USB_UNLINK_URB(9)                         USB Core APIs                         USB_UNLINK_URB(9)



NAME
       usb_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 asynchronous, however the HCD might call the ->complete callback during
       unlink. Therefore when drivers call usb_unlink_urb, they must not hold any locks that may
       be taken by the completion function. 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.

       The URB must not be deallocated while this routine is running. In particular, when a
       driver calls this routine, it must insure that the completion handler cannot deallocate
       the URB.

RETURN
       -EINPROGRESS on success. See description for other values on failure.

       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.

COPYRIGHT
Kernel Hackers Manual 4.8.                 January 2017                         USB_UNLINK_URB(9)