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>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
---
drivers/gpu/drm/drm_vblank.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
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 {
--
2.18.0
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