DRM/KMS: Add Bail-Out Conditions for Loop.

When trying to obtain an accurate timestamp for the last vsync interrupt
in vblank_disable_and_save() we loop until the vsync counter after reading
the time stamp is identical to the one before.
In the case where no hardware timestamp can be obtained there is probably
no point in trying to make sure we remain within the same vsync during
the time we obtain the counter.
Furthermore we should make sure there's an 'emergency exit' so that we
don't end up in an endless loop when the driver get_vblank_timestamp()
function doesn't manage to return within the same vsync.
This may happen when this function prints out debugging information over
a slow (ie serial) line.
Signed-off-by: NEgbert Eich <eich@suse.de>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -106,6 +106,7 @@ static void vblank_disable_and_save(struct drm_device *dev, int crtc)
 	s64 diff_ns;
 	int vblrc;
 	struct timeval tvblank;
+	int count = DRM_TIMESTAMP_MAXRETRIES;
 
 	/* Prevent vblank irq processing while disabling vblank irqs,
 	 * so no updates of timestamps or count can happen after we've
@@ -131,7 +132,10 @@ static void vblank_disable_and_save(struct drm_device *dev, int crtc)
 	do {
 		dev->last_vblank[crtc] = dev->driver->get_vblank_counter(dev, crtc);
 		vblrc = drm_get_last_vbltimestamp(dev, crtc, &tvblank, 0);
-	} while (dev->last_vblank[crtc] != dev->driver->get_vblank_counter(dev, crtc));
+	} while (dev->last_vblank[crtc] != dev->driver->get_vblank_counter(dev, crtc) && (--count) && vblrc);
+
+	if (!count)
+		vblrc = 0;
 
 	/* Compute time difference to stored timestamp of last vblank
 	 * as updated by last invocation of drm_handle_vblank() in vblank irq.