Provided by: linux-doc-2.6.15_2.6.15-23.39_all
usb_unlink_urb - abort/cancel a transfer request for an endpoint
int usb_unlink_urb (struct urb * urb);
urb pointer to urb describing a previously submitted request, may be
This routine cancels an in-progress request. URBs complete only once
per submission, and may be canceled only once per submission.
Successful cancellation means the requests’s completion handler will be
called with a status code indicating that the request has been canceled
(rather than any other code) and will quickly be removed from host
controller data structures.
This request is always asynchronous. Success is indicated by returning
-EINPROGRESS, at which time the URB will normally have been unlinked
but not yet given back to the device driver. When it is called, the
completion function will see urb->status == -ECONNRESET. Failure is
indicated by any other return value. Unlinking will fail when the URB
is not currently ‘‘linked’’ (i.e., it was never submitted, or it was
unlinked before, or the hardware is already finished with it), even if
the completion handler has not yet run.
UNLINKING AND ENDPOINT QUEUES
Host Controller Drivers (HCDs) place all the URBs for a particular
endpoint in a queue. Normally the queue advances as the controller
hardware processes each request. But when an URB terminates with an
error its queue stops, at least until that URB’s completion routine
returns. It is guaranteed that the queue will not restart until all its
unlinked URBs have been fully retired, with their completion routines
run, even if that’s not until some time after the original completion
handler returns. Normally the same behavior and guarantees apply when
an URB terminates because it was unlinked; however if an URB is
unlinked before the hardware has started to execute it, then its queue
is not guaranteed to stop until all the preceding URBs have completed.
This means that USB device drivers can safely build deep queues for
large or complex transfers, and clean them up reliably after any sort
of aborted transfer by unlinking all pending URBs at the first fault.
Note that an URB terminating early because a short packet was received
will count as an error if and only if the URB_SHORT_NOT_OK flag is set.
Also, that all unlinks performed in any URB completion handler must be
Queues for isochronous endpoints are treated differently, because they
advance at fixed rates. Such queues do not stop when an URB is
unlinked. An unlinked URB may leave a gap in the stream of packets. It
is undefined whether such gaps can be filled in.
When a control URB terminates with an error, it is likely that the
status stage of the transfer will not take place, even if it is merely
a soft error resulting from a short-packet with URB_SHORT_NOT_OK set.