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

[Leandro Ribeiro <leandro.ribeiro@collabora.com>]

On 5/5/26 4:57 AM, Jani Nikula wrote:
> 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.
> 

Yes, the function name choice wasn't good.

drm_mode_config_validate() only produces warnings, while
drm_mode_config_enforce() returns error codes and should be used to
prevent initializing a driver that does not comply with the required
rules. In this case, the rule being added is supporting plane alpha but
not the blend mode property.

I could add such kind of rule to drm_mode_config_validate() itself, but
I wasn't sure if people would be happy with it producing both warnings
and errors. But maybe that would be a better approach than what I did.

> 
>> +		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;
>> +}
> 

-- 
Leandro Ribeiro