Re: [PATCH v6 07/14] drm/mediatek: Support alpha blending in display driver

[CK Hu (胡俊光)]

Hi, Shawn:

On Fri, 2024-03-22 at 13:28 +0800, Shawn Sung wrote:
> From: Hsiao Chien Sung <shawn.sung@xxxxxxxxxxxx>
> 
> Support "Pre-multiplied" and "None" blend mode on MediaTek's chips by
> adding correct blend mode property when the planes init.
> Before this patch, only the "Coverage" mode (default) is supported.
> 
> For more information, there are three pixel blend modes in DRM
> driver:
> "None", "Pre-multiplied", and "Coverage".
> 
> To understand the difference between these modes, let's take a look
> at
> the following two approaches to do alpha blending:
> 
> 1. Straight:
> dst.RGB = src.RGB * src.A + dst.RGB * (1 - src.A)
> This is straightforward and easy to understand, when the source layer
> is
> compositing with the destination layer, it's alpha will affect the
> result. This is also known as "post-multiplied", or "Coverage" mode.
> 
> 2. Pre-multiplied:
> dst.RGB = src.RGB + dst.RGB * (1 - src.A)
> Since the source RGB have already multiplied its alpha, only
> destination
> RGB need to multiply it. This is the "Pre-multiplied" mode in DRM.
> 
> For the "None" blend mode in DRM, it means the pixel alpha is ignored
> when compositing the layers, only the constant alpha for the
> composited
> layer will take effects.
> 
> Reviewed-by: AngeloGioacchino Del Regno <
> angelogioacchino.delregno@xxxxxxxxxxxxx>
> Reviewed-by: CK Hu <ck.hu@xxxxxxxxxxxx>
> Signed-off-by: Hsiao Chien Sung <shawn.sung@xxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/mediatek/mtk_plane.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_plane.c
> b/drivers/gpu/drm/mediatek/mtk_plane.c
> index 1723d4333f371..5bf757a3ef202 100644
> --- a/drivers/gpu/drm/mediatek/mtk_plane.c
> +++ b/drivers/gpu/drm/mediatek/mtk_plane.c
> @@ -346,6 +346,17 @@ int mtk_plane_init(struct drm_device *dev,
> struct drm_plane *plane,
>  			DRM_INFO("Create rotation property failed\n");
>  	}
>  
> +	err = drm_plane_create_alpha_property(plane);
> +	if (err)
> +		DRM_ERROR("failed to create property: alpha\n");
> +
> +	err = drm_plane_create_blend_mode_property(plane,
> +						   BIT(DRM_MODE_BLEND_P
> REMULTI) |
> +						   BIT(DRM_MODE_BLEND_C
> OVERAGE) |
> +						   BIT(DRM_MODE_BLEND_P
> IXEL_NONE));

After all sub driver support these function, then create blend
property. So move this patch after all sub driver support these blend
mode.

Regards,
CK

> +	if (err)
> +		DRM_ERROR("failed to create property: blend_mode\n");
> +
>  	drm_plane_helper_add(plane, &mtk_plane_helper_funcs);
>  
>  	return 0;