Re: [PATCH v2 4/7] media: v4l2-subdev: Refactor warnings in v4l2_subdev_link_validate()

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

Hi,

On 22/08/2024 18:45, Laurent Pinchart wrote:
> The v4l2_subdev_link_validate() function prints a one-time warning if it
> gets called on a link whose source or sink is not a subdev. As links get
> validated in the context of their sink, a call to the helper when the
> link's sink is not a subdev indicates that the driver has set its
> .link_validate() handler to v4l2_subdev_link_validate() on a non-subdev
> entity, which is a clear driver bug. On the other hand, the link's
> source not being a subdev indicates that the helper is used for a subdev
> connected to a video output device, which is a lesser issue, if only
> because this is currently common practice.

Hmm, what does this mean... So we have a sink subdev, which might be 
linked to a source subdev (in which case everything is fine), or it 
might be linked to a non-subdev source.

Why is it a bug to be linked to a non-subdev source? And if it is a bug, 
why is ok (only pr_warn_once)?

  Tomi

> There are no drivers left in the kernel that use
> v4l2_subdev_link_validate() in a context where it may get called on a
> non-subdev sink. Replace the pr_warn_once() with a WARN_ON_ONCE() in
> this case to make sure that new offenders won't be introduced.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> ---
> Changes since v1:
> 
> - Switch from WARN_ON() to WARN_ON_ONCE()
> ---
>   drivers/media/v4l2-core/v4l2-subdev.c | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-subdev.c b/drivers/media/v4l2-core/v4l2-subdev.c
> index 7c5812d55315..d3196042d5c5 100644
> --- a/drivers/media/v4l2-core/v4l2-subdev.c
> +++ b/drivers/media/v4l2-core/v4l2-subdev.c
> @@ -1443,11 +1443,15 @@ int v4l2_subdev_link_validate(struct media_link *link)
>   	bool states_locked;
>   	int ret;
>   
> -	if (!is_media_entity_v4l2_subdev(link->sink->entity) ||
> -	    !is_media_entity_v4l2_subdev(link->source->entity)) {
> -		pr_warn_once("%s of link '%s':%u->'%s':%u is not a V4L2 sub-device, driver bug!\n",
> -			     !is_media_entity_v4l2_subdev(link->sink->entity) ?
> -			     "sink" : "source",
> +	/*
> +	 * Links are validated in the context of the sink entity. Usage of this
> +	 * helper on a sink that is not a subdev is a clear driver bug.
> +	 */
> +	if (WARN_ON_ONCE(!is_media_entity_v4l2_subdev(link->sink->entity)))
> +		return -EINVAL;
> +
> +	if (!is_media_entity_v4l2_subdev(link->source->entity)) {
> +		pr_warn_once("source of link '%s':%u->'%s':%u is not a V4L2 sub-device, driver bug!\n",
>   			     link->source->entity->name, link->source->index,
>   			     link->sink->entity->name, link->sink->index);
>   		return 0;