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/Documentation/usb/ehci.txt

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  127-Dec-2002
  2
  3The EHCI driver is used to talk to high speed USB 2.0 devices using
  4USB 2.0-capable host controller hardware.  The USB 2.0 standard is
  5compatible with the USB 1.1 standard. It defines three transfer speeds:
  6
  7    - "High Speed" 480 Mbit/sec (60 MByte/sec)
  8    - "Full Speed" 12 Mbit/sec (1.5 MByte/sec)
  9    - "Low Speed" 1.5 Mbit/sec
 10
 11USB 1.1 only addressed full speed and low speed.  High speed devices
 12can be used on USB 1.1 systems, but they slow down to USB 1.1 speeds. 
 13
 14USB 1.1 devices may also be used on USB 2.0 systems.  When plugged
 15into an EHCI controller, they are given to a USB 1.1 "companion"
 16controller, which is a OHCI or UHCI controller as normally used with
 17such devices.  When USB 1.1 devices plug into USB 2.0 hubs, they
 18interact with the EHCI controller through a "Transaction Translator"
 19(TT) in the hub, which turns low or full speed transactions into
 20high speed "split transactions" that don't waste transfer bandwidth.
 21
 22At this writing, this driver has been seen to work with implementations
 23of EHCI from (in alphabetical order):  Intel, NEC, Philips, and VIA.
 24Other EHCI implementations are becoming available from other vendors;
 25you should expect this driver to work with them too.
 26
 27While usb-storage devices have been available since mid-2001 (working
 28quite speedily on the 2.4 version of this driver), hubs have only
 29been available since late 2001, and other kinds of high speed devices
 30appear to be on hold until more systems come with USB 2.0 built-in.
 31Such new systems have been available since early 2002, and became much
 32more typical in the second half of 2002.
 33
 34Note that USB 2.0 support involves more than just EHCI.  It requires
 35other changes to the Linux-USB core APIs, including the hub driver,
 36but those changes haven't needed to really change the basic "usbcore"
 37APIs exposed to USB device drivers.
 38
 39- David Brownell
 40  <dbrownell@users.sourceforge.net>
 41
 42
 43FUNCTIONALITY
 44
 45This driver is regularly tested on x86 hardware, and has also been
 46used on PPC hardware so big/little endianness issues should be gone.
 47It's believed to do all the right PCI magic so that I/O works even on
 48systems with interesting DMA mapping issues.
 49
 50Transfer Types
 51
 52At this writing the driver should comfortably handle all control, bulk,
 53and interrupt transfers, including requests to USB 1.1 devices through
 54transaction translators (TTs) in USB 2.0 hubs.  But you may find bugs.
 55
 56High Speed Isochronous (ISO) transfer support is also functional, but
 57at this writing no Linux drivers have been using that support.
 58
 59Full Speed Isochronous transfer support, through transaction translators,
 60is not yet available.  Note that split transaction support for ISO
 61transfers can't share much code with the code for high speed ISO transfers,
 62since EHCI represents these with a different data structure.  So for now,
 63most USB audio and video devices can't be connected to high speed buses.
 64
 65Driver Behavior
 66
 67Transfers of all types can be queued.  This means that control transfers
 68from a driver on one interface (or through usbfs) won't interfere with
 69ones from another driver, and that interrupt transfers can use periods
 70of one frame without risking data loss due to interrupt processing costs.
 71
 72The EHCI root hub code hands off USB 1.1 devices to its companion
 73controller.  This driver doesn't need to know anything about those
 74drivers; a OHCI or UHCI driver that works already doesn't need to change
 75just because the EHCI driver is also present.
 76
 77There are some issues with power management; suspend/resume doesn't
 78behave quite right at the moment.
 79
 80Also, some shortcuts have been taken with the scheduling periodic
 81transactions (interrupt and isochronous transfers).  These place some
 82limits on the number of periodic transactions that can be scheduled,
 83and prevent use of polling intervals of less than one frame.
 84
 85
 86USE BY
 87
 88Assuming you have an EHCI controller (on a PCI card or motherboard)
 89and have compiled this driver as a module, load this like:
 90
 91    # modprobe ehci-hcd
 92
 93and remove it by:
 94
 95    # rmmod ehci-hcd
 96
 97You should also have a driver for a "companion controller", such as
 98"ohci-hcd"  or "uhci-hcd".  In case of any trouble with the EHCI driver,
 99remove its module and then the driver for that companion controller will
100take over (at lower speed) all the devices that were previously handled
101by the EHCI driver.
102
103Module parameters (pass to "modprobe") include:
104
105    log2_irq_thresh (default 0):
106	Log2 of default interrupt delay, in microframes.  The default
107	value is 0, indicating 1 microframe (125 usec).  Maximum value
108	is 6, indicating 2^6 = 64 microframes.  This controls how often
109	the EHCI controller can issue interrupts.
