[PATCH v4 1/2] drm/vblank: warn on sending stale event

[Robert Beckett]

Warn when about to send stale vblank info and add advice to
documentation on how to avoid.

Signed-off-by: Robert Beckett <bob.beckett@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/drm_vblank.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 603ab105125d..7dabb2bdb733 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -918,6 +918,19 @@ EXPORT_SYMBOL(drm_crtc_arm_vblank_event);
  *
  * See drm_crtc_arm_vblank_event() for a helper which can be used in certain
  * situation, especially to send out events for atomic commit operations.
+ *
+ * Care should be taken to avoid stale timestamps. If:
+ *   - your driver has vblank support (i.e. dev->num_crtcs > 0)
+ *   - the vblank irq is off (i.e. no one called drm_crtc_vblank_get)
+ *   - from the vblank code's pov the pipe is still running (i.e. not
+ *     in-between a drm_crtc_vblank_off()/on() pair)
+ * If all of these conditions hold then drm_crtc_send_vblank_event is
+ * going to give you a garbage timestamp and and sequence number (the last
+ * recorded before the irq was disabled). If you call drm_crtc_vblank_get/put
+ * around it, or after vblank_off, then either of those will have rolled things
+ * forward for you.
+ * So, drivers should call drm_crtc_vblank_off() before this function in their
+ * crtc atomic_disable handlers.
  */
 void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
 				struct drm_pending_vblank_event *e)
@@ -925,8 +938,12 @@ void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
 	struct drm_device *dev = crtc->dev;
 	u64 seq;
 	unsigned int pipe = drm_crtc_index(crtc);
+	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
 	ktime_t now;
 
+	WARN_ONCE(dev->num_crtcs > 0 && !vblank->enabled && !vblank->inmodeset,
+		  "sending stale vblank info\n");
+
 	if (dev->num_crtcs > 0) {
 		seq = drm_vblank_count_and_time(dev, pipe, &now);
 	} else {
-- 
2.18.0