[PATCH v4 4/4] RFC: drm/sun4i: Process alpha channel of most bottom layer

[Roman Stratiienko <r.stratiienko@gmail.com>]

Allwinner display engine blender consists of 3 pipelined blending units.

PIPE0->\
        BLD0-\
PIPE1->/      BLD1-\
PIPE2->------/      BLD2->OUT
PIPE3->------------/

This pipeline produces incorrect composition if PIPE0 buffer has alpha.
Correct solution is to add one more blending step and mix PIPE0 with
background, but it is not supported by the hardware.

Use premultiplied alpha buffer of PIPE0 overlay channel as is.
In this case we got same effect as mixing PIPE0 with black background.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>

---

v4:
- Initial version, depends on other unmerged patches in the patchset.
---
 drivers/gpu/drm/sun4i/sun8i_ui_layer.c | 2 +-
 drivers/gpu/drm/sun4i/sun8i_vi_layer.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/sun4i/sun8i_ui_layer.c b/drivers/gpu/drm/sun4i/sun8i_ui_layer.c
index dd6145f80c36..d94f4d8b9128 100644
--- a/drivers/gpu/drm/sun4i/sun8i_ui_layer.c
+++ b/drivers/gpu/drm/sun4i/sun8i_ui_layer.c
@@ -106,7 +106,7 @@ static void sun8i_ui_layer_update_alpha(struct sun8i_mixer *mixer, int channel,
 	regmap_update_bits(mixer->engine.regs,
 			   SUN8I_MIXER_BLEND_PREMULTIPLY(bld_base),
 			   SUN8I_MIXER_BLEND_PREMULTIPLY_EN(zpos),
-			   SUN8I_MIXER_BLEND_PREMULTIPLY_EN(zpos));
+			   zpos ? SUN8I_MIXER_BLEND_PREMULTIPLY_EN(zpos) : 0);
 }
 
 static int sun8i_ui_layer_update_coord(struct sun8i_mixer *mixer, int channel,
diff --git a/drivers/gpu/drm/sun4i/sun8i_vi_layer.c b/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
index e6d8a539614f..68a6843db4ab 100644
--- a/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
+++ b/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
@@ -108,7 +108,7 @@ static void sun8i_vi_layer_update_alpha(struct sun8i_mixer *mixer, int channel,
 	regmap_update_bits(mixer->engine.regs,
 			   SUN8I_MIXER_BLEND_PREMULTIPLY(bld_base),
 			   SUN8I_MIXER_BLEND_PREMULTIPLY_EN(zpos),
-			   (mixer->cfg->is_de3) ?
+			   (zpos != 0 && mixer->cfg->is_de3) ?
 				SUN8I_MIXER_BLEND_PREMULTIPLY_EN(zpos) : 0);
 
 }
-- 
2.17.1