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

Sean Anderson
2025-11-13T20:37:44Z

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@linux.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