From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
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@xxxxxxxxx>
Cc: Stefan Schake <stschake@xxxxxxxxx>
Cc: Eric Anholt <eric@xxxxxxxxxx>
Cc: Adrian Salido <salidoa@xxxxxxxxxx>
Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
Signed-off-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
---
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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