linux-kernel - [PATCH v7 15/18] drm/mediatek: Support alpha blending in display driver

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Message-ID: <20240502103848.5845-16-shawn.sung@mediatek.com>
Date: Thu, 2 May 2024 18:38:45 +0800
From: Shawn Sung <shawn.sung@...iatek.com>
To: Chun-Kuang Hu <chunkuang.hu@...nel.org>, AngeloGioacchino Del Regno
	<angelogioacchino.delregno@...labora.com>
CC: Philipp Zabel <p.zabel@...gutronix.de>, David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>, Matthias Brugger <matthias.bgg@...il.com>,
	Bibby Hsieh <bibby.hsieh@...iatek.com>, CK Hu <ck.hu@...iatek.com>, "Nancy .
 Lin" <nancy.lin@...iatek.com>, Fei Shao <fshao@...omium.org>, Sean Paul
	<sean@...rly.run>, Jason Chen
	<jason-ch.chen@...iatek.corp-partner.google.com>,
	<dri-devel@...ts.freedesktop.org>, <linux-mediatek@...ts.infradead.org>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>, Hsiao
 Chien Sung <shawn.sung@...iatek.com>
Subject: [PATCH v7 15/18] drm/mediatek: Support alpha blending in display driver

From: Hsiao Chien Sung <shawn.sung@...iatek.com>

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@...labora.com>
Reviewed-by: CK Hu <ck.hu@...iatek.com>
Signed-off-by: Hsiao Chien Sung <shawn.sung@...iatek.com>
---
 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_PREMULTI) |
+						   BIT(DRM_MODE_BLEND_COVERAGE) |
+						   BIT(DRM_MODE_BLEND_PIXEL_NONE));
+	if (err)
+		DRM_ERROR("failed to create property: blend_mode\n");
+
 	drm_plane_helper_add(plane, &mtk_plane_helper_funcs);

 	return 0;
-- 
2.18.0