USB: URB documentation: claim complete() will be run with IRQs enabled

There is no good reason to run complete() in hard interrupt
disabled context.

After switch to run complete() in tasklet, we will enable local IRQs
when calling complete() since we can do it at that time.

Even though we still disable IRQs now when calling complete()
in tasklet, the URB documentation is updated to claim complete()
will be run in tasklet context and local IRQs will be enabled, so
that USB drivers can know the change and avoid one deadlock caused
by: assume IRQs disabled in complete() and call spin_lock() to
hold lock which might be acquired in interrupt context.

Current spin_lock() usages in drivers' complete() will be cleaned
up at the same time, and once the cleanup is finished, local IRQs
will be enabled when calling complete() in tasklet.

Also fix description about type of usb_complete_t, and remove the
advice of running completion handler in tasklet for decreasing
system latency.

Cc: Oliver Neukum <oliver@neukum.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/usb/URB.txt

@@ -195,13 +195,12 @@ by the completion handler.
 
 The handler is of the following type:
 
-	typedef void (*usb_complete_t)(struct urb *, struct pt_regs *)
+	typedef void (*usb_complete_t)(struct urb *)
 
-I.e., it gets the URB that caused the completion call, plus the
-register values at the time of the corresponding interrupt (if any).
-In the completion handler, you should have a look at urb->status to
-detect any USB errors. Since the context parameter is included in the URB,
-you can pass information to the completion handler. 
+I.e., it gets the URB that caused the completion call. In the completion
+handler, you should have a look at urb->status to detect any USB errors.
+Since the context parameter is included in the URB, you can pass
+information to the completion handler.
 
 Note that even when an error (or unlink) is reported, data may have been
 transferred.  That's because USB transfers are packetized; it might take
@@ -210,12 +209,12 @@ have transferred successfully before the completion was called.
 
 
 NOTE:  ***** WARNING *****
-NEVER SLEEP IN A COMPLETION HANDLER.  These are normally called
-during hardware interrupt processing.  If you can, defer substantial
-work to a tasklet (bottom half) to keep system latencies low.  You'll
-probably need to use spinlocks to protect data structures you manipulate
-in completion handlers.
+NEVER SLEEP IN A COMPLETION HANDLER.  These are often called in atomic
+context.
 
+In the current kernel, completion handlers run with local interrupts
+disabled, but in the future this will be changed, so don't assume that
+local IRQs are always disabled inside completion handlers.
 
 1.8. How to do isochronous (ISO) transfers?