drm_vblank_get
drm_vblank_get(drm, event->pipe);
extern int drm_vblank_get(struct drm_device *dev, int crtc);
if (drm_vblank_get(dev, crtc) == 0)
ret = drm_vblank_get(dev, crtc);
EXPORT_SYMBOL(drm_vblank_get);