media: vb2: reorder checks in vb2_poll()

When reaching the end of stream, V4L2 clients may expect the
V4L2_EOS_EVENT before being able to dequeue the last buffer, which has
the V4L2_BUF_FLAG_LAST flag set.

If the vb2_poll() function first checks for events and afterwards if
buffers are available, a driver can queue the V4L2_EOS_EVENT event and
return the buffer after the check for events but before the check for
buffers. This causes vb2_poll() to signal that the buffer with
V4L2_BUF_FLAG_LAST can be read without the V4L2_EOS_EVENT being
available.

First, check for available buffers and afterwards for events to ensure
that if vb2_poll() signals POLLIN | POLLRDNORM for the
V4L2_BUF_FLAG_LAST buffer, it also signals POLLPRI for the
V4L2_EOS_EVENT.
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

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

@@ -872,17 +872,19 @@ 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 res = 0;
+	__poll_t res;
+
+	res = vb2_core_poll(q, file, wait);
 
 	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;
+			res |= EPOLLPRI;
 	}
 
-	return res | vb2_core_poll(q, file, wait);
+	return res;
 }
 EXPORT_SYMBOL_GPL(vb2_poll);