128 lines
5.3 KiB
ReStructuredText
128 lines
5.3 KiB
ReStructuredText
============================
|
|
Linux kernel SLIMbus support
|
|
============================
|
|
|
|
Overview
|
|
========
|
|
|
|
What is SLIMbus?
|
|
----------------
|
|
SLIMbus (Serial Low Power Interchip Media Bus) is a specification developed by
|
|
MIPI (Mobile Industry Processor Interface) alliance. The bus uses master/slave
|
|
configuration, and is a 2-wire multi-drop implementation (clock, and data).
|
|
|
|
Currently, SLIMbus is used to interface between application processors of SoCs
|
|
(System-on-Chip) and peripheral components (typically codec). SLIMbus uses
|
|
Time-Division-Multiplexing to accommodate multiple data channels, and
|
|
a control channel.
|
|
|
|
The control channel is used for various control functions such as bus
|
|
management, configuration and status updates. These messages can be unicast (e.g.
|
|
reading/writing device specific values), or multicast (e.g. data channel
|
|
reconfiguration sequence is a broadcast message announced to all devices)
|
|
|
|
A data channel is used for data-transfer between 2 SLIMbus devices. Data
|
|
channel uses dedicated ports on the device.
|
|
|
|
Hardware description:
|
|
---------------------
|
|
SLIMbus specification has different types of device classifications based on
|
|
their capabilities.
|
|
A manager device is responsible for enumeration, configuration, and dynamic
|
|
channel allocation. Every bus has 1 active manager.
|
|
|
|
A generic device is a device providing application functionality (e.g. codec).
|
|
|
|
Framer device is responsible for clocking the bus, and transmitting frame-sync
|
|
and framing information on the bus.
|
|
|
|
Each SLIMbus component has an interface device for monitoring physical layer.
|
|
|
|
Typically each SoC contains SLIMbus component having 1 manager, 1 framer device,
|
|
1 generic device (for data channel support), and 1 interface device.
|
|
External peripheral SLIMbus component usually has 1 generic device (for
|
|
functionality/data channel support), and an associated interface device.
|
|
The generic device's registers are mapped as 'value elements' so that they can
|
|
be written/read using SLIMbus control channel exchanging control/status type of
|
|
information.
|
|
In case there are multiple framer devices on the same bus, manager device is
|
|
responsible to select the active-framer for clocking the bus.
|
|
|
|
Per specification, SLIMbus uses "clock gears" to do power management based on
|
|
current frequency and bandwidth requirements. There are 10 clock gears and each
|
|
gear changes the SLIMbus frequency to be twice its previous gear.
|
|
|
|
Each device has a 6-byte enumeration-address and the manager assigns every
|
|
device with a 1-byte logical address after the devices report presence on the
|
|
bus.
|
|
|
|
Software description:
|
|
---------------------
|
|
There are 2 types of SLIMbus drivers:
|
|
|
|
slim_controller represents a 'controller' for SLIMbus. This driver should
|
|
implement duties needed by the SoC (manager device, associated
|
|
interface device for monitoring the layers and reporting errors, default
|
|
framer device).
|
|
|
|
slim_device represents the 'generic device/component' for SLIMbus, and a
|
|
slim_driver should implement driver for that slim_device.
|
|
|
|
Device notifications to the driver:
|
|
-----------------------------------
|
|
Since SLIMbus devices have mechanisms for reporting their presence, the
|
|
framework allows drivers to bind when corresponding devices report their
|
|
presence on the bus.
|
|
However, it is possible that the driver needs to be probed
|
|
first so that it can enable corresponding SLIMbus device (e.g. power it up and/or
|
|
take it out of reset). To support that behavior, the framework allows drivers
|
|
to probe first as well (e.g. using standard DeviceTree compatibility field).
|
|
This creates the necessity for the driver to know when the device is functional
|
|
(i.e. reported present). device_up callback is used for that reason when the
|
|
device reports present and is assigned a logical address by the controller.
|
|
|
|
Similarly, SLIMbus devices 'report absent' when they go down. A 'device_down'
|
|
callback notifies the driver when the device reports absent and its logical
|
|
address assignment is invalidated by the controller.
|
|
|
|
Another notification "boot_device" is used to notify the slim_driver when
|
|
controller resets the bus. This notification allows the driver to take necessary
|
|
steps to boot the device so that it's functional after the bus has been reset.
|
|
|
|
Driver and Controller APIs:
|
|
---------------------------
|
|
.. kernel-doc:: include/linux/slimbus.h
|
|
:internal:
|
|
|
|
.. kernel-doc:: drivers/slimbus/slimbus.h
|
|
:internal:
|
|
|
|
.. kernel-doc:: drivers/slimbus/core.c
|
|
:export:
|
|
|
|
Clock-pause:
|
|
------------
|
|
SLIMbus mandates that a reconfiguration sequence (known as clock-pause) be
|
|
broadcast to all active devices on the bus before the bus can enter low-power
|
|
mode. Controller uses this sequence when it decides to enter low-power mode so
|
|
that corresponding clocks and/or power-rails can be turned off to save power.
|
|
Clock-pause is exited by waking up framer device (if controller driver initiates
|
|
exiting low power mode), or by toggling the data line (if a slave device wants
|
|
to initiate it).
|
|
|
|
Clock-pause APIs:
|
|
~~~~~~~~~~~~~~~~~
|
|
.. kernel-doc:: drivers/slimbus/sched.c
|
|
:export:
|
|
|
|
Messaging:
|
|
----------
|
|
The framework supports regmap and read/write apis to exchange control-information
|
|
with a SLIMbus device. APIs can be synchronous or asynchronous.
|
|
The header file <linux/slimbus.h> has more documentation about messaging APIs.
|
|
|
|
Messaging APIs:
|
|
~~~~~~~~~~~~~~~
|
|
.. kernel-doc:: drivers/slimbus/messaging.c
|
|
:export:
|