USB: EHCI: stagger frames for interrupt transfers

This patch (as1243) tries to improve ehci-hcd's scheduling of
interrupt transfers.  Instead of trying to cram all transfers with the
same period into the same frame, the new code will spread the
transfers out among lots of different frames.  This should reduce the
periodic schedule load in any one frame -- some host controllers have
trouble when there's too much work to do.

A more thorough approach would stagger the uframe values as well.  But
this is enough to make a big improvement.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Dwayne Fontenot <dwayne.fontenot@att.net>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/ehci-sched.c

@@ -760,16 +760,18 @@ static int qh_schedule(struct ehci_hcd *ehci, struct ehci_qh *qh)
 	if (status) {
 		/* "normal" case, uframing flexible except with splits */
 		if (qh->period) {
-			frame = qh->period - 1;
-			do {
+			int		i;
+
+			for (i = qh->period; status && i > 0; --i) {
+				frame = ++ehci->random_frame % qh->period;
 				for (uframe = 0; uframe < 8; uframe++) {
 					status = check_intr_schedule (ehci,
 							frame, uframe, qh,
 							&c_mask);
 					if (status == 0)
 						break;
 				}
-			} while (status && frame--);
+			}
 
 		/* qh->period == 0 means every uframe */
 		} else {

drivers/usb/host/ehci.h

@@ -116,6 +116,7 @@ struct ehci_hcd {			/* one per controller */
 	struct timer_list	watchdog;
 	unsigned long		actions;
 	unsigned		stamp;
+	unsigned		random_frame;
 	unsigned long		next_statechange;
 	u32			command;