[RFC PATCH] drm/amd/display: dont ignore alpha property

[Melissa Wen]

Hi all,

I'm examining the IGT kms_plane_alpha_blend test, specifically the
alpha-7efc. It fails on AMD and Intel gen8 hw, but passes on Intel
gen11. At first, I thought it was a rounding issue. In fact, it may be
the problem for different results between intel hw generations.

However, I changed the test locally to compare CRCs for all alpha values
in the range before the test fails. Interestingly, it fails for all
values when running on AMD, even when comparing planes with zero alpha
(fully transparent). Moreover, I see the same CRC values regardless of
the value set in the alpha property.

To ensure that the blending mode is as expected, I explicitly set the
Pre-multiplied blending mode in the test. Then I tried using different
framebuffer data and alpha values. I've tried obvious comparisons too,
such as fully opaque and fully transparent.

As far as I could verify and understand, the value set for the ALPHA
property is totally ignored by AMD drivers. I'm not sure if this is a
matter of how we interpret the meaning of the premultiplied blend mode
or the driver's assumptions about the data in these blend modes.
For example, I made a change in the test as here:
https://paste.debian.net/1235620/
That basically means same framebuffer, but different alpha values for
each plane. And the result was succesful (but I expected it fails).

Besides that, I see that other subtests in kms_plane_alpha_blend are
skipped, use "None" pixel blend mode, or are not changing the
IGT_PLANE_ALPHA property. So, this alpha-7efc seems to be the only one
in the subset that is checking changes on alpha property under a
Pre-multiplied blend mode, and it is failing.

I see some inputs in this issue:
https://gitlab.freedesktop.org/drm/amd/-/issues/1769.
But them, I guessed there are different interpretations for handling
plane alpha in the pre-multiplied blend mode. Tbh, I'm not clear, but
there's always a chance of different interpretations, and I don't have
a third driver with CRC capabilities for further comparisons.

I made some experiments on blnd_cfg values, changing alpha_mode vs
global_gain and global_alpha. I think the expected behaviour for the
Pre-multiplied blend mode is achieved by applying this RFC patch (for
Cezanne).

Does it seems reasonable? Can anyone help me with more inputs to guide
me the right direction or point out what I misunderstood about these
concepts?

Thanks,

Signed-off-by: Melissa Wen <mwen at igalia.com>
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c  | 2 +-
 drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 6633df7682ce..821ffafa441e 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -5438,7 +5438,7 @@ fill_blending_from_plane_state(const struct drm_plane_state *plane_state,
 
 	if (plane_state->alpha < 0xffff) {
 		*global_alpha = true;
-		*global_alpha_value = plane_state->alpha >> 8;
+		*global_alpha_value = plane_state->alpha;
 	}
 }
 
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..b4888f91a9d0 100644
--- a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
+++ b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
@@ -2367,6 +2367,10 @@ void dcn20_update_mpcc(struct dc *dc, struct pipe_ctx *pipe_ctx)
 			== SURFACE_PIXEL_FORMAT_GRPH_RGBE_ALPHA)
 		blnd_cfg.pre_multiplied_alpha = false;
 
+	if (blnd_cfg.pre_multiplied_alpha) {
+		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA_COMBINED_GLOBAL_GAIN;
+		blnd_cfg.global_gain = blnd_cfg.global_alpha;
+	}
 	/*
 	 * TODO: remove hack
 	 * Note: currently there is a bug in init_hw such that
-- 
2.35.1

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