[v5,1/2] drm/vblank: warn on sending stale event

Commit Message

Bob Beckett June 28, 2019, 2:15 p.m. UTC

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

Signed-off-by: Robert Beckett <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
---
 drivers/gpu/drm/drm_vblank.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

Patch

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 603ab105125d..9395b8c690b8 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -918,6 +918,20 @@  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 all of the following are
+ * true:
+ *   - 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 after the
+ *     call to drm_crtc_vblank_off() but before the next call to
+ *     drm_crtc_vblank_on())
+ * then drm_crtc_send_vblank_event is going to give you a garbage timestamp and
+ * sequence number (the last recorded before the irq was disabled).
+ *
+ * Drivers must either hold a vblank reference acquired through
+ * drm_crtc_vblank_get() or the vblank must have been shut off by calling
+ * drm_crtc_vblank_off().
  */
 void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
 				struct drm_pending_vblank_event *e)
@@ -925,8 +939,12 @@  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 {