drm: Avoid random vblank counter jumps if the hardware counter has been reset

When drm_vblank_on() is called the hardware vblank counter may have
been reset, so we can't trust that the old values sampled prior to
drm_vblank_off() have anything to do with the new values.

So update the .last count in drm_vblank_on() to make the first
drm_vblank_enable() consider that as the reference point. This
will correct the user space visible counter to account for the
time between drm_vblank_on() and the first drm_vblank_enable()
calls.

For extra safety subtract one from the .last count in drm_vblank_on()
to make sure that user space will never see the same counter value
before and after modeset.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 1f86f6c6ecc6..fc1525a499d9 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1095,6 +1095,18 @@ void drm_vblank_on(struct drm_device *dev, int crtc)
                 atomic_dec(&dev->vblank[crtc].refcount);
                 dev->vblank[crtc].inmodeset = 0;
         }
+
+        /*
+         * sample the current counter to avoid random jumps
+         * when drm_vblank_enable() applies the diff
+         *
+         * -1 to make sure user will never see the same
+         * vblank counter value before and after a modeset
+         */
+        dev->vblank[crtc].last =
+                (dev->driver->get_vblank_counter(dev, crtc) - 1) &
+                dev->max_vblank_count;
+
         /* re-enable interrupts if there's are users left */
         if (atomic_read(&dev->vblank[crtc].refcount) != 0)
                 WARN_ON(drm_vblank_enable(dev, crtc));