EHCI: don't rescan interrupt QHs needlessly

This patch (as1466) speeds up processing of ehci-hcd's periodic list.
The existing code will pointlessly rescan an interrupt endpoint queue
each time it encounters the queue's QH in the periodic list, which can
happen quite a few times if the endpoint's period is low. On some
embedded systems, this useless overhead can waste so much time that
the driver falls hopelessly behind and loses events.

The patch introduces a "periodic_stamp" variable, which gets
incremented each time scan_periodic() runs and each time the scan
advances to a new frame. If the corresponding stamp in an interrupt
QH is equal to the current periodic_stamp, we assume the QH has
already been scanned and skip over it. Otherwise we scan the QH as
usual, and if none of its URBs have completed then we store the
current periodic_stamp in the QH's stamp, preventing it from being
scanned again.

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

drivers/usb/host/ehci-q.c

···
 				is_input, 0,
 				hb_mult(maxp) * max_packet(maxp)));
 		qh->start = NO_FRAME;
+		qh->stamp = ehci->periodic_stamp;
 
 		if (urb->dev->speed == USB_SPEED_HIGH) {
 			qh->c_usecs = 0;

drivers/usb/host/ehci-sched.c

···
 	}
 	clock &= mod - 1;
 	clock_frame = clock >> 3;
+	++ehci->periodic_stamp;
 
 	for (;;) {
 		union ehci_shadow	q, *q_p;
···
 				temp.qh = qh_get (q.qh);
 				type = Q_NEXT_TYPE(ehci, q.qh->hw->hw_next);
 				q = q.qh->qh_next;
-				modified = qh_completions (ehci, temp.qh);
-				if (unlikely(list_empty(&temp.qh->qtd_list) ||
-						temp.qh->needs_rescan))
-					intr_deschedule (ehci, temp.qh);
+				if (temp.qh->stamp != ehci->periodic_stamp) {
+					modified = qh_completions(ehci, temp.qh);
+					if (!modified)
+						temp.qh->stamp = ehci->periodic_stamp;
+					if (unlikely(list_empty(&temp.qh->qtd_list) ||
+							temp.qh->needs_rescan))
+						intr_deschedule(ehci, temp.qh);
+				}
 				qh_put (temp.qh);
 				break;
 			case Q_TYPE_FSTN:
···
 			if (ehci->clock_frame != clock_frame) {
 				free_cached_lists(ehci);
 				ehci->clock_frame = clock_frame;
+				++ehci->periodic_stamp;
 			}
 		} else {
 			now_uframe++;

drivers/usb/host/ehci.h

···
 	struct timer_list	watchdog;
 	unsigned long		actions;
 	unsigned		stamp;
+	unsigned		periodic_stamp;
 	unsigned		random_frame;
 	unsigned long		next_statechange;
 	ktime_t			last_periodic_enable;