Re: [PATCH 4/8] media: v4l2-subdev: Refactor warnings in v4l2_subdev_link_validate()

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

On Sat, Jun 22, 2024 at 04:47:33PM +0000, Sakari Ailus wrote:
> Hi Laurent,
> 
> Thanks for the patch.
> 
> On Wed, Jun 19, 2024 at 04:23:52AM +0300, 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.
> > 
> > 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() in this
> > case to make sure that new offenders won't be introduced.
> > 
> > Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> > ---
> >  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 4f71199bf592..2d5e39c79620 100644
> > --- a/drivers/media/v4l2-core/v4l2-subdev.c
> > +++ b/drivers/media/v4l2-core/v4l2-subdev.c
> > @@ -1451,11 +1451,15 @@ 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(!is_media_entity_v4l2_subdev(link->sink->entity)))
> > +		return -EINVAL;
> 
> WARN*() is nowadays deprecated.

Why so ? I've followed a recent discussion where some people pointed out
they disliked WARN*() due to panic-on-warn, but I didn't see any
conclusion there that deprecated WARN*().

> Could you continue to use pr_warn_once()
> for this (or dev_warn_one())?

I think a WARN_ON() is the right option here. A failure indicates a
clear driver bug, we should be as loud as possible to make sure it gets
noticed during development.

> > +
> > +	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