[PATCH v2 3/4] drm/virtio: create blend mode property on cursor plane

[Qinyun Tan <qinyuntan@linux.alibaba.com>]

Since commit 860e748bddcc ("drm: ensure blend mode supported if pixel
format with alpha exposed"), drm_mode_config_validate() warns when a
plane exposes an alpha pixel format but not the "pixel blend mode"
property. The virtio-gpu cursor plane (HOST_ARGB8888) trips this.

The virtio-gpu specification does not define the cursor alpha
semantics. The host forwards the cursor pixels verbatim to its display
frontends, and the remote cursor protocols among them (SPICE alpha
cursors, the VNC "Cursor With Alpha" encoding) both define
pre-multiplied alpha, matching what userspace has always assumed when
the property is not attached. Expose a "pixel blend mode" property
advertising only DRM_MODE_BLEND_PREMULTI to make these semantics
explicit and silence the warning. The primary plane only exposes
HOST_XRGB8888, so the call is gated to the cursor. No functional
change.

Fixes: 860e748bddcc ("drm: ensure blend mode supported if pixel format with alpha exposed")
Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
---
v2: no code change; explain the protocol-level justification for
PREMULTI in the commit message.

 drivers/gpu/drm/virtio/virtgpu_plane.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/virtio/virtgpu_plane.c b/drivers/gpu/drm/virtio/virtgpu_plane.c
index 1d1b27ece62a7..640815af40980 100644
--- a/drivers/gpu/drm/virtio/virtgpu_plane.c
+++ b/drivers/gpu/drm/virtio/virtgpu_plane.c
@@ -24,6 +24,7 @@
  */
 
 #include <drm/drm_atomic_helper.h>
+#include <drm/drm_blend.h>
 #include <drm/drm_damage_helper.h>
 #include <drm/drm_fourcc.h>
 #include <drm/drm_gem_atomic_helper.h>
@@ -609,6 +610,9 @@ struct drm_plane *virtio_gpu_plane_init(struct virtio_gpu_device *vgdev,
 
 	if (type == DRM_PLANE_TYPE_PRIMARY)
 		drm_plane_enable_fb_damage_clips(plane);
+	else if (type == DRM_PLANE_TYPE_CURSOR)
+		drm_plane_create_blend_mode_property(plane,
+						     BIT(DRM_MODE_BLEND_PREMULTI));
 
 	return plane;
 }
-- 
2.43.7