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diff --git a/Documentation/usb/ehci.rst b/Documentation/usb/ehci.rst new file mode 100644 index 000000000..31f650e7c --- /dev/null +++ b/Documentation/usb/ehci.rst @@ -0,0 +1,230 @@ +=========== +EHCI driver +=========== + +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. |