[PATCH 6.18 146/175] drm/amd/display: fix wrong color value mapping on MCM shaper LUT

Greg Kroah-Hartman
2026-02-09T14:33:13Z

From: Melissa Wen <mwen@igalia.com>

[ Upstream commit 8f959d37c1f2efec6dac55915ee82302e98101fb ]

Some shimmer/colorful points appears when using the steamOS color
pipeline for HDR on gaming with DCN32. These points look like black
values being wrongly mapped to red/blue/green values. It was caused
because the number of hw points in regular LUTs and in a shaper LUT was
treated as the same.

DCN3+ regular LUTs have 257 bases and implicit deltas (i.e. HW
calculates them), but shaper LUT is a special case: it has 256 bases and
256 deltas, as in DCN1-2 regular LUTs, and outputs 14-bit values.

Fix that by setting by decreasing in 1 the number of HW points computed
in the LUT segmentation so that shaper LUT (i.e. fixpoint == true) keeps
the same DCN10 CM logic and regular LUTs go with `hw_points + 1`.

CC: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Fixes: 4d5fd3d08ea9 ("drm/amd/display: PQ tail accuracy")
Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 5006505b19a2119e71c008044d59f6d753c858b9)
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c b/drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c
index 0690c346f2c52..a4f14b16564c2 100644
--- a/drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c
+++ b/drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c
@@ -163,6 +163,11 @@ bool cm3_helper_translate_curve_to_hw_format(
hw_points += (1 << seg_distr[k]);
}

+ // DCN3+ have 257 pts in lieu of no separate slope registers
+ // Prior HW had 256 base+slope pairs
+ // Shaper LUT (i.e. fixpoint == true) is still 256 bases and 256 deltas
+ hw_points = fixpoint ? (hw_points - 1) : hw_points;
+
j = 0;
for (k = 0; k < (region_end - region_start); k++) {
increment = NUMBER_SW_SEGMENTS / (1 << seg_distr[k]);
@@ -223,8 +228,6 @@ bool cm3_helper_translate_curve_to_hw_format(
corner_points[1].green.slope = dc_fixpt_zero;
corner_points[1].blue.slope = dc_fixpt_zero;

- // DCN3+ have 257 pts in lieu of no separate slope registers
- // Prior HW had 256 base+slope pairs
lut_params->hw_points_num = hw_points + 1;

k = 0;
--
2.51.0