drm/blend: Add a generic alpha property

Some drivers duplicate the logic to create a property to store a per-plane
alpha.

This is especially useful if we ever want to support extra protocols for
Wayland like:
https://lists.freedesktop.org/archives/wayland-devel/2017-August/034741.html

Let's create a helper in order to move that to the core.

Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6e1ce0db78fcfc407e94913c64819e65109d034d.1523432341.git-series.maxime.ripard@bootlin.com

1 files changed, 39 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
index 5a81e1b4c076..a16a74d7e15e 100644
--- a/drivers/gpu/drm/drm_blend.c
+++ b/drivers/gpu/drm/drm_blend.c
@@ -88,6 +88,13 @@
  * On top of this basic transformation additional properties can be exposed by
  * the driver:
  *
+ * alpha:
+ * 	Alpha is setup with drm_plane_create_alpha_property(). It controls the
+ * 	plane-wide opacity, from transparent (0) to opaque (0xffff). It can be
+ * 	combined with pixel alpha.
+ *	The pixel values in the framebuffers are expected to not be
+ *	pre-multiplied by the global alpha associated to the plane.
+ *
  * rotation:
  *	Rotation is set up with drm_plane_create_rotation_property(). It adds a
  *	rotation and reflection step between the source and destination rectangles.
@@ -106,6 +113,38 @@
  */
 
 /**
+ * drm_plane_create_alpha_property - create a new alpha property
+ * @plane: drm plane
+ *
+ * This function creates a generic, mutable, alpha property and enables support
+ * for it in the DRM core. It is attached to @plane.
+ *
+ * The alpha property will be allowed to be within the bounds of 0
+ * (transparent) to 0xffff (opaque).
+ *
+ * Returns:
+ * 0 on success, negative error code on failure.
+ */
+int drm_plane_create_alpha_property(struct drm_plane *plane)
+{
+	struct drm_property *prop;
+
+	prop = drm_property_create_range(plane->dev, 0, "alpha",
+					 0, DRM_BLEND_ALPHA_OPAQUE);
+	if (!prop)
+		return -ENOMEM;
+
+	drm_object_attach_property(&plane->base, prop, DRM_BLEND_ALPHA_OPAQUE);
+	plane->alpha_property = prop;
+
+	if (plane->state)
+		plane->state->alpha = DRM_BLEND_ALPHA_OPAQUE;
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_plane_create_alpha_property);
+
+/**
  * drm_plane_create_rotation_property - create a new rotation property
  * @plane: drm plane
  * @rotation: initial value of the rotation property