media: vb2: fix epoll() by calling poll_wait first

The epoll function expects that whenever the poll file op is
called, the poll_wait function is also called. That didn't
always happen in vb2_core_poll() and vb2_poll(). Fix this,
otherwise epoll() would timeout when it shouldn't.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by: Yi Qingliang <niqingliang2003@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/common/videobuf2/videobuf2-core.c

@@ -2291,6 +2291,8 @@ __poll_t vb2_core_poll(struct vb2_queue *q, struct file *file,
 	if (q->is_output && !(req_events & (EPOLLOUT | EPOLLWRNORM)))
 		return 0;
 
+	poll_wait(file, &q->done_wq, wait);
+
 	/*
 	 * Start file I/O emulator only if streaming API has not been used yet.
 	 */
@@ -2342,8 +2344,6 @@ __poll_t vb2_core_poll(struct vb2_queue *q, struct file *file,
 		 */
 		if (q->last_buffer_dequeued)
 			return EPOLLIN | EPOLLRDNORM;
-
-		poll_wait(file, &q->done_wq, wait);
 	}
 
 	/*

drivers/media/common/videobuf2/videobuf2-v4l2.c

@@ -868,16 +868,14 @@ EXPORT_SYMBOL_GPL(vb2_queue_release);
 __poll_t vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
 {
 	struct video_device *vfd = video_devdata(file);
-	__poll_t req_events = poll_requested_events(wait);
 	__poll_t res = 0;
 
 	if (test_bit(V4L2_FL_USES_V4L2_FH, &vfd->flags)) {
 		struct v4l2_fh *fh = file->private_data;
 
+		poll_wait(file, &fh->wait, wait);
 		if (v4l2_event_pending(fh))
 			res = EPOLLPRI;
-		else if (req_events & EPOLLPRI)
-			poll_wait(file, &fh->wait, wait);
 	}
 
 	return res | vb2_core_poll(q, file, wait);