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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.
+