch/mips/jz4740/platform.c. Note
that we are not using device tree bindings here.
In order to register itself to the controller driver, the glue layer
goes through a few steps, basically allocating the controller hardware
resources and initialising a couple of circuits. To do so, it needs to
keep track of the information used throughout these steps. This is done
by defining a private jz4740_glue structure:
::
struct jz4740_glue {
struct device *dev;
struct platform_device *musb;
struct clk *clk;
};
The dev and musb members are both device structure variables. The first
one holds generic information about the device, since it's the basic
device structure, and the latter holds information more closely related
to the subsystem the device is registered to. The clk variable keeps
information related to the device clock operation.
Let's go through the steps of the probe function that leads the glue
layer to register itself to the controller driver.
N.B.: For the sake of readability each function will be split in logical
parts, each part being shown as if it was independent from the others.
::
static int jz4740_probe(struct platform_device *pdev)
{
struct platform_device *musb;
struct jz4740_glue *glue;
struct clk *clk;
int ret;
glue = devm_kzalloc(&pdev->dev, sizeof(*glue), GFP_KERNEL);
if (!glue)
return -ENOMEM;
musb = platform_device_alloc("musb-hdrc", PLATFORM_DEVID_AUTO);
if (!musb) {
dev_err(&pdev->dev, "failed to allocate musb device\n");
return -ENOMEM;
}
clk = devm_clk_get(&pdev->dev, "udc");
if (IS_ERR(clk)) {
dev_err(&pdev->dev, "failed to get clock\n");
ret = PTR_ERR(clk);
goto err_platform_device_put;
}
ret = clk_prepare_enable(clk);
if (ret) {
dev_err(&pdev->dev, "failed to enable clock\n");
goto err_platform_device_put;
}
musb->dev.parent = &pdev->dev;
glue->dev = &pdev->dev;
glue->musb = musb;
glue->clk = clk;
return 0;
err_platform_device_put:
platform_device_put(musb);
return ret;
}
The first few lines of the probe function allocate and assign the glue,
musb and clk variables. The GFP_KERNEL flag (line 8) allows the
allocation process to sleep and wait for memory, thus being usable in a
blocking situation. The PLATFORM_DEVID_AUTO flag (line 12) allows
automatic allocation and management of device IDs in order to avoid
device namespace collisions with explicit IDs. With devm_clk_get()
(line 18) the glue layer allocates the clock -- the ``devm_`` prefix
indicates that clk_get() is managed: it automatically frees the
allocated clock resource data when the device is released -- and enable
it.
Then comes the registration steps:
::
static int jz4740_probe(struct platform_device *pdev)
{
struct musb_hdrc_platform_data *pdata = &jz4740_musb_platform_data;
pdata->platform_ops = &jz4740_musb_ops;
platform_set_drvdata(pdev, glue);
ret = platform_device_add_resources(musb, pdev->resource,
pdev->num_resources);
if (ret) {
dev_err(&pdev->dev, "failed to add resources\n");
goto err_clk_disable;
}
ret = platform_device_add_data(musb, pdata, sizeof(*pdata));
if (ret) {
dev_err(&pdev->dev, "failed to add platform_data\n");
goto err_clk_disable;
}
return 0;
err_clk_disable:
clk_disable_unprepare(clk);
err_platform_device_put:
platform_device_put(musb);
return ret;
}
The first step is to pass the device data privately held by the glue
layer on to the controller driver through platform_set_drvdata() (line
7). Next is passing on the device resources information, also privately
held at that point, through platform_device_add_resources() (line 9).
Finally comes passing on the platform specific data to the controller
driver (line 16). Platform data will be discussed in `Chapter
4 <#device-platform-data>`__, but here we are looking at the
platform_ops function pointer (line 5) in musb_hdrc_platform_data
structure (line 3). This function pointer allows the MUSB controller
driver to know which function to call for device operation:
::
static const struct musb_platform_ops jz4740_musb_ops = {
.init = jz4740_musb_init,
.exit = jz4740_musb_exit,
};
Here we have the minimal case where only init and exit functions are
called by the controller driver when needed. Fact is the JZ4740 MUSB
controller is a basic controller, lacking some features found in other
controllers, otherwise we may also have pointers to a few other
functions like a power management function or a function to switch
between OTG and non-OTG modes, for instance.
