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

[Alex Hung <alex.hung@amd.com>]

Reviewed-by: Alex Hung <alex.hung@amd.com>

On 1/22/26 08:20, Melissa Wen wrote:
> 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>
> ---
>   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 0690c346f2c5..a4f14b16564c 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;