[media] media: vb2: Fix regression on poll() for RW mode

When using a device is read/write mode, vb2 does not handle properly the
first select/poll operation.

The reason for this, is that when this code has been refactored, some of
the operations have changed their order, and now fileio emulator is not
started.

The reintroduced check to the core is enabled by a quirk flag, that
avoids this check by other subsystems like DVB.

Fixes: 49d8ab9f ("media] media: videobuf2: Separate vb2_poll()")
Reported-by: Dimitrios Katsaros <patcherwork@gmail.com>
Cc: Junghak Sung <jh1009.sung@samsung.com>
Cc: <stable@vger.kernel.org>      # for v4.5 and up
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

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

@@ -2297,6 +2297,16 @@ unsigned int vb2_core_poll(struct vb2_queue *q, struct file *file,
 	if (!vb2_is_streaming(q) || q->error)
 		return POLLERR;
 
+	/*
+	 * If this quirk is set and QBUF hasn't been called yet then
+	 * return POLLERR as well. This only affects capture queues, output
+	 * queues will always initialize waiting_for_buffers to false.
+	 * This quirk is set by V4L2 for backwards compatibility reasons.
+	 */
+	if (q->quirk_poll_must_check_waiting_for_buffers &&
+	    q->waiting_for_buffers && (req_events & (POLLIN | POLLRDNORM)))
+		return POLLERR;
+
 	/*
 	 * For output streams you can call write() as long as there are fewer
 	 * buffers queued than there are buffers available.

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

@@ -771,6 +771,12 @@ int vb2_queue_init(struct vb2_queue *q)
 	q->is_output = V4L2_TYPE_IS_OUTPUT(q->type);
 	q->copy_timestamp = (q->timestamp_flags & V4L2_BUF_FLAG_TIMESTAMP_MASK)
 			== V4L2_BUF_FLAG_TIMESTAMP_COPY;
+	/*
+	 * For compatibility with vb1: if QBUF hasn't been called yet, then
+	 * return POLLERR as well. This only affects capture queues, output
+	 * queues will always initialize waiting_for_buffers to false.
+	 */
+	q->quirk_poll_must_check_waiting_for_buffers = true;
 
 	return vb2_core_queue_init(q);
 }
@@ -824,14 +830,6 @@ unsigned int vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
 			poll_wait(file, &fh->wait, wait);
 	}
 
-	/*
-	 * For compatibility with vb1: if QBUF hasn't been called yet, then
-	 * return POLLERR as well. This only affects capture queues, output
-	 * queues will always initialize waiting_for_buffers to false.
-	 */
-	if (q->waiting_for_buffers && (req_events & (POLLIN | POLLRDNORM)))
-		return POLLERR;
-
 	return res | vb2_core_poll(q, file, wait);
 }
 EXPORT_SYMBOL_GPL(vb2_poll);

include/media/videobuf2-core.h

@@ -404,6 +404,9 @@ struct vb2_buf_ops {
  * @fileio_read_once:		report EOF after reading the first buffer
  * @fileio_write_immediately:	queue buffer after each write() call
  * @allow_zero_bytesused:	allow bytesused == 0 to be passed to the driver
+ * @quirk_poll_must_check_waiting_for_buffers: Return POLLERR at poll when QBUF
+ *              has not been called. This is a vb1 idiom that has been adopted
+ *              also by vb2.
  * @lock:	pointer to a mutex that protects the vb2_queue struct. The
  *		driver can set this to a mutex to let the v4l2 core serialize
  *		the queuing ioctls. If the driver wants to handle locking
@@ -467,6 +470,7 @@ struct vb2_queue {
 	unsigned			fileio_read_once:1;
 	unsigned			fileio_write_immediately:1;
 	unsigned			allow_zero_bytesused:1;
+	unsigned		   quirk_poll_must_check_waiting_for_buffers:1;
 
 	struct mutex			*lock;
 	void				*owner;