drm: Add per-plane pixel blend mode property

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

Adds 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.

Changes since v1:
 - Moves the blending equation into the DOC comment
 - Refines the comments of drm_plane_create_blend_mode_property to not
   enumerate the #defines, but instead the string values
 - Uses fg.* instead of pixel.* and plane_alpha instead of plane.alpha
Changes since v2:
 - Refines the comments of drm_plane_create_blend_mode_property:
      1) Puts the descriptions (after the ":") on a new line
      2) Adds explaining why @supported_modes need PREMUL as default
Changes since v3:
 - Refines drm_plane_create_blend_mode_property(). drm_property_add_enum()
   can calculate the index itself just fine, so no point in having the
   caller pass it in.
 - Since the current DRM assumption is that alpha is premultiplied
   as default, define DRM_MODE_BLEND_PREMULTI as 0 will be better.
 - Refines some comments.
Changes since v4:
 - Adds comments in drm_blend.h.
 - Removes setting default value in drm_plane_create_blend_mode_property()
   as it is already in __drm_atomic_helper_plane_reset().
 - Fixes to use state->pixel_blend_mode instead of using
   plane->state->pixel_blend_mode in reset function.
 - Rebases on drm-misc-next.
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Lowry Li <lowry.li@arm.com>
Signed-off-by: Ayan Kumar Halder <ayan.halder@arm.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/245734/

drivers/gpu/drm/drm_atomic.c

@@ -895,6 +895,8 @@ 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)) {
 			DRM_DEBUG_ATOMIC("[PLANE:%d:%s] bad rotation bitmask: 0x%llx\n",
@@ -968,6 +970,8 @@ drm_atomic_plane_get_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) {

drivers/gpu/drm/drm_atomic_helper.c

@@ -3569,6 +3569,7 @@ void __drm_atomic_helper_plane_reset(struct drm_plane *plane,
 	/* Reset the alpha value to fully opaque if it matters */
 	if (plane->alpha_property)
 		state->alpha = plane->alpha_property->values[1];
+	state->pixel_blend_mode = DRM_MODE_BLEND_PREMULTI;
 
 	plane->state = state;
 }

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 +494,80 @@ 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 anything else.
+ *
+ * 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;
+
+	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, 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;
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_plane_create_blend_mode_property);

include/drm/drm_blend.h

@@ -27,6 +27,10 @@
 #include <linux/ctype.h>
 #include <drm/drm_mode.h>
 
+#define DRM_MODE_BLEND_PREMULTI		0
+#define DRM_MODE_BLEND_COVERAGE		1
+#define DRM_MODE_BLEND_PIXEL_NONE	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

include/drm/drm_plane.h

@@ -117,6 +117,7 @@ struct drm_plane_state {
 	 * details.
 	 */
 	u16 alpha;
+	uint16_t pixel_blend_mode;
 
 	/**
 	 * @rotation:
@@ -659,6 +660,14 @@ struct drm_plane {
 	 * drm_plane_create_rotation_property().
 	 */
 	struct drm_property *rotation_property;
+	/**
+	 * @blend_mode_property:
+	 * Optional "pixel blend mode" enum property for this plane.
+	 * Blend mode property represents the alpha blending equation selection,
+	 * describing how the pixels from the current plane are composited with
+	 * the background.
+	 */
+	struct drm_property *blend_mode_property;
 
 	/**
 	 * @color_encoding_property: