[PATCH v4 1/4] v4l: vsp1: Prevent multiple streamon race commencing pipeline early

[Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>]

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>

---

v4:
 - Revert and rework back to v1 implementation style
 - Provide detailed comments on the race

v3:
 - Move 'flag reset' to be inside the vsp1_reset_wpf() function call
 - Tidy up the wpf->pipe reference for the configured flag

To test this race, I have used the vsp-unit-test-0007.sh from Laurent's
VSP-Tests [0] in iteration. Without this patch, failures can be seen be
seen anywhere up to the 150 iterations mark.

With this patch in place, tests have successfully iterated over 1500
loops.

The function affected by this change appears to have been around since
v4.6-rc2-105-g351bbf99f245 and thus could be included in stable trees
from that point forward. The issue may have been prevalent before that
but the solution would need reworking for earlier version.

[0] http://git.ideasonboard.com/renesas/vsp-tests.git
---
 drivers/media/platform/vsp1/vsp1_video.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/media/platform/vsp1/vsp1_video.c b/drivers/media/platform/vsp1/vsp1_video.c
index e6592b576ca3..f7dc249eb398 100644
--- a/drivers/media/platform/vsp1/vsp1_video.c
+++ b/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);
-- 
git-series 0.9.1