At that point of the registration process, the controller driver
actually calls the init function:
::
static int jz4740_musb_init(struct musb *musb)
{
musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
if (!musb->xceiv) {
pr_err("HS UDC: no transceiver configured\n");
return -ENODEV;
}
/* Silicon does not implement ConfigData register.
* Set dyn_fifo to avoid reading EP config from hardware.
*/
musb->dyn_fifo = true;
musb->isr = jz4740_musb_interrupt;
return 0;
}
The goal of jz4740_musb_init() is to get hold of the transceiver
driver data of the MUSB controller hardware and pass it on to the MUSB
controller driver, as usual. The transceiver is the circuitry inside the
controller hardware responsible for sending/receiving the USB data.
Since it is an implementation of the physical layer of the OSI model,
the transceiver is also referred to as PHY.
Getting hold of the MUSB PHY driver data is done with usb_get_phy()
which returns a pointer to the structure containing the driver instance
data. The next couple of instructions (line 12 and 14) are used as a
quirk and to setup IRQ handling respectively. Quirks and IRQ handling
will be discussed later in `Chapter 5 <#device-quirks>`__ and `Chapter
3 <#handling-irqs>`__.
::
static int jz4740_musb_exit(struct musb *musb)
{
usb_put_phy(musb->xceiv);
return 0;
}
Acting as the counterpart of init, the exit function releases the MUSB
PHY driver when the controller hardware itself is about to be released.
Again, note that init and exit are fairly simple in this case due to the
basic set of features of the JZ4740 controller hardware. When writing an
musb glue layer for a more complex controller hardware, you might need
to take care of more processing in those two functions.
Returning from the init function, the MUSB controller driver jumps back
into the probe function:
::
static int jz4740_probe(struct platform_device *pdev)
{
ret = platform_device_add(musb);
if (ret) {
dev_err(&pdev->dev, "failed to register musb device\n");
goto err_clk_disable;
}
return 0;
err_clk_disable:
clk_disable_unprepare(clk);
err_platform_device_put:
platform_device_put(musb);
return ret;
}
This is the last part of the device registration process where the glue
layer adds the controller hardware device to Linux kernel device
hierarchy: at this stage, all known information about the device is
passed on to the Linux USB core stack.
::
static int jz4740_remove(struct platform_device *pdev)
{
struct jz4740_glue *glue = platform_get_drvdata(pdev);
platform_device_unregister(glue->musb);
clk_disable_unprepare(glue->clk);
return 0;
}
Acting as the counterpart of probe, the remove function unregister the
MUSB controller hardware (line 5) and disable the clock (line 6),
allowing it to be gated.
Handling IRQs
=============
Additionally to the MUSB controller hardware basic setup and
registration, the glue layer is also responsible for handling the IRQs:
::
static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
{
unsigned long flags;
irqreturn_t retval = IRQ_NONE;
struct musb *musb = __hci;
spin_lock_irqsave(&musb->lock, flags);
musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
/*
* The controller is gadget only, the state of the host mode IRQ bits is
* undefined. Mask them to make sure that the musb driver core will
* never see them set
*/
musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
MUSB_INTR_RESET | MUSB_INTR_SOF;
if (musb->int_usb || musb->int_tx || musb->int_rx)
retval = musb_interrupt(musb);
spin_unlock_irqrestore(&musb->lock, flags);
return retval;
}
Here the glue layer mostly has to read the relevant hardware registers
and pass their values on to the controller driver which will handle the
actual event that triggered the IRQ.
The interrupt handler critical section is protected by the
spin_lock_irqsave() and counterpart spin_unlock_irqrestore()
functions (line 7 and 24 respectively), which prevent the interrupt
handler code to be run by two different threads at the same time.
Then the relevant interrupt registers are read (line 9 to 11):
- MUSB_INTRUSB: indicates which USB interrupts are currently active,
- MUSB_INTRTX: indicates which of the interrupts for TX endpoints are
currently active,
- MUSB_INTRRX: indicates which of the interrupts for TX endpoints are
currently active.
Note that musb_readb() is used to read 8-bit registers at most, while
musb_readw() allows us to read at most 16-bit registers. There are
other functions that can be used depending on the size of your device
registers. See musb_io.h for more information.
Instruction on line 18 is another quirk specific to the JZ4740 USB
device controller, which will be discussed later in `Chapter
5 <#device-quirks>`__.
