|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
|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.
|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.
|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.
|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:
|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!)
|dumps the periodic schedule, used for interrupt
|and isochronous transfers. Doesn't show qtds.
|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.
|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.
|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).
|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.
|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.
|More than standard 80% periodic bandwidth allocation is possible
|through sysfs uframe_periodic_max parameter. Describe that.