Re: [PATCH v2 2/2] drm: ensure blend mode supported if alpha exposed

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

On Mon, 04 May 2026, Leandro Ribeiro <leandro.ribeiro@collabora.com> wrote:
> Before "drm/drm_blend: allow blend mode property without PREMULTI",
> userspace would have to assume that only PREMULTI was supported by
> drivers that didn't expose the blend mode property. But now userspace
> shouldn't relly on that, as they can't count with drivers always
> supporting PREMULTI.
>
> Error out if a driver expose alpha property or pixel formats with alpha
> and does not expose the blend mode property. This way userspace don't
> have to guess. Drivers that hit such error must be fixed.
>
> Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
> ---
>  drivers/gpu/drm/drm_crtc_internal.h |  1 +
>  drivers/gpu/drm/drm_drv.c           |  7 ++++-
>  drivers/gpu/drm/drm_mode_config.c   | 42 +++++++++++++++++++++++++++++
>  3 files changed, 49 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_crtc_internal.h b/drivers/gpu/drm/drm_crtc_internal.h
> index c09409229644..bdbb6b9b94ea 100644
> --- a/drivers/gpu/drm/drm_crtc_internal.h
> +++ b/drivers/gpu/drm/drm_crtc_internal.h
> @@ -96,6 +96,7 @@ int drm_mode_setcrtc(struct drm_device *dev,
>  int drm_modeset_register_all(struct drm_device *dev);
>  void drm_modeset_unregister_all(struct drm_device *dev);
>  void drm_mode_config_validate(struct drm_device *dev);
> +int drm_mode_config_enforce(struct drm_device *dev);
>  
>  /* drm_modes.c */
>  const char *drm_get_mode_status_name(enum drm_mode_status status);
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 985c283cf59f..76766a370650 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -1059,9 +1059,14 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
>  	const struct drm_driver *driver = dev->driver;
>  	int ret;
>  
> -	if (!driver->load)
> +	if (!driver->load) {
>  		drm_mode_config_validate(dev);
>  
> +		ret = drm_mode_config_enforce(dev);

Reading the code, I'll have no idea what "enforce" means here. Reading
the function itself isn't helping either.

BR,
Jani.


> +		if (ret)
> +			return ret;
> +	}
> +
>  	WARN_ON(!dev->managed.final_kfree);
>  
>  	if (drm_dev_needs_global_mutex(dev))
> diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> index 66f7dc37b597..20fd26ecb957 100644
> --- a/drivers/gpu/drm/drm_mode_config.c
> +++ b/drivers/gpu/drm/drm_mode_config.c
> @@ -738,3 +738,45 @@ void drm_mode_config_validate(struct drm_device *dev)
>  	     "Must have as many primary planes as there are CRTCs, but have %u primary planes and %u CRTCs",
>  	     num_primary, dev->mode_config.num_crtc);
>  }
> +
> +static int plane_require_blend_mode_for_alpha(struct drm_plane *plane)
> +{
> +	struct drm_device *dev = plane->dev;
> +	const struct drm_format_info *fmt;
> +	u32 i;
> +
> +	/* blend mode property supported, no need to check anything */
> +	if (plane->blend_mode_property)
> +		return 0;
> +
> +	if (plane->alpha_property) {
> +		drm_err(dev, "[PLANE:%d:%s] alpha property exposed but blend mode not setup",
> +			plane->base.id, plane->name);
> +		return -EINVAL;
> +	}
> +
> +	for (i = 0; i < plane->format_count; i++) {
> +		fmt = drm_format_info(plane->format_types[i]);
> +		if (fmt->has_alpha) {
> +			drm_err(dev, "[PLANE:%d:%s] pixel format with alpha exposed but blend mode not setup",
> +			        plane->base.id, plane->name);
> +			return -EINVAL;
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +int drm_mode_config_enforce(struct drm_device *dev)
> +{
> +	struct drm_plane *plane;
> +	int ret;
> +
> +	drm_for_each_plane(plane, dev) {
> +		ret = plane_require_blend_mode_for_alpha(plane);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	return 0;
> +}

-- 
Jani Nikula, Intel