Re: [PATCH V2] drm/loongson: Create blend mode property for cursor plane

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The cursor plane's blend mode property advertises support for DRM_MODE_BLEND_PIXEL_NONE, but the driver never reads or implements this mode, causing it to incorrectly behave as DRM_MODE_BLEND_COVERAGE.
--

commit 0fb989e37bfbacfc53639e07453dacd08e37bac4
Author: Huacai Chen <chenhuacai@kernel.org>

drm/loongson: Create blend mode property for cursor plane

This commit fixes boot warnings by exposing a blend mode property for cursor
planes that use pixel formats with alpha. It configures the planes to expose
support for both DRM_MODE_BLEND_PIXEL_NONE and DRM_MODE_BLEND_COVERAGE.

> diff --git a/drivers/gpu/drm/loongson/lsdc_plane.c b/drivers/gpu/drm/loongson/lsdc_plane.c
> index bea42215796d4..969b2aaa4fd09 100644
> --- a/drivers/gpu/drm/loongson/lsdc_plane.c
> +++ b/drivers/gpu/drm/loongson/lsdc_plane.c
[ ... ]
> @@ -765,15 +767,16 @@ int ls7a1000_cursor_plane_init(struct drm_device *ddev,
>  
>  	drm_plane_helper_add(plane, &ls7a1000_cursor_plane_helper_funcs);
>  
> -	return 0;
> +	return drm_plane_create_blend_mode_property(plane, blend_caps);
>  }

[Severity: Medium]
Does this driver actually support DRM_MODE_BLEND_PIXEL_NONE?

The blend caps advertise DRM_MODE_BLEND_PIXEL_NONE, allowing userspace to
request disabling alpha blending on the cursor plane. However, looking at
the atomic update path for this hardware:

ls7a1000_cursor_plane_atomic_update() {
        ...
        ops->update_cfg(cursor, CURSOR_SIZE_32X32, CURSOR_FORMAT_ARGB8888);
        ...
}

And similarly for the ls7a2000 hardware:

ls7a2000_cursor_plane_atomic_update() {
        ...
        ops->update_cfg(cursor, cursor_size, CURSOR_FORMAT_ARGB8888);
        ...
}

It appears the hardware is unconditionally programmed to use alpha blending
(CURSOR_FORMAT_ARGB8888) regardless of the plane_state->pixel_blend_mode
value.

If userspace sets the pixel blend mode to None, won't it silently continue
using alpha coverage instead of rendering an opaque cursor?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260902132152.3170934-1-chenhuacai@loongson.cn?part=1