The glue layer still needs to register the IRQ handler though. Remember
the instruction on line 14 of the init function:
::
static int jz4740_musb_init(struct musb *musb)
{
musb->isr = jz4740_musb_interrupt;
return 0;
}
This instruction sets a pointer to the glue layer IRQ handler function,
in order for the controller hardware to call the handler back when an
IRQ comes from the controller hardware. The interrupt handler is now
implemented and registered.
Device Platform Data
====================
In order to write an MUSB glue layer, you need to have some data
describing the hardware capabilities of your controller hardware, which
is called the platform data.
Platform data is specific to your hardware, though it may cover a broad
range of devices, and is generally found somewhere in the arch/
directory, depending on your device architecture.
For instance, platform data for the JZ4740 SoC is found in
arch/mips/jz4740/platform.c. In the platform.c file each device of the
JZ4740 SoC is described through a set of structures.
Here is the part of arch/mips/jz4740/platform.c that covers the USB
Device Controller (UDC):
::
/* USB Device Controller */
struct platform_device jz4740_udc_xceiv_device = {
.name = "usb_phy_gen_xceiv",
.id = 0,
};
static struct resource jz4740_udc_resources[] = {
[0] = {
.start = JZ4740_UDC_BASE_ADDR,
.end = JZ4740_UDC_BASE_ADDR + 0x10000 - 1,
.flags = IORESOURCE_MEM,
},
[1] = {
.start = JZ4740_IRQ_UDC,
.end = JZ4740_IRQ_UDC,
.flags = IORESOURCE_IRQ,
.name = "mc",
},
};
struct platform_device jz4740_udc_device = {
.name = "musb-jz4740",
.id = -1,
.dev = {
.dma_mask = &jz4740_udc_device.dev.coherent_dma_mask,
.coherent_dma_mask = DMA_BIT_MASK(32),
},
.num_resources = ARRAY_SIZE(jz4740_udc_resources),
.resource = jz4740_udc_resources,
};
The jz4740_udc_xceiv_device platform device structure (line 2)
describes the UDC transceiver with a name and id number.
At the time of this writing, note that "usb_phy_gen_xceiv" is the
specific name to be used for all transceivers that are either built-in
with reference USB IP or autonomous and doesn't require any PHY
programming. You will need to set CONFIG_NOP_USB_XCEIV=y in the
kernel configuration to make use of the corresponding transceiver
driver. The id field could be set to -1 (equivalent to
PLATFORM_DEVID_NONE), -2 (equivalent to PLATFORM_DEVID_AUTO) or
start with 0 for the first device of this kind if we want a specific id
number.
The jz4740_udc_resources resource structure (line 7) defines the UDC
registers base addresses.
The first array (line 9 to 11) defines the UDC registers base memory
addresses: start points to the first register memory address, end points
to the last register memory address and the flags member defines the
type of resource we are dealing with. So IORESOURCE_MEM is used to
define the registers memory addresses. The second array (line 14 to 17)
defines the UDC IRQ registers addresses. Since there is only one IRQ
register available for the JZ4740 UDC, start and end point at the same
address. The IORESOURCE_IRQ flag tells that we are dealing with IRQ
resources, and the name "mc" is in fact hard-coded in the MUSB core in
order for the controller driver to retrieve this IRQ resource by
querying it by its name.
Finally, the jz4740_udc_device platform device structure (line 21)
describes the UDC itself.
The "musb-jz4740" name (line 22) defines the MUSB driver that is used
for this device; remember this is in fact the name that we used in the
jz4740_driver platform driver structure in `Chapter
2 <#linux-musb-basics>`__. The id field (line 23) is set to -1
(equivalent to PLATFORM_DEVID_NONE) since we do not need an id for the
device: the MUSB controller driver was already set to allocate an
automatic id in `Chapter 2 <#linux-musb-basics>`__. In the dev field we
care for DMA related information here. The dma_mask field (line 25)
defines the width of the DMA mask that is going to be used, and
coherent_dma_mask (line 26) has the same purpose but for the
alloc_coherent DMA mappings: in both cases we are using a 32 bits mask.
Then the resource field (line 29) is simply a pointer to the resource
structure defined before, while the num_resources field (line 28) keeps
track of the number of arrays defined in the resource structure (in this
case there were two resource arrays defined before).
