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drm/mediatek: Support alpha blending in display driver
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: CK Hu <ck.hu@mediatek.com> Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com> Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com> Link: https://patchwork.kernel.org/project/dri-devel/patch/20240717-alpha-blending-v4-5-4b1c806c0749@mediatek.com/ Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
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@ -346,6 +346,17 @@ int mtk_plane_init(struct drm_device *dev, struct drm_plane *plane,
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DRM_INFO("Create rotation property failed\n");
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}
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err = drm_plane_create_alpha_property(plane);
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if (err)
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DRM_ERROR("failed to create property: alpha\n");
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err = drm_plane_create_blend_mode_property(plane,
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BIT(DRM_MODE_BLEND_PREMULTI) |
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BIT(DRM_MODE_BLEND_COVERAGE) |
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BIT(DRM_MODE_BLEND_PIXEL_NONE));
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if (err)
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DRM_ERROR("failed to create property: blend_mode\n");
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drm_plane_helper_add(plane, &mtk_plane_helper_funcs);
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return 0;
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