[PATCH v2] drm/amd/display: don't ignore alpha property on pre-multiplied mode

[Melissa Wen]

"Pre-multiplied" is the default pixel blend mode for KMS/DRM, as
documented in supported_modes of drm_plane_create_blend_mode_property():
https://cgit.freedesktop.org/drm/drm-misc/tree/drivers/gpu/drm/drm_blend.c

In this mode, both 'pixel alpha' and 'plane alpha' participate in the
calculation, as described by the pixel blend mode formula in KMS/DRM
documentation:

out.rgb = plane_alpha * fg.rgb +
          (1 - (plane_alpha * fg.alpha)) * bg.rgb

Considering the blend config mechanisms we have in the driver so far,
the alpha mode that better fits this blend mode is the
_PER_PIXEL_ALPHA_COMBINED_GLOBAL_GAIN, where the value for global_gain
is the plane alpha (global_alpha).

With this change, alpha property stops to be ignored. It also addresses
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1734

v2:
 * keep the 8-bit value for global_alpha_value (Nicholas)
 * correct the logical ordering for combined global gain (Nicholas)
 * apply to dcn10 too (Nicholas)

Signed-off-by: Melissa Wen <mwen at igalia.com>
---
 .../drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c  | 14 +++++++++-----
 drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c | 14 +++++++++-----
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c b/drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c
index ad757b59e00e..b1034e6258c8 100644
--- a/drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c
+++ b/drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c
@@ -2528,14 +2528,18 @@ void dcn10_update_mpcc(struct dc *dc, struct pipe_ctx *pipe_ctx)
 	struct mpc *mpc = dc->res_pool->mpc;
 	struct mpc_tree *mpc_tree_params = &(pipe_ctx->stream_res.opp->mpc_tree_params);
 
-	if (per_pixel_alpha)
-		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA;
-	else
-		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_GLOBAL_ALPHA;
-
 	blnd_cfg.overlap_only = false;
 	blnd_cfg.global_gain = 0xff;
 
+	if (per_pixel_alpha && pipe_ctx->plane_state->global_alpha) {
+		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA_COMBINED_GLOBAL_GAIN;
+		blnd_cfg.global_gain = pipe_ctx->plane_state->global_alpha_value;
+	} else if (per_pixel_alpha) {
+		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA;
+	} else {
+		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_GLOBAL_ALPHA;
+	}
+
 	if (pipe_ctx->plane_state->global_alpha)
 		blnd_cfg.global_alpha = pipe_ctx->plane_state->global_alpha_value;
 	else
diff --git a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
index 4290eaf11a04..b627c41713cc 100644
--- a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
+++ b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
@@ -2344,14 +2344,18 @@ void dcn20_update_mpcc(struct dc *dc, struct pipe_ctx *pipe_ctx)
 	struct mpc *mpc = dc->res_pool->mpc;
 	struct mpc_tree *mpc_tree_params = &(pipe_ctx->stream_res.opp->mpc_tree_params);
 
-	if (per_pixel_alpha)
-		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA;
-	else
-		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_GLOBAL_ALPHA;
-
 	blnd_cfg.overlap_only = false;
 	blnd_cfg.global_gain = 0xff;
 
+	if (per_pixel_alpha && pipe_ctx->plane_state->global_alpha) {
+		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA_COMBINED_GLOBAL_GAIN;
+		blnd_cfg.global_gain = pipe_ctx->plane_state->global_alpha_value;
+	} else if (per_pixel_alpha) {
+		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA;
+	} else {
+		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_GLOBAL_ALPHA;
+	}
+
 	if (pipe_ctx->plane_state->global_alpha)
 		blnd_cfg.global_alpha = pipe_ctx->plane_state->global_alpha_value;
 	else
-- 
2.35.1