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>

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));