Re: [PATCH] media: mc: Remove spurious WARN_ON in __media_pipeline_start

On Fri, Aug 21, 2026 at 10:28:07PM +0530, Niharika Khare wrote:
> Vimc driver's entity topology has three capture nodes and two of
> those (Raw Capture 0 and RGB/YUV Capture) are reachable from each
> other via enabled links. The crash report and the C repro show that
> the two capture nodes attempt to invoke a STREAMON session one after
> the other, which resulted in the WARNING getting triggered by the
> media controller.
> 
>   vimc link validate: Sensor A:src:640x480 (0x42474752, 0, 0, 0, 0) Raw Capture 0:snk:640x480 (0x42474752, 8, 0, 0, 0)
>   vimc link validate: Scaler:src:640x480 (0x33424752, 8, 0, 0, 0) RGB/YUV Capture:snk:640x480 (0x33424752, 8, 0, 0, 0)
>   origin->pipe && origin->pipe != pipe
>   WARNING: drivers/media/mc/mc-entity.c:785 at __media_pipeline_start+0x1add/0x22b0, CPU#1: syz.0.17/5912
> 
> The graph walk originating from Raw Capture 0 (first STREAMON in C
> repro on /dev/video0) reaches RGB/YUV Capture and marks its origin
> pad's pipe with its own pipe. When RGB/YUV Capture's graph walk starts
> (second STREAMON in C repro on /dev/video4), the pad is already
> claimed. This is a legitimate userspace request but not something that

No, that's not supposed to be a legitimate use case. The WARN_ON
indicates a driver bug.

> kernel should allow while the pad remains claimed. Hence, the kernel
> checks the pipe and returns -EBUSY instead of raising a bug, same as
> the other non-origin pads. The check on origin pad's pipe needs to stay
> otherwise the origin pad case would go through the increment on the
> start_count path, even though there exists a loop few lines below this
> block to check the pad and return -EBUSY. The check also provides early
> failure detection and prevents the complete graph walk.
> 
> Fixes: ae219872834a ("media: mc: entity: Rewrite media_pipeline_start()")
> Reported-by: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=68e901d044baaea1f60c
> Tested-by: [email protected]
> Signed-off-by: Niharika Khare <[email protected]>
> ---
>  drivers/media/mc/mc-entity.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/media/mc/mc-entity.c b/drivers/media/mc/mc-entity.c
> index 3fa0bc687..7b4ef2c95 100644
> --- a/drivers/media/mc/mc-entity.c
> +++ b/drivers/media/mc/mc-entity.c
> @@ -778,12 +778,18 @@ __must_check int __media_pipeline_start(struct media_pad *origin,
>  
>  	lockdep_assert_held(&mdev->graph_mutex);
>  
> -	/*
> -	 * If the pad is already part of a pipeline, that pipeline must be the
> -	 * same as the pipe given to media_pipeline_start().
> -	 */
> -	if (WARN_ON(origin->pipe && origin->pipe != pipe))
> -		return -EINVAL;
> +	/*
> +	 * If the pad is part of a different pipeline than the one passed on to
> +	 * media_pipeline_start(), the pad must wait for the ongoing session on
> +	 * this previous pipeline to complete before it starts a new session.
> +	 * The request for starting the pipeline, although valid, cannot be
> +	 * served until the ongoing request finishes.
> +	 */
> +	if (origin->pipe && origin->pipe != pipe) {
> +		dev_dbg(mdev->dev, "Failed to start pipeline: pad '%s':%u busy\n",
> +			origin->entity->name, origin->index);
> +		return -EBUSY;
> +	}
>  
>  	/*
>  	 * If the pipeline has already been started, it is guaranteed to be

-- 
Regards,

Laurent Pinchart