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

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

On Tue, Jun 25, 2024 at 03:46:06PM +0000, Sakari Ailus wrote:
> Hi Laurent,
> 
> On Tue, Jun 25, 2024 at 05:02:47PM +0300, Laurent Pinchart wrote:
> > 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*().
> 
> Fair enough, there seems to have been more discussion.
> 
> As this is still user-triggerable, this should be WARN_ON_ONCE() to prevent
> the user from filling the logs, intentionally or not.

It's user-triggerable, but only when the driver is clearly buggy :-)
WARN_ON_ONCE() is OK with me though, I don't think warning every time
would provide much added value.

> > > 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