drm/nouveau/tmr: avoid processing completed alarms when adding a new one

BugLink: http://bugs.launchpad.net/bugs/1694621

commit 330bdf62 upstream.

The idea here was to avoid having to "manually" program the HW if there's
a new earliest alarm.  This was lazy and bad, as it leads to loads of fun
races between inter-related callers (ie. therm).

Turns out, it's not so difficult after all.  Go figure ;)
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

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

@@ -80,12 +80,22 @@ nvkm_timer_alarm(struct nvkm_timer *tmr, u32 nsec, struct nvkm_alarm *alarm)
 			if (list->timestamp > alarm->timestamp)
 				break;
 		}
+
 		list_add_tail(&alarm->head, &list->head);
+
+		/* Update HW if this is now the earliest alarm. */
+		list = list_first_entry(&tmr->alarms, typeof(*list), head);
+		if (list == alarm) {
+			tmr->func->alarm_init(tmr, alarm->timestamp);
+			/* This shouldn't happen if callers aren't stupid.
+			 *
+			 * Worst case scenario is that it'll take roughly
+			 * 4 seconds for the next alarm to trigger.
+			 */
+			WARN_ON(alarm->timestamp <= nvkm_timer_read(tmr));
+		}
 	}
 	spin_unlock_irqrestore(&tmr->lock, flags);
-
-	/* process pending alarms */
-	nvkm_timer_alarm_trigger(tmr);
 }
 
 void