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

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>
Cc: stable@vger.kernel.org

1 files changed, 16 insertions, 10 deletions

diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/timer/base.c b/drivers/gpu/drm/nouveau/nvkm/subdev/timer/base.c
index 07dc82bfe346..4e696627fdcd 100644
--- a/drivers/gpu/drm/nouveau/nvkm/subdev/timer/base.c
+++ b/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))
-                        list_move_tail(&alarm->head, &exec);
+                /* 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);