231 lines
9.9 KiB
Plaintext
231 lines
9.9 KiB
Plaintext
===========
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EHCI driver
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===========
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27-Dec-2002
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The EHCI driver is used to talk to high speed USB 2.0 devices using
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USB 2.0-capable host controller hardware. The USB 2.0 standard is
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compatible with the USB 1.1 standard. It defines three transfer speeds:
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- "High Speed" 480 Mbit/sec (60 MByte/sec)
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- "Full Speed" 12 Mbit/sec (1.5 MByte/sec)
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- "Low Speed" 1.5 Mbit/sec
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USB 1.1 only addressed full speed and low speed. High speed devices
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can be used on USB 1.1 systems, but they slow down to USB 1.1 speeds.
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USB 1.1 devices may also be used on USB 2.0 systems. When plugged
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into an EHCI controller, they are given to a USB 1.1 "companion"
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controller, which is a OHCI or UHCI controller as normally used with
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such devices. When USB 1.1 devices plug into USB 2.0 hubs, they
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interact with the EHCI controller through a "Transaction Translator"
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(TT) in the hub, which turns low or full speed transactions into
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high speed "split transactions" that don't waste transfer bandwidth.
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At this writing, this driver has been seen to work with implementations
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of EHCI from (in alphabetical order): Intel, NEC, Philips, and VIA.
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Other EHCI implementations are becoming available from other vendors;
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you should expect this driver to work with them too.
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While usb-storage devices have been available since mid-2001 (working
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quite speedily on the 2.4 version of this driver), hubs have only
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been available since late 2001, and other kinds of high speed devices
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appear to be on hold until more systems come with USB 2.0 built-in.
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Such new systems have been available since early 2002, and became much
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more typical in the second half of 2002.
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Note that USB 2.0 support involves more than just EHCI. It requires
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other changes to the Linux-USB core APIs, including the hub driver,
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but those changes haven't needed to really change the basic "usbcore"
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APIs exposed to USB device drivers.
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- David Brownell
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<dbrownell@users.sourceforge.net>
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Functionality
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=============
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This driver is regularly tested on x86 hardware, and has also been
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used on PPC hardware so big/little endianness issues should be gone.
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It's believed to do all the right PCI magic so that I/O works even on
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systems with interesting DMA mapping issues.
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Transfer Types
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--------------
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At this writing the driver should comfortably handle all control, bulk,
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and interrupt transfers, including requests to USB 1.1 devices through
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transaction translators (TTs) in USB 2.0 hubs. But you may find bugs.
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High Speed Isochronous (ISO) transfer support is also functional, but
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at this writing no Linux drivers have been using that support.
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Full Speed Isochronous transfer support, through transaction translators,
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is not yet available. Note that split transaction support for ISO
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transfers can't share much code with the code for high speed ISO transfers,
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since EHCI represents these with a different data structure. So for now,
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most USB audio and video devices can't be connected to high speed buses.
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Driver Behavior
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---------------
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Transfers of all types can be queued. This means that control transfers
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from a driver on one interface (or through usbfs) won't interfere with
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ones from another driver, and that interrupt transfers can use periods
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of one frame without risking data loss due to interrupt processing costs.
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The EHCI root hub code hands off USB 1.1 devices to its companion
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controller. This driver doesn't need to know anything about those
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drivers; a OHCI or UHCI driver that works already doesn't need to change
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just because the EHCI driver is also present.
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There are some issues with power management; suspend/resume doesn't
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behave quite right at the moment.
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Also, some shortcuts have been taken with the scheduling periodic
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transactions (interrupt and isochronous transfers). These place some
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limits on the number of periodic transactions that can be scheduled,
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and prevent use of polling intervals of less than one frame.
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Use by
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======
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Assuming you have an EHCI controller (on a PCI card or motherboard)
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and have compiled this driver as a module, load this like::
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# modprobe ehci-hcd
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and remove it by::
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# rmmod ehci-hcd
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You should also have a driver for a "companion controller", such as
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"ohci-hcd" or "uhci-hcd". In case of any trouble with the EHCI driver,
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remove its module and then the driver for that companion controller will
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take over (at lower speed) all the devices that were previously handled
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by the EHCI driver.
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Module parameters (pass to "modprobe") include:
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log2_irq_thresh (default 0):
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Log2 of default interrupt delay, in microframes. The default
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value is 0, indicating 1 microframe (125 usec). Maximum value
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is 6, indicating 2^6 = 64 microframes. This controls how often
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the EHCI controller can issue interrupts.
