[RFC] vb2: add support for (un)prepare_streaming queue ops

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index b203c1e26353..2f95d2102de4 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -544,6 +544,7 @@  static int __vb2_queue_free(struct vb2_queue *q, unsigned int buffers)
 	 */
 	if (q->num_buffers) {
 		bool unbalanced = q->cnt_start_streaming != q->cnt_stop_streaming ||
+				  q->cnt_prepare_streaming != q->cnt_unprepare_streaming;
 				  q->cnt_wait_prepare != q->cnt_wait_finish;

 		if (unbalanced || debug) {
@@ -552,14 +553,18 @@  static int __vb2_queue_free(struct vb2_queue *q, unsigned int buffers)
 			pr_info("     setup: %u start_streaming: %u stop_streaming: %u\n",
 				q->cnt_queue_setup, q->cnt_start_streaming,
 				q->cnt_stop_streaming);
+			pr_info("     prepare_streaming: %u unprepare_streaming: %u\n",
+				q->cnt_prepare_streaming, q->cnt_unprepare_streaming);
 			pr_info("     wait_prepare: %u wait_finish: %u\n",
 				q->cnt_wait_prepare, q->cnt_wait_finish);
 		}
 		q->cnt_queue_setup = 0;
 		q->cnt_wait_prepare = 0;
 		q->cnt_wait_finish = 0;
+		q->cnt_prepare_streaming = 0;
 		q->cnt_start_streaming = 0;
 		q->cnt_stop_streaming = 0;
+		q->cnt_unprepare_streaming = 0;
 	}
 	for (buffer = 0; buffer < q->num_buffers; ++buffer) {
 		struct vb2_buffer *vb = q->bufs[buffer];
@@ -1991,6 +1996,9 @@  static void __vb2_queue_cancel(struct vb2_queue *q)
 	if (q->start_streaming_called)
 		call_void_qop(q, stop_streaming, q);

+	if (q->streaming)
+		call_void_qop(q, unprepare_streaming, q);
+
 	/*
 	 * If you see this warning, then the driver isn't cleaning up properly
 	 * in stop_streaming(). See the stop_streaming() documentation in
@@ -2102,6 +2110,10 @@  int vb2_core_streamon(struct vb2_queue *q, unsigned int type)
 		return -EINVAL;
 	}

+	ret = call_qop(q, prepare_streaming, q);
+	if (ret)
+		return ret;
+
 	/*
 	 * Tell driver to start streaming provided sufficient buffers
 	 * are available.
@@ -2109,16 +2121,20 @@  int vb2_core_streamon(struct vb2_queue *q, unsigned int type)
 	if (q->queued_count >= q->min_buffers_needed) {
 		ret = v4l_vb2q_enable_media_source(q);
 		if (ret)
-			return ret;
+			goto unprepare;
 		ret = vb2_start_streaming(q);
 		if (ret)
-			return ret;
+			goto unprepare;
 	}

 	q->streaming = 1;

 	dprintk(q, 3, "successful\n");
 	return 0;
+
+unprepare:
+	call_void_qop(q, unprepare_streaming, q);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(vb2_core_streamon);

diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
index 5468b633b9d2..623146476157 100644
--- a/include/media/videobuf2-core.h
+++ b/include/media/videobuf2-core.h
@@ -386,6 +386,9 @@  struct vb2_buffer {
  *			the buffer contents will be ignored anyway.
  * @buf_cleanup:	called once before the buffer is freed; drivers may
  *			perform any additional cleanup; optional.
+ * @prepare_streaming:	called once to prepare for 'streaming' state; this is
+ *			where validation can be done to verify everything is
+ *			okay to start streaming later. Optional.
  * @start_streaming:	called once to enter 'streaming' state; the driver may
  *			receive buffers with @buf_queue callback
  *			before @start_streaming is called; the driver gets the
@@ -405,6 +408,7 @@  struct vb2_buffer {
  *			callback by calling vb2_buffer_done() with either
  *			%VB2_BUF_STATE_DONE or %VB2_BUF_STATE_ERROR; may use
  *			vb2_wait_for_all_buffers() function
+ * @unprepare_streaming:called as counterpart to @prepare_streaming. Optional.
  * @buf_queue:		passes buffer vb to the driver; driver may start
  *			hardware operation on this buffer; driver should give
  *			the buffer back by calling vb2_buffer_done() function;
@@ -432,8 +436,10 @@  struct vb2_ops {
 	void (*buf_finish)(struct vb2_buffer *vb);
 	void (*buf_cleanup)(struct vb2_buffer *vb);

+	int (*prepare_streaming)(struct vb2_queue *q);
 	int (*start_streaming)(struct vb2_queue *q, unsigned int count);
 	void (*stop_streaming)(struct vb2_queue *q);
+	void (*unprepare_streaming)(struct vb2_queue *q);

 	void (*buf_queue)(struct vb2_buffer *vb);

@@ -641,8 +647,10 @@  struct vb2_queue {
 	u32				cnt_queue_setup;
 	u32				cnt_wait_prepare;
 	u32				cnt_wait_finish;
+	u32				cnt_prepare_streaming;
 	u32				cnt_start_streaming;
 	u32				cnt_stop_streaming;
+	u32				cnt_unprepare_streaming;
 #endif
 };