[PATCH v2] drm: add some comments to drm_wait_vblank and drm_queue_vblank_event

[Ilija Hadzic]

during the review of the fix for locks problems in drm_wait_vblank,
a couple of false concerns were raised about how the drm_vblank_get
and drm_vblank_put are used in this function; it turned out that the
code is correct and that it cannot be simplified

add a few comments to explain non-obvious flows in the code,
to prevent "false alarms" in the future

v2: incorporate comments received from Daniel Vetter

Signed-off-by: Ilija Hadzic <ihadzic at research.bell-labs.com>
---
 drivers/gpu/drm/drm_irq.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 3830e9e..79c02da 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1124,6 +1124,7 @@ static int drm_queue_vblank_event(struct drm_device *dev, int pipe,
 		trace_drm_vblank_event_delivered(current->pid, pipe,
 						 vblwait->request.sequence);
 	} else {
+		/* drm_handle_vblank_events will call drm_vblank_put */
 		list_add_tail(&e->base.link, &dev->vblank_event_list);
 		vblwait->reply.sequence = vblwait->request.sequence;
 	}
@@ -1204,8 +1205,12 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 		goto done;
 	}
 
-	if (flags & _DRM_VBLANK_EVENT)
+	if (flags & _DRM_VBLANK_EVENT) {
+		/* must hold on to the vblank ref until the event fires
+		 * drm_vblank_put will be called asynchronously
+		 */
 		return drm_queue_vblank_event(dev, crtc, vblwait, file_priv);
+	}
 
 	if ((flags & _DRM_VBLANK_NEXTONMISS) &&
 	    (seq - vblwait->request.sequence) <= (1<<23)) {
-- 
1.7.7