linux-kernel - [PATCH 3/3] drm: zynqmp: Add blend mode property to graphics plane

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Message-Id: <20251113203715.2768107-4-sean.anderson@linux.dev>
Date: Thu, 13 Nov 2025 15:37:14 -0500
From: Sean Anderson <sean.anderson@...ux.dev>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
	dri-devel@...ts.freedesktop.org
Cc: linux-kernel@...r.kernel.org,
	Mike Looijmans <mike.looijmans@...ic.nl>,
	David Airlie <airlied@...il.com>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Anatoliy Klymenko <anatoliy.klymenko@....com>,
	Maxime Ripard <mripard@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Simona Vetter <simona@...ll.ch>,
	Michal Simek <michal.simek@....com>,
	Sean Anderson <sean.anderson@...ux.dev>
Subject: [PATCH 3/3] drm: zynqmp: Add blend mode property to graphics plane

When global alpha is enabled, per-pixel alpha is ignored. Allow
userspace to explicitly specify whether to use per-pixel alpha by
exposing it through the blend mode property. I'm not sure whether the
per-pixel alpha is pre-multiplied or not [1], but apparently it *must* be
pre-multiplied so I guess we have to advertise it.

[1] All we get is "The alpha value available with the graphics stream
    will define the transparency of the graphics."

Signed-off-by: Sean Anderson <sean.anderson@...ux.dev>
---

 drivers/gpu/drm/xlnx/zynqmp_kms.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/xlnx/zynqmp_kms.c b/drivers/gpu/drm/xlnx/zynqmp_kms.c
index 456ada9ac003..fa1cfc16db36 100644
--- a/drivers/gpu/drm/xlnx/zynqmp_kms.c
+++ b/drivers/gpu/drm/xlnx/zynqmp_kms.c
@@ -61,6 +61,13 @@ static int zynqmp_dpsub_plane_atomic_check(struct drm_plane *plane,
 	if (!new_plane_state->crtc)
 		return 0;
 
+	if (new_plane_state->pixel_blend_mode != DRM_MODE_BLEND_PIXEL_NONE &&
+	    new_plane_state->alpha >> 8 != 0xff) {
+		drm_dbg_kms(plane->dev,
+			    "Plane alpha must be 1.0 when using pixel alpha\n");
+		return -EINVAL;
+	}
+
 	crtc_state = drm_atomic_get_crtc_state(state, new_plane_state->crtc);
 	if (IS_ERR(crtc_state))
 		return PTR_ERR(crtc_state);
@@ -117,9 +124,13 @@ static void zynqmp_dpsub_plane_atomic_update(struct drm_plane *plane,
 
 	zynqmp_disp_layer_update(layer, new_state);
 
-	if (plane->index == ZYNQMP_DPSUB_LAYER_GFX)
-		zynqmp_disp_blend_set_global_alpha(dpsub->disp, true,
+	if (plane->index == ZYNQMP_DPSUB_LAYER_GFX) {
+		bool blend = plane->state->pixel_blend_mode ==
+			     DRM_MODE_BLEND_PIXEL_NONE;
+
+		zynqmp_disp_blend_set_global_alpha(dpsub->disp, blend,
 						   plane->state->alpha >> 8);
+	}
 
 	/*
 	 * Unconditionally enable the layer, as it may have been disabled
@@ -179,9 +190,18 @@ static int zynqmp_dpsub_create_planes(struct zynqmp_dpsub *dpsub)
 			return ret;
 
 		if (i == ZYNQMP_DPSUB_LAYER_GFX) {
+			unsigned int blend_modes =
+				BIT(DRM_MODE_BLEND_PIXEL_NONE) |
+				BIT(DRM_MODE_BLEND_PREMULTI);
+
 			ret = drm_plane_create_alpha_property(plane);
 			if (ret)
 				return ret;
+
+			ret = drm_plane_create_blend_mode_property(plane,
+								   blend_modes);
+			if (ret)
+				return ret;
 		}
 	}
 
-- 
2.35.1.1320.gc452695387.dirty