[PATCH v2] drm/vkms: Add pixel blend mode property

[oushixiong1025]

From: Shixiong Ou <oushixiong@xxxxxxxxxx>

The vkms planes expose formats with an alpha channel but do not
create the pixel blend mode property. Since
commit 860e748bddcc ("drm: ensure blend mode supported if pixel
format with alpha exposed") this triggers a warning during
drm_mode_config_validate():

[  993.538979] ------------[ cut here ]------------
[  993.539000] [PLANE:35:plane-0] pixel format with alpha exposed but blend mode not setup
[  993.539063] WARNING: drivers/gpu/drm/drm_mode_config.c:872 at drm_mode_config_validate
......
[  993.539578] Call trace:
[  993.539580]  drm_mode_config_validate+0x398/0x558 [drm] (P)
[  993.539707]  drm_dev_register+0x1cc/0x2a0 [drm]
[  993.539832]  vkms_create+0x184/0x1d0 [vkms]
[  993.539854]  vkms_init+0x78/0xff8 [vkms]
......

The vkms composer only blends premultiplied alpha, see
pre_mul_alpha_blend(), so create the property with
DRM_MODE_BLEND_PREMULTI as the only supported mode.


Reported-by: Ye Liu <liuye@xxxxxxxxxx>
Signed-off-by: Shixiong Ou <oushixiong@xxxxxxxxxx>
Reviewed-by: Leandro Ribeiro <leandro.ribeiro@xxxxxxxxxxxxx>
---
v1->v2:
- Update the stale comment above pre_mul_alpha_blend(): the DRM-wide
  premultiplied-alpha assumption no longer holds, vkms only implements
  the pre-multiplied blend mode.(Leandro Ribeiro)

 drivers/gpu/drm/vkms/vkms_composer.c |  8 ++++----
 drivers/gpu/drm/vkms/vkms_plane.c    | 10 ++++++++++
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index a238d40a49f9..c9e1cb10849e 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -36,10 +36,10 @@ static u16 pre_mul_blend_channel(u16 src, u16 dst, u16 alpha)
  * The pixels [@x_start;@x_start+@pixel_count) in stage_buffer are blended at
  * [@x_start;@x_start+@pixel_count) in output_buffer.
  *
- * The current DRM assumption is that pixel color values have been already
- * pre-multiplied with the alpha channel values. See more
- * drm_plane_create_blend_mode_property(). Also, this formula assumes a
- * completely opaque background.
+ * vkms only implements the pre-multiplied blend mode (see
+ * drm_plane_create_blend_mode_property()), so the pixel color values are
+ * expected to be already pre-multiplied with the alpha channel values.
+ * Also, this formula assumes a completely opaque background.
  */
 static void pre_mul_alpha_blend(const struct line_buffer *stage_buffer,
 				struct line_buffer *output_buffer, int x_start, int pixel_count)
diff --git a/drivers/gpu/drm/vkms/vkms_plane.c b/drivers/gpu/drm/vkms/vkms_plane.c
index 6ee5c3f3207c..c4272f5f0354 100644
--- a/drivers/gpu/drm/vkms/vkms_plane.c
+++ b/drivers/gpu/drm/vkms/vkms_plane.c
@@ -276,6 +276,7 @@ struct vkms_plane *vkms_plane_init(struct vkms_device *vkmsdev,
 {
 	struct drm_device *dev = &vkmsdev->drm;
 	struct vkms_plane *plane;
+	int ret;
 
 	plane = drmm_universal_plane_alloc(dev, struct vkms_plane, base, 0,
 					   &vkms_plane_funcs,
@@ -287,6 +288,15 @@ struct vkms_plane *vkms_plane_init(struct vkms_device *vkmsdev,
 
 	drm_plane_helper_add(&plane->base, &vkms_plane_helper_funcs);
 
+	/*
+	 * The vkms composer only blends premultiplied alpha, see
+	 * pre_mul_alpha_blend(), so that is the only supported mode.
+	 */
+	ret = drm_plane_create_blend_mode_property(&plane->base,
+						   BIT(DRM_MODE_BLEND_PREMULTI));
+	if (ret)
+		return ERR_PTR(ret);
+
 	drm_plane_create_rotation_property(&plane->base, DRM_MODE_ROTATE_0,
 					   DRM_MODE_ROTATE_MASK | DRM_MODE_REFLECT_MASK);
 
-- 
2.25.1


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