With this quick overview of the UDC platform data at the arch/ level now
done, let's get back to the MUSB glue layer specific platform data in
drivers/usb/musb/jz4740.c:
::
static struct musb_hdrc_config jz4740_musb_config = {
/* Silicon does not implement USB OTG. */
.multipoint = 0,
/* Max EPs scanned, driver will decide which EP can be used. */
.num_eps = 4,
/* RAMbits needed to configure EPs from table */
.ram_bits = 9,
.fifo_cfg = jz4740_musb_fifo_cfg,
.fifo_cfg_size = ARRAY_SIZE(jz4740_musb_fifo_cfg),
};
static struct musb_hdrc_platform_data jz4740_musb_platform_data = {
.mode = MUSB_PERIPHERAL,
.config = &jz4740_musb_config,
};
First the glue layer configures some aspects of the controller driver
operation related to the controller hardware specifics. This is done
through the jz4740_musb_config musb_hdrc_config structure.
Defining the OTG capability of the controller hardware, the multipoint
member (line 3) is set to 0 (equivalent to false) since the JZ4740 UDC
is not OTG compatible. Then num_eps (line 5) defines the number of USB
endpoints of the controller hardware, including endpoint 0: here we have
3 endpoints + endpoint 0. Next is ram_bits (line 7) which is the width
of the RAM address bus for the MUSB controller hardware. This
information is needed when the controller driver cannot automatically
configure endpoints by reading the relevant controller hardware
registers. This issue will be discussed when we get to device quirks in
`Chapter 5 <#device-quirks>`__. Last two fields (line 8 and 9) are also
about device quirks: fifo_cfg points to the USB endpoints configuration
table and fifo_cfg_size keeps track of the size of the number of
entries in that configuration table. More on that later in `Chapter
5 <#device-quirks>`__.
Then this configuration is embedded inside jz4740_musb_platform_data
musb_hdrc_platform_data structure (line 11): config is a pointer to
the configuration structure itself, and mode tells the controller driver
if the controller hardware may be used as MUSB_HOST only,
MUSB_PERIPHERAL only or MUSB_OTG which is a dual mode.
Remember that jz4740_musb_platform_data is then used to convey
platform data information as we have seen in the probe function in
`Chapter 2 <#linux-musb-basics>`__
Device Quirks
=============
Completing the platform data specific to your device, you may also need
to write some code in the glue layer to work around some device specific
limitations. These quirks may be due to some hardware bugs, or simply be
the result of an incomplete implementation of the USB On-the-Go
specification.
The JZ4740 UDC exhibits such quirks, some of which we will discuss here
for the sake of insight even though these might not be found in the
controller hardware you are working on.
Let's get back to the init function first:
::
static int jz4740_musb_init(struct musb *musb)
{
musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
if (!musb->xceiv) {
pr_err("HS UDC: no transceiver configured\n");
return -ENODEV;
}
/* Silicon does not implement ConfigData register.
* Set dyn_fifo to avoid reading EP config from hardware.
*/
musb->dyn_fifo = true;
musb->isr = jz4740_musb_interrupt;
return 0;
}
Instruction on line 12 helps the MUSB controller driver to work around
the fact that the controller hardware is missing registers that are used
for USB endpoints configuration.
Without these registers, the controller driver is unable to read the
endpoints configuration from the hardware, so we use line 12 instruction
to bypass reading the configuration from silicon, and rely on a
hard-coded table that describes the endpoints configuration instead:
::
static struct musb_fifo_cfg jz4740_musb_fifo_cfg[] = {
{ .hw_ep_num = 1, .style = FIFO_TX, .maxpacket = 512, },
{ .hw_ep_num = 1, .style = FIFO_RX, .maxpacket = 512, },
{ .hw_ep_num = 2, .style = FIFO_TX, .maxpacket = 64, },
};
Looking at the configuration table above, we see that each endpoints is
described by three fields: hw_ep_num is the endpoint number, style is
its direction (either FIFO_TX for the controller driver to send packets
in the controller hardware, or FIFO_RX to receive packets from
hardware), and maxpacket defines the maximum size of each data packet
that can be transmitted over that endpoint. Reading from the table, the
controller driver knows that endpoint 1 can be used to send and receive
USB data packets of 512 bytes at once (this is in fact a bulk in/out
endpoint), and endpoint 2 can be used to send data packets of 64 bytes
at once (this is in fact an interrupt endpoint).
Note that there is no information about endpoint 0 here: that one is
implemented by default in every silicon design, with a predefined
configuration according to the USB specification. For more examples of
endpoint configuration tables, see musb_core.c.
