Re: [PATCH] drm/vkms: Add pixel blend mode property

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

On 8/28/26 6:02 AM, oushixiong1025@163.com wrote:
> From: Shixiong Ou <oushixiong@kylinos.cn>
> 
> The vkms planes expose formats with an alpha channel but do not
> create the pixel blend mode property. Since
> commit 860e748bddcc ("drm: ensure blend mode supported if pixel
> format with alpha exposed") this triggers a warning during
> drm_mode_config_validate():
> 
> [  993.538979] ------------[ cut here ]------------
> [  993.539000] [PLANE:35:plane-0] pixel format with alpha exposed but blend mode not setup
> [  993.539063] WARNING: drivers/gpu/drm/drm_mode_config.c:872 at drm_mode_config_validate
> ......
> [  993.539578] Call trace:
> [  993.539580]  drm_mode_config_validate+0x398/0x558 [drm] (P)
> [  993.539707]  drm_dev_register+0x1cc/0x2a0 [drm]
> [  993.539832]  vkms_create+0x184/0x1d0 [vkms]
> [  993.539854]  vkms_init+0x78/0xff8 [vkms]
> ......
> 
> The vkms composer only blends premultiplied alpha, see
> pre_mul_alpha_blend(), so create the property with
> DRM_MODE_BLEND_PREMULTI as the only supported mode.
> 
> Reported-by: Ye Liu <liuye@kylinos.cn>
> Signed-off-by: Shixiong Ou <oushixiong@kylinos.cn>
> ---
>  drivers/gpu/drm/vkms/vkms_plane.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_plane.c b/drivers/gpu/drm/vkms/vkms_plane.c
> index 6ee5c3f3207c..c4272f5f0354 100644
> --- a/drivers/gpu/drm/vkms/vkms_plane.c
> +++ b/drivers/gpu/drm/vkms/vkms_plane.c
> @@ -276,6 +276,7 @@ struct vkms_plane *vkms_plane_init(struct vkms_device *vkmsdev,
>  {
>  	struct drm_device *dev = &vkmsdev->drm;
>  	struct vkms_plane *plane;
> +	int ret;
>  
>  	plane = drmm_universal_plane_alloc(dev, struct vkms_plane, base, 0,
>  					   &vkms_plane_funcs,
> @@ -287,6 +288,15 @@ struct vkms_plane *vkms_plane_init(struct vkms_device *vkmsdev,
>  
>  	drm_plane_helper_add(&plane->base, &vkms_plane_helper_funcs);
>  
> +	/*
> +	 * The vkms composer only blends premultiplied alpha, see
> +	 * pre_mul_alpha_blend(), so that is the only supported mode.
> +	 */
> +	ret = drm_plane_create_blend_mode_property(&plane->base,
> +						   BIT(DRM_MODE_BLEND_PREMULTI));
> +	if (ret)
> +		return ERR_PTR(ret);
> +

Hello,

This looks good to me! But I think the comment in pre_mul_alpha_blend()
could also be updated. It states:

"The current DRM assumption is that pixel color values have been already
pre-multiplied with the alpha channel values"

Which is not true anymore.

With this change:

Reviewed-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

>  	drm_plane_create_rotation_property(&plane->base, DRM_MODE_ROTATE_0,
>  					   DRM_MODE_ROTATE_MASK | DRM_MODE_REFLECT_MASK);
>  

-- 
Leandro Ribeiro