drm/nouveau/tmr: handle races with hw when updating the next alarm time

commit 1b0f8438 upstream.

If the time to the next alarm is short enough, we could race with HW and
end up with an ~4 second delay until it triggers.

Fix this by checking again after we update HW.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/nouveau/nvkm/subdev/timer/base.c

@@ -36,23 +36,29 @@ nvkm_timer_alarm_trigger(struct nvkm_timer *tmr)
 	unsigned long flags;
 	LIST_HEAD(exec);
 
-	/* move any due alarms off the pending list */
+	/* Process pending alarms. */
 	spin_lock_irqsave(&tmr->lock, flags);
 	list_for_each_entry_safe(alarm, atemp, &tmr->alarms, head) {
-		if (alarm->timestamp <= nvkm_timer_read(tmr))
+		/* Have we hit the earliest alarm that hasn't gone off? */
+		if (alarm->timestamp > nvkm_timer_read(tmr)) {
+			/* Schedule it.  If we didn't race, we're done. */
+			tmr->func->alarm_init(tmr, alarm->timestamp);
+			if (alarm->timestamp > nvkm_timer_read(tmr))
+				break;
+		}
+
+		/* Move to completed list.  We'll drop the lock before
+		 * executing the callback so it can reschedule itself.
+		 */
 		list_move_tail(&alarm->head, &exec);
 	}
 
-	/* reschedule interrupt for next alarm time */
-	if (!list_empty(&tmr->alarms)) {
-		alarm = list_first_entry(&tmr->alarms, typeof(*alarm), head);
-		tmr->func->alarm_init(tmr, alarm->timestamp);
-	} else {
+	/* Shut down interrupt if no more pending alarms. */
+	if (list_empty(&tmr->alarms))
 		tmr->func->alarm_fini(tmr);
-	}
 	spin_unlock_irqrestore(&tmr->lock, flags);
 
-	/* execute any pending alarm handlers */
+	/* Execute completed callbacks. */
 	list_for_each_entry_safe(alarm, atemp, &exec, head) {
 		list_del_init(&alarm->head);
 		alarm->func(alarm);