[PATCH] drm/drm_blend: allow blend mode property without PREMULTI

[Leandro Ribeiro <leandro.ribeiro@collabora.com>]

Some hardware only supports the COVERAGE blend mode and lacks PREMULTI
support entirely. DRM currently requires that PREMULTI is present when
creating a blend mode property, which prevents such drivers from being
properly upstreamed.

Remove this restriction and allow drivers to create a blend mode
property without PREMULTI, enabling support for hardware that
implements only COVERAGE blend mode.

This does not introduce a regression, as no existing upstream drivers
expose only COVERAGE. However, userspace that wants to support such kind
of hardware in the future will have to check the supported blend modes
instead of assuming PREMULTI is always supported.

If the driver does not support the blend mode property, userspace must
assume that only PREMULTI is supported (as was the case before this
patch).

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
---
 drivers/gpu/drm/drm_blend.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
index 1f3af27d2418..e5015d0fb219 100644
--- a/drivers/gpu/drm/drm_blend.c
+++ b/drivers/gpu/drm/drm_blend.c
@@ -563,10 +563,10 @@ EXPORT_SYMBOL(drm_atomic_normalize_zpos);
 /**
  * drm_plane_create_blend_mode_property - create a new blend mode property
  * @plane: drm plane
- * @supported_modes: bitmask of supported modes, must include
- *		     BIT(DRM_MODE_BLEND_PREMULTI). Current DRM assumption is
- *		     that alpha is premultiplied, and old userspace can break if
- *		     the property defaults to anything else.
+ * @supported_modes: bitmask of supported modes. When
+ *		     BIT(DRM_MODE_BLEND_PREMULTI) is included, it will be used
+ *		     as the default. Otherwise, the default will fallback to one
+ *		     of the supported modes.
  *
  * This creates a new property describing the blend mode.
  *
@@ -586,6 +586,9 @@ EXPORT_SYMBOL(drm_atomic_normalize_zpos);
  *	pre-multiplied and will do so when blending them to the background color
  *	values.
  *
+ * Note: if the blend mode property is not present, userspace must assume that
+ * the driver supports only "Pre-multiplied".
+ *
  * RETURNS:
  * Zero for success or -errno
  */
@@ -599,13 +602,13 @@ int drm_plane_create_blend_mode_property(struct drm_plane *plane,
 		{ DRM_MODE_BLEND_PREMULTI, "Pre-multiplied" },
 		{ DRM_MODE_BLEND_COVERAGE, "Coverage" },
 	};
+	unsigned int default_mode;
 	unsigned int valid_mode_mask = BIT(DRM_MODE_BLEND_PIXEL_NONE) |
 				       BIT(DRM_MODE_BLEND_PREMULTI)   |
 				       BIT(DRM_MODE_BLEND_COVERAGE);
 	int i;
 
-	if (WARN_ON((supported_modes & ~valid_mode_mask) ||
-		    ((supported_modes & BIT(DRM_MODE_BLEND_PREMULTI)) == 0)))
+	if (WARN_ON((supported_modes & ~valid_mode_mask)))
 		return -EINVAL;
 
 	prop = drm_property_create(dev, DRM_MODE_PROP_ENUM,
@@ -630,7 +633,14 @@ int drm_plane_create_blend_mode_property(struct drm_plane *plane,
 		}
 	}
 
-	drm_object_attach_property(&plane->base, prop, DRM_MODE_BLEND_PREMULTI);
+	if (supported_modes & BIT(DRM_MODE_BLEND_PREMULTI))
+		default_mode = DRM_MODE_BLEND_PREMULTI;
+	else if (supported_modes & BIT(DRM_MODE_BLEND_COVERAGE))
+		default_mode = DRM_MODE_BLEND_COVERAGE;
+	else
+		default_mode = DRM_MODE_BLEND_PIXEL_NONE;
+
+	drm_object_attach_property(&plane->base, prop, default_mode);
 	plane->blend_mode_property = prop;
 
 	return 0;
-- 
2.54.0