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If you're using this driver on a 2.5 kernel, and you've enabled USB
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debugging support, you'll see three files in the "sysfs" directory for
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any EHCI controller:
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"async"
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dumps the asynchronous schedule, used for control
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and bulk transfers. Shows each active qh and the qtds
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pending, usually one qtd per urb. (Look at it with
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usb-storage doing disk I/O; watch the request queues!)
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"periodic"
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dumps the periodic schedule, used for interrupt
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and isochronous transfers. Doesn't show qtds.
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"registers"
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show controller register state, and
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The contents of those files can help identify driver problems.
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Device drivers shouldn't care whether they're running over EHCI or not,
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but they may want to check for "usb_device->speed == USB_SPEED_HIGH".
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High speed devices can do things that full speed (or low speed) ones
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can't, such as "high bandwidth" periodic (interrupt or ISO) transfers.
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Also, some values in device descriptors (such as polling intervals for
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periodic transfers) use different encodings when operating at high speed.
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However, do make a point of testing device drivers through USB 2.0 hubs.
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Those hubs report some failures, such as disconnections, differently when
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transaction translators are in use; some drivers have been seen to behave
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badly when they see different faults than OHCI or UHCI report.
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Performance
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===========
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USB 2.0 throughput is gated by two main factors: how fast the host
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controller can process requests, and how fast devices can respond to
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them. The 480 Mbit/sec "raw transfer rate" is obeyed by all devices,
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but aggregate throughput is also affected by issues like delays between
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individual high speed packets, driver intelligence, and of course the
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overall system load. Latency is also a performance concern.
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Bulk transfers are most often used where throughput is an issue. It's
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good to keep in mind that bulk transfers are always in 512 byte packets,
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and at most 13 of those fit into one USB 2.0 microframe. Eight USB 2.0
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microframes fit in a USB 1.1 frame; a microframe is 1 msec/8 = 125 usec.
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So more than 50 MByte/sec is available for bulk transfers, when both
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hardware and device driver software allow it. Periodic transfer modes
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(isochronous and interrupt) allow the larger packet sizes which let you
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approach the quoted 480 MBit/sec transfer rate.
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Hardware Performance
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--------------------
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At this writing, individual USB 2.0 devices tend to max out at around
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20 MByte/sec transfer rates. This is of course subject to change;
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and some devices now go faster, while others go slower.
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The first NEC implementation of EHCI seems to have a hardware bottleneck
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at around 28 MByte/sec aggregate transfer rate. While this is clearly
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enough for a single device at 20 MByte/sec, putting three such devices
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onto one bus does not get you 60 MByte/sec. The issue appears to be
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that the controller hardware won't do concurrent USB and PCI access,
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so that it's only trying six (or maybe seven) USB transactions each
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microframe rather than thirteen. (Seems like a reasonable trade off
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for a product that beat all the others to market by over a year!)
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It's expected that newer implementations will better this, throwing
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more silicon real estate at the problem so that new motherboard chip
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sets will get closer to that 60 MByte/sec target. That includes an
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updated implementation from NEC, as well as other vendors' silicon.
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There's a minimum latency of one microframe (125 usec) for the host
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to receive interrupts from the EHCI controller indicating completion
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of requests. That latency is tunable; there's a module option. By
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default ehci-hcd driver uses the minimum latency, which means that if
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you issue a control or bulk request you can often expect to learn that
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it completed in less than 250 usec (depending on transfer size).
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Software Performance
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--------------------
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To get even 20 MByte/sec transfer rates, Linux-USB device drivers will
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need to keep the EHCI queue full. That means issuing large requests,
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or using bulk queuing if a series of small requests needs to be issued.
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When drivers don't do that, their performance results will show it.
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In typical situations, a usb_bulk_msg() loop writing out 4 KB chunks is
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going to waste more than half the USB 2.0 bandwidth. Delays between the
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I/O completion and the driver issuing the next request will take longer
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than the I/O. If that same loop used 16 KB chunks, it'd be better; a
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sequence of 128 KB chunks would waste a lot less.
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But rather than depending on such large I/O buffers to make synchronous
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I/O be efficient, it's better to just queue up several (bulk) requests
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to the HC, and wait for them all to complete (or be canceled on error).
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Such URB queuing should work with all the USB 1.1 HC drivers too.
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In the Linux 2.5 kernels, new usb_sg_*() api calls have been defined; they
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queue all the buffers from a scatterlist. They also use scatterlist DMA
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mapping (which might apply an IOMMU) and IRQ reduction, all of which will
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help make high speed transfers run as fast as they can.
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TBD:
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Interrupt and ISO transfer performance issues. Those periodic
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transfers are fully scheduled, so the main issue is likely to be how
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to trigger "high bandwidth" modes.
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TBD:
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More than standard 80% periodic bandwidth allocation is possible
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through sysfs uframe_periodic_max parameter. Describe that.
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