drm: Only update final vblank count when precise ts is available

Drivers without a hardware vblank counter simply can't account for the
vblanks that happened while the vblank interrupt was off. To check
this grab a vblank timestamp and if the result is dubious follow the
normal save-and-disable logic.

Drivers should prevent this by setting vblank_disable_allowed = false,
but since running vblank interrupts constantly is not good for power
consumption most drivers lie. Testing for precise vblank timestamps is
the next best thing we can check for.

Suggested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 87d148cec877..ad699b729278 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -148,8 +148,15 @@ static void vblank_disable_and_save(struct drm_device *dev, int crtc)
          * has been ticking all along until this time. This makes the
          * count account for the entire time between drm_vblank_on() and
          * drm_vblank_off().
+         *
+         * But only do this if precise vblank timestamps are available.
+         * Otherwise we might read a totally bogus timestamp since drivers
+         * lacking precise timestamp support rely upon sampling the system clock
+         * at vblank interrupt time. Which obviously won't work out well if the
+         * vblank interrupt is disabled.
          */
-        if (!vblank->enabled) {
+        if (!vblank->enabled &&
+            drm_get_last_vbltimestamp(dev, crtc, &tvblank, 0) > 0) {
                 drm_update_vblank_count(dev, crtc);
                 spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
                 return;