USB_UNLINK_URB(9) USB Core APIs USB_UNLINK_URB(9)NAMEusb_unlink_urb - abort/cancel a transfer request for an endpoint
SYNOPSIS
int usb_unlink_urb(struct urb * urb);
ARGUMENTS
urb
pointer to urb describing a previously submitted request, may be
NULL
DESCRIPTION
This routine cancels an in-progress request. URBs complete only once
per submission, and may be canceled only once per submission.
Successful cancellation means termination of urb will be expedited and
the completion handler will be called with a status code indicating
that the request has been canceled (rather than any other code).
Drivers should not call this routine or related routines, such as
usb_kill_urb or usb_unlink_anchored_urbs, after their disconnect method
has returned. The disconnect function should synchronize with a
driver´s I/O routines to insure that all URB-related activity has
completed before it returns.
This request is always asynchronous. Success is indicated by returning
-EINPROGRESS, at which time the URB will probably not yet have been
given back to the device driver. When it is eventually called, the
completion function will see urb->status == -ECONNRESET. Failure is
indicated by usb_unlink_urb returning any other value. Unlinking will
fail when 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
[The behaviors and guarantees described below do not apply to virtual
root hubs but only to endpoint queues for physical USB devices.]
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 generally stops (see below), at least until that URB´s
completion routine returns. It is guaranteed that a stopped 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. The same behavior and
guarantee apply when an URB terminates because it was unlinked.
Bulk and interrupt endpoint queues are guaranteed to stop whenever an
URB terminates with any sort of error, including -ECONNRESET, -ENOENT,
and -EREMOTEIO. Control endpoint queues behave the same way except that
they are not guaranteed to stop for -EREMOTEIO errors. Queues for
isochronous endpoints are treated differently, because they must
advance at fixed rates. Such queues do not stop when an URB encounters
an error or is unlinked. An unlinked isochronous URB may leave a gap in
the stream of packets; it is undefined whether such gaps can be filled
in.
Note that early termination of an URB because a short packet was
received will generate a -EREMOTEIO error if and only if the
URB_SHORT_NOT_OK flag is set. By setting this flag, USB device drivers
can build deep queues for large or complex bulk transfers and clean
them up reliably after any sort of aborted transfer by unlinking all
pending URBs at the first fault.
When a control URB terminates with an error other than -EREMOTEIO, it
is quite likely that the status stage of the transfer will not take
place.
COPYRIGHTKernel Hackers Manual 2.6. November 2013 USB_UNLINK_URB(9)
