USB: documentation update for usb_unlink_urb

This patch (as936) updates the kerneldoc for usb_unlink_urb. The
explanation of how endpoint queues are meant to work is now clearer
and in better agreement with reality.

Signed-off-by: Alan Stern <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/core/urb.c

···
  * @urb: pointer to urb describing a previously submitted request,
  *	may be NULL
  *
- * 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 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).
  *
- * 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.
+ * 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 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.
+ * 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.
  *
- * 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.
+ * 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 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 asynchronous.
+ * 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.
  *
- * 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.
+ * 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.
  */
 int usb_unlink_urb(struct urb *urb)
 {