[media] v4l: vsp1: Prevent multiple streamon race commencing pipeline early

With multiple inputs through the BRU it is feasible for the streams to
race each other at stream-on.

Multiple VIDIOC_STREAMON calls racing each other could have process
N-1 skipping over the pipeline setup section and then start the pipeline
early, if videobuf2 has already enqueued buffers to the driver for
process N but not called the .start_streaming() operation yet

In the case of the video pipelines, this
can present two serious issues.

 1) A null-dereference if the pipe->dl is committed at the same time as
    the vsp1_video_setup_pipeline() is processing

 2) A hardware hang, where a display list is committed without having
    called vsp1_video_setup_pipeline() first

Repair this issue, by ensuring that only the stream which configures the
pipeline is able to start it.
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/platform/vsp1/vsp1_video.c

@@ -797,6 +797,7 @@ static int vsp1_video_start_streaming(struct vb2_queue *vq, unsigned int count)
 {
 	struct vsp1_video *video = vb2_get_drv_priv(vq);
 	struct vsp1_pipeline *pipe = video->rwpf->pipe;
+	bool start_pipeline = false;
 	unsigned long flags;
 	int ret;
 
@@ -807,11 +808,23 @@ static int vsp1_video_start_streaming(struct vb2_queue *vq, unsigned int count)
 			mutex_unlock(&pipe->lock);
 			return ret;
 		}
+
+		start_pipeline = true;
 	}
 
 	pipe->stream_count++;
 	mutex_unlock(&pipe->lock);
 
+	/*
+	 * vsp1_pipeline_ready() is not sufficient to establish that all streams
+	 * are prepared and the pipeline is configured, as multiple streams
+	 * can race through streamon with buffers already queued; Therefore we
+	 * don't even attempt to start the pipeline until the last stream has
+	 * called through here.
+	 */
+	if (!start_pipeline)
+		return 0;
+
 	spin_lock_irqsave(&pipe->irqlock, flags);
 	if (vsp1_pipeline_ready(pipe))
 		vsp1_video_pipeline_run(pipe);