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Michael Reutman reports that an AMD/ATI EHCI host controller on one of his computers does not stop transferring data when an active bulk QH is unlinked from the async schedule. Apparently that host controller fails to implement the IAA mechanism correctly when an active QH is unlinked. This leads to data corruption, because the controller continues to update the QH in memory when the driver doesn't expect it. As a result, the next URB submitted for that QH can hang, because the link pointers for the TD queue have been messed up. This misbehavior is observed quite regularly. To be fair, the EHCI spec (section 4.8.2) says that active QHs should not be unlinked. It goes on to recommend a procedure that involves waiting for the QH to go inactive before unlinking it. In the real world this is impractical, not least because the QH may _never_ go inactive. (What were they thinking?) Sometimes we have no choice but to unlink an active QH. In an attempt to avoid the problems that can ensue, this patch changes how the driver decides when the unlink is complete. In addition to waiting through two IAA cycles, in cases where the QH was not known to be inactive beforehand we now wait until a 2-ms period has elapsed with the host controller making no change to the QH data structure (the hw_current and hw_token fields in particular). The intuition here is that after such a long period, the endpoint must be NAKing and hopefully the QH has been dropped from the host controller's internal cache. There's no way to know if this reasoning is really valid -- the spec is no help in this regard -- but at least this approach fixes Michael's problem. The test for whether the QH is already known to be inactive involves the reason for unlinking the QH originally. If it was unlinked because it had halted, or it stopped in response to a short read, or it overlaid a dummy TD (a silicon bug), then it certainly is inactive. If it was unlinked because the TD queue was empty and no TDs have been added to the queue in the meantime, then it must be inactive. Or if the hardware status indicates that the QH is currently halted (even if that wasn't the reason for unlinking it), then it is inactive. Otherwise, if none of those checks apply, we go through the 2-ms delay. Signed-off-by: Alan Stern <stern@rowland.harvard.edu> Reported-by: Michael Reutman <mreutman@epiqsolutions.com> Tested-by: Michael Reutman <mreutman@epiqsolutions.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
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| usb-skeleton.c | ||
To understand all the Linux-USB framework, you'll use these resources:
* This source code. This is necessarily an evolving work, and
includes kerneldoc that should help you get a current overview.
("make pdfdocs", and then look at "usb.pdf" for host side and
"gadget.pdf" for peripheral side.) Also, Documentation/usb has
more information.
* The USB 2.0 specification (from www.usb.org), with supplements
such as those for USB OTG and the various device classes.
The USB specification has a good overview chapter, and USB
peripherals conform to the widely known "Chapter 9".
* Chip specifications for USB controllers. Examples include
host controllers (on PCs, servers, and more); peripheral
controllers (in devices with Linux firmware, like printers or
cell phones); and hard-wired peripherals like Ethernet adapters.
* Specifications for other protocols implemented by USB peripheral
functions. Some are vendor-specific; others are vendor-neutral
but just standardized outside of the www.usb.org team.
Here is a list of what each subdirectory here is, and what is contained in
them.
core/ - This is for the core USB host code, including the
usbfs files and the hub class driver ("hub_wq").
host/ - This is for USB host controller drivers. This
includes UHCI, OHCI, EHCI, and others that might
be used with more specialized "embedded" systems.
gadget/ - This is for USB peripheral controller drivers and
the various gadget drivers which talk to them.
Individual USB driver directories. A new driver should be added to the
first subdirectory in the list below that it fits into.
image/ - This is for still image drivers, like scanners or
digital cameras.
../input/ - This is for any driver that uses the input subsystem,
like keyboard, mice, touchscreens, tablets, etc.
../media/ - This is for multimedia drivers, like video cameras,
radios, and any other drivers that talk to the v4l
subsystem.
../net/ - This is for network drivers.
serial/ - This is for USB to serial drivers.
storage/ - This is for USB mass-storage drivers.
class/ - This is for all USB device drivers that do not fit
into any of the above categories, and work for a range
of USB Class specified devices.
misc/ - This is for all USB device drivers that do not fit
into any of the above categories.