Let's now get back to the interrupt handler function:
::
static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
{
unsigned long flags;
irqreturn_t retval = IRQ_NONE;
struct musb *musb = __hci;
spin_lock_irqsave(&musb->lock, flags);
musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
/*
* The controller is gadget only, the state of the host mode IRQ bits is
* undefined. Mask them to make sure that the musb driver core will
* never see them set
*/
musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
MUSB_INTR_RESET | MUSB_INTR_SOF;
if (musb->int_usb || musb->int_tx || musb->int_rx)
retval = musb_interrupt(musb);
spin_unlock_irqrestore(&musb->lock, flags);
return retval;
}
Instruction on line 18 above is a way for the controller driver to work
around the fact that some interrupt bits used for USB host mode
operation are missing in the MUSB_INTRUSB register, thus left in an
undefined hardware state, since this MUSB controller hardware is used in
peripheral mode only. As a consequence, the glue layer masks these
missing bits out to avoid parasite interrupts by doing a logical AND
operation between the value read from MUSB_INTRUSB and the bits that
are actually implemented in the register.
These are only a couple of the quirks found in the JZ4740 USB device
controller. Some others were directly addressed in the MUSB core since
the fixes were generic enough to provide a better handling of the issues
for others controller hardware eventually.
Conclusion
==========
Writing a Linux MUSB glue layer should be a more accessible task, as
this documentation tries to show the ins and outs of this exercise.
The JZ4740 USB device controller being fairly simple, I hope its glue
layer serves as a good example for the curious mind. Used with the
current MUSB glue layers, this documentation should provide enough
guidance to get started; should anything gets out of hand, the linux-usb
mailing list archive is another helpful resource to browse through.
Acknowledgements
================
Many thanks to Lars-Peter Clausen and Maarten ter Huurne for answering
my questions while I was writing the JZ4740 glue layer and for helping
me out getting the code in good shape.
I would also like to thank the Qi-Hardware community at large for its
cheerful guidance and support.
Resources
=========
USB Home Page: http://www.usb.org
linux-usb Mailing List Archives: http://marc.info/?l=linux-usb
USB On-the-Go Basics:
http://www.maximintegrated.com/app-notes/index.mvp/id/1822
Writing USB Device Drivers:
https://www.kernel.org/doc/htmldocs/writing_usb_driver/index.html
Texas Instruments USB Configuration Wiki Page:
http://processors.wiki.ti.com/index.php/Usbgeneralpage
Analog Devices Blackfin MUSB Configuration:
http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:drivers:musb

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==========================
Writing USB Device Drivers
==========================
:Author: Greg Kroah-Hartman
Introduction
============
The Linux USB subsystem has grown from supporting only two different
types of devices in the 2.2.7 kernel (mice and keyboards), to over 20
different types of devices in the 2.4 kernel. Linux currently supports
almost all USB class devices (standard types of devices like keyboards,
mice, modems, printers and speakers) and an ever-growing number of
vendor-specific devices (such as USB to serial converters, digital
cameras, Ethernet devices and MP3 players). For a full list of the
different USB devices currently supported, see Resources.
The remaining kinds of USB devices that do not have support on Linux are
almost all vendor-specific devices. Each vendor decides to implement a
custom protocol to talk to their device, so a custom driver usually
needs to be created. Some vendors are open with their USB protocols and
help with the creation of Linux drivers, while others do not publish
them, and developers are forced to reverse-engineer. See Resources for
some links to handy reverse-engineering tools.
Because each different protocol causes a new driver to be created, I
have written a generic USB driver skeleton, modelled after the
pci-skeleton.c file in the kernel source tree upon which many PCI
network drivers have been based. This USB skeleton can be found at
drivers/usb/usb-skeleton.c in the kernel source tree. In this article I
will walk through the basics of the skeleton driver, explaining the
different pieces and what needs to be done to customize it to your
specific device.
Linux USB Basics
================
If you are going to write a Linux USB driver, please become familiar
with the USB protocol specification. It can be found, along with many
other useful documents, at the USB home page (see Resources). An
excellent introduction to the Linux USB subsystem can be found at the
USB Working Devices List (see Resources). It explains how the Linux USB
subsystem is structured and introduces the reader to the concept of USB
urbs (USB Request Blocks), which are essential to USB drivers.
The first thing a Linux USB driver needs to do is register itself with
the Linux USB subsystem, giving it some information about which devices
the driver supports and which functions to call when a device supported
by the driver is inserted or removed from the system. All of this
information is passed to the USB subsystem in the usb_driver structure.
