[PATCH v3 1/2] drm: Add per-plane pixel blend mode property

[Lowry Li]

Pixel blend modes represent the alpha blending equation
selection, describing how the pixels from the current
plane are composited with the background.

Add a pixel_blend_mode to drm_plane_state and a
blend_mode_property to drm_plane, and related support
functions.

Defines three blend modes in drm_blend.h.

Signed-off-by: Lowry Li <lowry.li@xxxxxxx>
---
 drivers/gpu/drm/drm_atomic.c        |   4 ++
 drivers/gpu/drm/drm_atomic_helper.c |   1 +
 drivers/gpu/drm/drm_blend.c         | 126 ++++++++++++++++++++++++++++++++++++
 include/drm/drm_blend.h             |   6 ++
 include/drm/drm_plane.h             |   5 ++
 5 files changed, 142 insertions(+)

diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 07fef42..1d18389 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -785,6 +785,8 @@ static int drm_atomic_plane_set_property(struct drm_plane *plane,
 		state->src_h = val;
 	} else if (property == plane->alpha_property) {
 		state->alpha = val;
+	} else if (property == plane->blend_mode_property) {
+		state->pixel_blend_mode = val;
 	} else if (property == plane->rotation_property) {
 		if (!is_power_of_2(val & DRM_MODE_ROTATE_MASK))
 			return -EINVAL;
@@ -852,6 +854,8 @@ static int drm_atomic_plane_set_property(struct drm_plane *plane,
 		*val = state->src_h;
 	} else if (property == plane->alpha_property) {
 		*val = state->alpha;
+	} else if (property == plane->blend_mode_property) {
+		*val = state->pixel_blend_mode;
 	} else if (property == plane->rotation_property) {
 		*val = state->rotation;
 	} else if (property == plane->zpos_property) {
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 130da51..7f5d463 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -3515,6 +3515,7 @@ void drm_atomic_helper_plane_reset(struct drm_plane *plane)
 		/* Reset the alpha value to fully opaque if it matters */
 		if (plane->alpha_property)
 			plane->state->alpha = plane->alpha_property->values[1];
+		plane->state->pixel_blend_mode = DRM_MODE_BLEND_PREMULTI;
 	}
 }
 EXPORT_SYMBOL(drm_atomic_helper_plane_reset);
diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
index a16a74d..ac6f19c 100644
--- a/drivers/gpu/drm/drm_blend.c
+++ b/drivers/gpu/drm/drm_blend.c
@@ -107,6 +107,52 @@
  *	planes. Without this property the primary plane is always below the cursor
  *	plane, and ordering between all other planes is undefined.
  *
+ * pixel blend mode:
+ *	Pixel blend mode is set up with drm_plane_create_blend_mode_property().
+ *	It adds a blend mode for alpha blending equation selection, describing
+ *	how the pixels from the current plane are composited with the
+ *	background.
+ *
+ *	 Three alpha blending equations are defined:
+ *
+ *	 "None":
+ *		 Blend formula that ignores the pixel alpha::
+ *
+ *			 out.rgb = plane_alpha * fg.rgb +
+ *				 (1 - plane_alpha) * bg.rgb
+ *
+ *	 "Pre-multiplied":
+ *		 Blend formula that assumes the pixel color values
+ *		 have been already pre-multiplied with the alpha
+ *		 channel values::
+ *
+ *			 out.rgb = plane_alpha * fg.rgb +
+ *				 (1 - (plane_alpha * fg.alpha)) * bg.rgb
+ *
+ *	 "Coverage":
+ *		 Blend formula that assumes the pixel color values have not
+ *		 been pre-multiplied and will do so when blending them to the
+ *		 background color values::
+ *
+ *			 out.rgb = plane_alpha * fg.alpha * fg.rgb +
+ *				 (1 - (plane_alpha * fg.alpha)) * bg.rgb
+ *
+ *	 Using the following symbols:
+ *
+ *	 ``fg.rgb``:
+ *		 Each of the RGB component values from the plane's pixel
+ *	 ``fg.alpha``:
+ *		 Alpha component value from the plane's pixel. If the plane's
+ *		 pixel format has no alpha component, then this is assumed to be
+ *		 1.0. In these cases, this property has no effect, as all three
+ *		 equations become equivalent.
+ *	 ``bg.rgb``:
+ *		 Each of the RGB component values from the background
+ *	 ``plane_alpha``:
+ *		 Plane alpha value set by the plane "alpha" property. If the
+ *		 plane does not expose the "alpha" property, then this is
+ *		 assumed to be 1.0
+ *
  * Note that all the property extensions described here apply either to the
  * plane or the CRTC (e.g. for the background color, which currently is not
  * exposed and assumed to be black).
@@ -448,3 +494,83 @@ int drm_atomic_normalize_zpos(struct drm_device *dev,
 	return 0;
 }
 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 coverage.
+ *
+ * This creates a new property describing the blend mode.
+ *
+ * The property exposed to userspace is an enumeration property (see
+ * drm_property_create_enum()) called "pixel blend mode" and has the
+ * following enumeration values:
+ *
+ * "None":
+ *	Blend formula that ignores the pixel alpha.
+ *
+ * "Pre-multiplied":
+ *	Blend formula that assumes the pixel color values have been already
+ *	pre-multiplied with the alpha channel values.
+ *
+ * "Coverage":
+ *	Blend formula that assumes the pixel color values have not been
+ *	pre-multiplied and will do so when blending them to the background color
+ *	values.
+ *
+ * RETURNS:
+ * Zero for success or -errno
+ */
+int drm_plane_create_blend_mode_property(struct drm_plane *plane,
+					 unsigned int supported_modes)
+{
+	struct drm_device *dev = plane->dev;
+	struct drm_property *prop;
+	static const struct drm_prop_enum_list props[] = {
+		{ DRM_MODE_BLEND_PIXEL_NONE, "None" },
+		{ DRM_MODE_BLEND_PREMULTI, "Pre-multiplied" },
+		{ DRM_MODE_BLEND_COVERAGE, "Coverage" },
+	};
+	unsigned int valid_mode_mask = BIT(DRM_MODE_BLEND_PIXEL_NONE) |
+				       BIT(DRM_MODE_BLEND_PREMULTI)   |
+				       BIT(DRM_MODE_BLEND_COVERAGE);
+	int i, j = 0;
+
+	if (WARN_ON((supported_modes & ~valid_mode_mask) ||
+		    ((supported_modes & BIT(DRM_MODE_BLEND_PREMULTI)) == 0)))
+		return -EINVAL;
+
+	prop = drm_property_create(dev, DRM_MODE_PROP_ENUM,
+				   "pixel blend mode",
+				   hweight32(supported_modes));
+	if (!prop)
+		return -ENOMEM;
+
+	for (i = 0; i < ARRAY_SIZE(props); i++) {
+		int ret;
+
+		if (!(BIT(props[i].type) & supported_modes))
+			continue;
+
+		ret = drm_property_add_enum(prop, j++, props[i].type,
+					    props[i].name);
+
+		if (ret) {
+			drm_property_destroy(dev, prop);
+
+			return ret;
+		}
+	}
+
+	drm_object_attach_property(&plane->base, prop, DRM_MODE_BLEND_PREMULTI);
+	plane->blend_mode_property = prop;
+
+	if (plane->state)
+		plane->state->pixel_blend_mode = DRM_MODE_BLEND_PREMULTI;
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_plane_create_blend_mode_property);
diff --git a/include/drm/drm_blend.h b/include/drm/drm_blend.h
index 330c561..28c5076 100644
--- a/include/drm/drm_blend.h
+++ b/include/drm/drm_blend.h
@@ -27,6 +27,10 @@
 #include <linux/ctype.h>
 #include <drm/drm_mode.h>
 
