Warn when about to send stale vblank info and add advice to documentation on how to avoid.
Signed-off-by: Robert Beckett <[email protected]> --- 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