The skeleton driver declares a usb_driver as:
::
static struct usb_driver skel_driver = {
.name = "skeleton",
.probe = skel_probe,
.disconnect = skel_disconnect,
.fops = &skel_fops,
.minor = USB_SKEL_MINOR_BASE,
.id_table = skel_table,
};
The variable name is a string that describes the driver. It is used in
informational messages printed to the system log. The probe and
disconnect function pointers are called when a device that matches the
information provided in the id_table variable is either seen or
removed.
The fops and minor variables are optional. Most USB drivers hook into
another kernel subsystem, such as the SCSI, network or TTY subsystem.
These types of drivers register themselves with the other kernel
subsystem, and any user-space interactions are provided through that
interface. But for drivers that do not have a matching kernel subsystem,
such as MP3 players or scanners, a method of interacting with user space
is needed. The USB subsystem provides a way to register a minor device
number and a set of file_operations function pointers that enable this
user-space interaction. The skeleton driver needs this kind of
interface, so it provides a minor starting number and a pointer to its
file_operations functions.
The USB driver is then registered with a call to usb_register, usually
in the driver's init function, as shown here:
::
static int __init usb_skel_init(void)
{
int result;
/* register this driver with the USB subsystem */
result = usb_register(&skel_driver);
if (result < 0) {
err("usb_register failed for the "__FILE__ "driver."
"Error number %d", result);
return -1;
}
return 0;
}
module_init(usb_skel_init);
When the driver is unloaded from the system, it needs to deregister
itself with the USB subsystem. This is done with the usb_deregister
function:
::
static void __exit usb_skel_exit(void)
{
/* deregister this driver with the USB subsystem */
usb_deregister(&skel_driver);
}
module_exit(usb_skel_exit);
To enable the linux-hotplug system to load the driver automatically when
the device is plugged in, you need to create a MODULE_DEVICE_TABLE.
The following code tells the hotplug scripts that this module supports a
single device with a specific vendor and product ID:
::
/* table of devices that work with this driver */
static struct usb_device_id skel_table [] = {
{ USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) },
{ } /* Terminating entry */
};
MODULE_DEVICE_TABLE (usb, skel_table);
There are other macros that can be used in describing a usb_device_id
for drivers that support a whole class of USB drivers. See usb.h for
more information on this.
Device operation
================
When a device is plugged into the USB bus that matches the device ID
pattern that your driver registered with the USB core, the probe
function is called. The usb_device structure, interface number and the
interface ID are passed to the function:
::
static int skel_probe(struct usb_interface *interface,
const struct usb_device_id *id)
The driver now needs to verify that this device is actually one that it
can accept. If so, it returns 0. If not, or if any error occurs during
initialization, an errorcode (such as ``-ENOMEM`` or ``-ENODEV``) is
returned from the probe function.
In the skeleton driver, we determine what end points are marked as
bulk-in and bulk-out. We create buffers to hold the data that will be
sent and received from the device, and a USB urb to write data to the
device is initialized.
Conversely, when the device is removed from the USB bus, the disconnect
function is called with the device pointer. The driver needs to clean
any private data that has been allocated at this time and to shut down
any pending urbs that are in the USB system.
Now that the device is plugged into the system and the driver is bound
to the device, any of the functions in the file_operations structure
that were passed to the USB subsystem will be called from a user program
trying to talk to the device. The first function called will be open, as
the program tries to open the device for I/O. We increment our private
usage count and save a pointer to our internal structure in the file
structure. This is done so that future calls to file operations will
enable the driver to determine which device the user is addressing. All
of this is done with the following code:
::
/* increment our usage count for the module */
++skel->open_count;
/* save our object in the file's private structure */
file->private_data = dev;
After the open function is called, the read and write functions are
called to receive and send data to the device. In the skel_write
function, we receive a pointer to some data that the user wants to send
to the device and the size of the data. The function determines how much
data it can send to the device based on the size of the write urb it has
created (this size depends on the size of the bulk out end point that
the device has). Then it copies the data from user space to kernel
space, points the urb to the data and submits the urb to the USB
subsystem. This can be seen in the following code:
::
/* we can only write as much as 1 urb will hold */
bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count;
/* copy the data from user space into our urb */
copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written);
/* set up our urb */
usb_fill_bulk_urb(skel->write_urb,
skel->dev,
usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr),
skel->write_urb->transfer_buffer,
bytes_written,
skel_write_bulk_callback,
skel);
/* send the data out the bulk port */
result = usb_submit_urb(skel->write_urb);
if (result) {
err("Failed submitting write urb, error %d", result);
}
When the write urb is filled up with the proper information using the
usb_fill_bulk_urb function, we point the urb's completion callback to
call our own skel_write_bulk_callback function. This function is
called when the urb is finished by the USB subsystem. The callback
function is called in interrupt context, so caution must be taken not to
do very much processing at that time. Our implementation of
skel_write_bulk_callback merely reports if the urb was completed
successfully or not and then returns.
