Re: [PATCH 1/1] drm: use drm_warn() in validate_blend_mode_for_alpha_formats()

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 29 Jul 2026, Leandro Ribeiro <leandro.ribeiro@collabora.com> wrote:
> Commit 860e748bddcc ("drm: ensure blend mode supported if pixel format
> with alpha exposed") introduced a WARN() to let driver developers know
> that a previously valid behavior should now be changed.

I should've chimed in way back when, but in retrospect it's a bit
presumptuous to add a commit that puts the burden of the change on the
driver developers, without so much as identifying which drivers or
getting an ack from them.

This is really not unlike the __deprecated attribute, see commit
771c035372a0 ("deprecate the '__deprecated' attribute warnings entirely
and for good"), but this one's runtime not build.

> But WARN() should not be used for that, as it's a kernel warning report
> mechanism for conditions that are not expected to happen. It also
> produces a stack trace. Instead, a simple warning-level log message
> should have been used, as drivers were expected to trigger the
> condition.
>
> This is causing problems for fuzzers, as they may stop when encountering
> a "BUG:" or "WARNING:" in the logs.
>
> Replace WARN() with drm_warn() in this function, avoiding these issues.

Fixes: 860e748bddcc ("drm: ensure blend mode supported if pixel format with alpha exposed")

> Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
> ---
>  drivers/gpu/drm/drm_mode_config.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> index 3bcc7bf0900c..3d9c554e08a8 100644
> --- a/drivers/gpu/drm/drm_mode_config.c
> +++ b/drivers/gpu/drm/drm_mode_config.c
> @@ -869,8 +869,9 @@ static void validate_blend_mode_for_alpha_formats(struct drm_plane *plane)
>  	for (i = 0; i < plane->format_count; i++) {
>  		fmt = drm_format_info(plane->format_types[i]);
>  		if (fmt->has_alpha) {
> -			WARN(1, "[PLANE:%d:%s] pixel format with alpha exposed but blend mode not setup",
> -			     plane->base.id, plane->name);
> +			drm_warn(plane->dev,
> +				 "[PLANE:%d:%s] pixel format with alpha exposed but blend mode not setup",
> +				 plane->base.id, plane->name);
>  			break;
>  		}
>  	}

-- 
Jani Nikula, Intel