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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Tomi,

On Mon, Aug 26, 2024 at 02:10:57PM +0300, Tomi Valkeinen wrote:
> 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)?
>
> > 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.

I'll add here

A subsequent change will improve the v4l2_subdev_link_validate() helper
to properly support validating video device to subdev links.

> > 
> > 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;

-- 
Regards,

Laurent Pinchart