110
111If you're using this driver on a 2.5 kernel, and you've enabled USB
112debugging support, you'll see three files in the "sysfs" directory for
113any EHCI controller:
114
115	"async" dumps the asynchronous schedule, used for control
116		and bulk transfers.  Shows each active qh and the qtds
117		pending, usually one qtd per urb.  (Look at it with
118		usb-storage doing disk I/O; watch the request queues!)
119	"periodic" dumps the periodic schedule, used for interrupt
120		and isochronous transfers.  Doesn't show qtds.
121	"registers" show controller register state, and
122
123The contents of those files can help identify driver problems.
124
125
126Device drivers shouldn't care whether they're running over EHCI or not,
127but they may want to check for "usb_device->speed == USB_SPEED_HIGH".
128High speed devices can do things that full speed (or low speed) ones
129can't, such as "high bandwidth" periodic (interrupt or ISO) transfers.
130Also, some values in device descriptors (such as polling intervals for
131periodic transfers) use different encodings when operating at high speed.
132
133However, do make a point of testing device drivers through USB 2.0 hubs.
134Those hubs report some failures, such as disconnections, differently when
135transaction translators are in use; some drivers have been seen to behave
136badly when they see different faults than OHCI or UHCI report.
137
138
139PERFORMANCE
140
141USB 2.0 throughput is gated by two main factors:  how fast the host
142controller can process requests, and how fast devices can respond to
143them.  The 480 Mbit/sec "raw transfer rate" is obeyed by all devices,
144but aggregate throughput is also affected by issues like delays between
145individual high speed packets, driver intelligence, and of course the
146overall system load.  Latency is also a performance concern.
147
148Bulk transfers are most often used where throughput is an issue.  It's
149good to keep in mind that bulk transfers are always in 512 byte packets,
150and at most 13 of those fit into one USB 2.0 microframe.  Eight USB 2.0
151microframes fit in a USB 1.1 frame; a microframe is 1 msec/8 = 125 usec.
152
153So more than 50 MByte/sec is available for bulk transfers, when both
154hardware and device driver software allow it.  Periodic transfer modes
155(isochronous and interrupt) allow the larger packet sizes which let you
156approach the quoted 480 MBit/sec transfer rate.
157
158Hardware Performance
159
160At this writing, individual USB 2.0 devices tend to max out at around
16120 MByte/sec transfer rates.  This is of course subject to change;
162and some devices now go faster, while others go slower.
163
164The first NEC implementation of EHCI seems to have a hardware bottleneck
165at around 28 MByte/sec aggregate transfer rate.  While this is clearly
166enough for a single device at 20 MByte/sec, putting three such devices
167onto one bus does not get you 60 MByte/sec.  The issue appears to be
168that the controller hardware won't do concurrent USB and PCI access,
169so that it's only trying six (or maybe seven) USB transactions each
170microframe rather than thirteen.  (Seems like a reasonable trade off
171for a product that beat all the others to market by over a year!)
172
173It's expected that newer implementations will better this, throwing
174more silicon real estate at the problem so that new motherboard chip
175sets will get closer to that 60 MByte/sec target.  That includes an
176updated implementation from NEC, as well as other vendors' silicon.
177
178There's a minimum latency of one microframe (125 usec) for the host
179to receive interrupts from the EHCI controller indicating completion
180of requests.  That latency is tunable; there's a module option.  By
181default ehci-hcd driver uses the minimum latency, which means that if
182you issue a control or bulk request you can often expect to learn that
183it completed in less than 250 usec (depending on transfer size).
184
185Software Performance
186
187To get even 20 MByte/sec transfer rates, Linux-USB device drivers will
188need to keep the EHCI queue full.  That means issuing large requests,
189or using bulk queuing if a series of small requests needs to be issued.
190When drivers don't do that, their performance results will show it.
191
192In typical situations, a usb_bulk_msg() loop writing out 4 KB chunks is
193going to waste more than half the USB 2.0 bandwidth.  Delays between the
194I/O completion and the driver issuing the next request will take longer
195than the I/O.  If that same loop used 16 KB chunks, it'd be better; a
196sequence of 128 KB chunks would waste a lot less.
197
198But rather than depending on such large I/O buffers to make synchronous
199I/O be efficient, it's better to just queue up several (bulk) requests
200to the HC, and wait for them all to complete (or be canceled on error).
201Such URB queuing should work with all the USB 1.1 HC drivers too.
202
203In the Linux 2.5 kernels, new usb_sg_*() api calls have been defined; they
204queue all the buffers from a scatterlist.  They also use scatterlist DMA
205mapping (which might apply an IOMMU) and IRQ reduction, all of which will
206help make high speed transfers run as fast as they can.
207
208
209TBD:  Interrupt and ISO transfer performance issues.  Those periodic
210transfers are fully scheduled, so the main issue is likely to be how
211to trigger "high bandwidth" modes.
212