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