The read function works a bit differently from the write function in
that we do not use an urb to transfer data from the device to the
driver. Instead we call the usb_bulk_msg function, which can be used
to send or receive data from a device without having to create urbs and
handle urb completion callback functions. We call the usb_bulk_msg
function, giving it a buffer into which to place any data received from
the device and a timeout value. If the timeout period expires without
receiving any data from the device, the function will fail and return an
error message. This can be shown with the following code:
::
/* do an immediate bulk read to get data from the device */
retval = usb_bulk_msg (skel->dev,
usb_rcvbulkpipe (skel->dev,
skel->bulk_in_endpointAddr),
skel->bulk_in_buffer,
skel->bulk_in_size,
&count, HZ*10);
/* if the read was successful, copy the data to user space */
if (!retval) {
if (copy_to_user (buffer, skel->bulk_in_buffer, count))
retval = -EFAULT;
else
retval = count;
}
The usb_bulk_msg function can be very useful for doing single reads or
writes to a device; however, if you need to read or write constantly to
a device, it is recommended to set up your own urbs and submit them to
the USB subsystem.
When the user program releases the file handle that it has been using to
talk to the device, the release function in the driver is called. In
this function we decrement our private usage count and wait for possible
pending writes:
::
/* decrement our usage count for the device */
--skel->open_count;
One of the more difficult problems that USB drivers must be able to
handle smoothly is the fact that the USB device may be removed from the
system at any point in time, even if a program is currently talking to
it. It needs to be able to shut down any current reads and writes and
notify the user-space programs that the device is no longer there. The
following code (function :c:func:`skel_delete()`) is an example of
how to do this:
::
static inline void skel_delete (struct usb_skel *dev)
{
kfree (dev->bulk_in_buffer);
if (dev->bulk_out_buffer != NULL)
usb_free_coherent (dev->udev, dev->bulk_out_size,
dev->bulk_out_buffer,
dev->write_urb->transfer_dma);
usb_free_urb (dev->write_urb);
kfree (dev);
}
If a program currently has an open handle to the device, we reset the
flag ``device_present``. For every read, write, release and other
functions that expect a device to be present, the driver first checks
this flag to see if the device is still present. If not, it releases
that the device has disappeared, and a -ENODEV error is returned to the
user-space program. When the release function is eventually called, it
determines if there is no device and if not, it does the cleanup that
the skel_disconnect function normally does if there are no open files
on the device (see Listing 5).
Isochronous Data
================
This usb-skeleton driver does not have any examples of interrupt or
isochronous data being sent to or from the device. Interrupt data is
sent almost exactly as bulk data is, with a few minor exceptions.
Isochronous data works differently with continuous streams of data being
sent to or from the device. The audio and video camera drivers are very
good examples of drivers that handle isochronous data and will be useful
if you also need to do this.
Conclusion
==========
Writing Linux USB device drivers is not a difficult task as the
usb-skeleton driver shows. This driver, combined with the other current
USB drivers, should provide enough examples to help a beginning author
create a working driver in a minimal amount of time. The linux-usb-devel
mailing list archives also contain a lot of helpful information.
Resources
=========
The Linux USB Project:
`http://www.linux-usb.org/ <http://www.linux-usb.org>`__
Linux Hotplug Project:
`http://linux-hotplug.sourceforge.net/ <http://linux-hotplug.sourceforge.net>`__
Linux USB Working Devices List:
`http://www.qbik.ch/usb/devices/ <http://www.qbik.ch/usb/devices>`__
linux-usb-devel Mailing List Archives:
http://marc.theaimsgroup.com/?l=linux-usb-devel
Programming Guide for Linux USB Device Drivers:
http://usb.cs.tum.edu/usbdoc
USB Home Page: http://www.usb.org