+#define DRM_MODE_BLEND_PIXEL_NONE	0
+#define DRM_MODE_BLEND_PREMULTI		1
+#define DRM_MODE_BLEND_COVERAGE		2
+
 struct drm_device;
 struct drm_atomic_state;
 struct drm_plane;
@@ -52,4 +56,6 @@ int drm_plane_create_zpos_immutable_property(struct drm_plane *plane,
 					     unsigned int zpos);
 int drm_atomic_normalize_zpos(struct drm_device *dev,
 			      struct drm_atomic_state *state);
+int drm_plane_create_blend_mode_property(struct drm_plane *plane,
+					 unsigned int supported_modes);
 #endif
diff --git a/include/drm/drm_plane.h b/include/drm/drm_plane.h
index 14b1607..6e2c045 100644
--- a/include/drm/drm_plane.h
+++ b/include/drm/drm_plane.h
@@ -44,6 +44,8 @@
  * @src_w: width of visible portion of plane (in 16.16)
  * @src_h: height of visible portion of plane (in 16.16)
  * @alpha: opacity of the plane
+ * @pixel_blend_mode: how the plane's framebuffer alpha channel is used when
+ *	blending with the background colour.
  * @rotation: rotation of the plane
  * @zpos: priority of the given plane on crtc (optional)
  *	Note that multiple active planes on the same crtc can have an identical
@@ -116,6 +118,7 @@ struct drm_plane_state {
 
 	/* Plane opacity */
 	u16 alpha;
+	uint16_t pixel_blend_mode;
 
 	/* Plane rotation */
 	unsigned int rotation;
@@ -513,6 +516,7 @@ enum drm_plane_type {
  * @alpha_property: alpha property for this plane
  * @zpos_property: zpos property for this plane
  * @rotation_property: rotation property for this plane
+ * @blend_mode_property: blend mode property for this plane
  * @helper_private: mid-layer private data
  */
 struct drm_plane {
@@ -589,6 +593,7 @@ struct drm_plane {
 	struct drm_property *alpha_property;
 	struct drm_property *zpos_property;
 	struct drm_property *rotation_property;
+	struct drm_property *blend_mode_property;
 
 	/**
 	 * @color_encoding_property:
-- 
1.9.1