[PATCH] drm: Document the alpha blending rules

[Ville Syrjala <ville.syrjala@linux.intel.com>]

From: Ville Syrjälä <ville.syrjala@linux.intel.com>

Document the fact that the use of a framebuffer with alpha implies
pre-multipled alpha blending, and that the crtc background color
is assumed to be black.

Not sure drm_fourcc.h is the best place for the uapi docs, but
couldn't think of anything better really.

Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Stefan Schake <stschake@gmail.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Adrian Salido <salidoa@google.com>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
 drivers/gpu/drm/drm_plane.c   | 6 ++++++
 include/uapi/drm/drm_fourcc.h | 7 +++++++
 2 files changed, 13 insertions(+)

diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
index 143041666096..21e3a560811d 100644
--- a/drivers/gpu/drm/drm_plane.c
+++ b/drivers/gpu/drm/drm_plane.c
@@ -48,6 +48,12 @@
  *
  * The type of a plane is exposed in the immutable "type" enumeration property,
  * which has one of the following values: "Overlay", "Primary", "Cursor".
+ *
+ * Use of a framebuffer with alpha implies that the plane will use
+ * pre-multiplied alpha blending: Dc = Sc + (1.0 - Sa) * Dc,
+ * where Sa is source alpha, Sc is source color, and Dc is destination
+ * color. The crtc background color below all the planes is assumed to
+ * be black.
  */
 
 static unsigned int drm_num_planes(struct drm_device *dev)
diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
index e04613d30a13..8b889a6adcd4 100644
--- a/include/uapi/drm/drm_fourcc.h
+++ b/include/uapi/drm/drm_fourcc.h
@@ -35,6 +35,13 @@ extern "C" {
 
 #define DRM_FORMAT_BIG_ENDIAN (1<<31) /* format is big endian instead of little endian */
 
+/*
+ * Note that using any format with alpha (A) implies pre-multiplied
+ * alpha blending: Dc = Sc + (1.0 - Sa) * Dc, where Sa is source alpha,
+ * Sc is source color, and Dc is destination color. The crtc background
+ * color below all the planes is assumed to be black.
+ */
+
 /* color index */
 #define DRM_FORMAT_C8		fourcc_code('C', '8', ' ', ' ') /* [7:0] C */
 
-- 
2.16.1

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