drm: Correct unbalanced drm_vblank_put() during mode setting.

The first time we install a mode, the vblank will be disabled for a pipe
and so drm_vblank_get() in drm_vblank_pre_modeset() will fail. As we
unconditionally call drm_vblank_put() afterwards, the vblank reference
counter becomes unbalanced.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/gpu/drm/drm_irq.c

@@ -435,6 +435,8 @@ EXPORT_SYMBOL(drm_vblank_get);
  */
 void drm_vblank_put(struct drm_device *dev, int crtc)
 {
+	BUG_ON (atomic_read (&dev->vblank_refcount[crtc]) == 0);
+
 	/* Last user schedules interrupt disable */
 	if (atomic_dec_and_test(&dev->vblank_refcount[crtc]))
 		mod_timer(&dev->vblank_disable_timer, jiffies + 5*DRM_HZ);
@@ -460,8 +462,9 @@ void drm_vblank_pre_modeset(struct drm_device *dev, int crtc)
 	 * so that interrupts remain enabled in the interim.
 	 */
 	if (!dev->vblank_inmodeset[crtc]) {
-		dev->vblank_inmodeset[crtc] = 1;
-		drm_vblank_get(dev, crtc);
+		dev->vblank_inmodeset[crtc] = 0x1;
+		if (drm_vblank_get(dev, crtc) == 0)
+			dev->vblank_inmodeset[crtc] |= 0x2;
 	}
 }
 EXPORT_SYMBOL(drm_vblank_pre_modeset);
@@ -473,9 +476,12 @@ void drm_vblank_post_modeset(struct drm_device *dev, int crtc)
 	if (dev->vblank_inmodeset[crtc]) {
 		spin_lock_irqsave(&dev->vbl_lock, irqflags);
 		dev->vblank_disable_allowed = 1;
-		dev->vblank_inmodeset[crtc] = 0;
 		spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
+
+		if (dev->vblank_inmodeset[crtc] & 0x2)
 			drm_vblank_put(dev, crtc);
+
+		dev->vblank_inmodeset[crtc] = 0;
 	}
 }
 EXPORT_SYMBOL(drm_vblank_post_modeset);