[media] vb2: allow requeuing buffers while streaming

vb2_buffer_done() already allows STATE_QUEUED, but currently only when not
streaming. It is useful to allow it while streaming as well, as this makes
it possible for drivers to requeue buffers while waiting for a stable
video signal.

Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

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

···
 				 V4L2_BUF_FLAG_KEYFRAME | V4L2_BUF_FLAG_TIMECODE)
 
 static void __vb2_queue_cancel(struct vb2_queue *q);
+static void __enqueue_in_driver(struct vb2_buffer *vb);
 
 /**
  * __vb2_buf_mem_alloc() - allocate video memory for the given buffer
···
 /**
  * vb2_buffer_done() - inform videobuf that an operation on a buffer is finished
  * @vb:		vb2_buffer returned from the driver
- * @state:	either VB2_BUF_STATE_DONE if the operation finished successfully
- *		or VB2_BUF_STATE_ERROR if the operation finished with an error.
+ * @state:	either VB2_BUF_STATE_DONE if the operation finished successfully,
+ *		VB2_BUF_STATE_ERROR if the operation finished with an error or
+ *		VB2_BUF_STATE_QUEUED if the driver wants to requeue buffers.
  *		If start_streaming fails then it should return buffers with state
  *		VB2_BUF_STATE_QUEUED to put them back into the queue.
  *
···
 	atomic_dec(&q->owned_by_drv_count);
 	spin_unlock_irqrestore(&q->done_lock, flags);
 
-	if (state == VB2_BUF_STATE_QUEUED)
+	if (state == VB2_BUF_STATE_QUEUED) {
+		if (q->start_streaming_called)
+			__enqueue_in_driver(vb);
 		return;
+	}
 
 	/* Inform any processes that may be waiting for buffers */
 	wake_up(&q->done_wq);