Re: [PATCH v4 11/11] drm/amd/display: Force GAMCOR for subsampled surfaces with PQ/Gamma22/HLG

[Robert Mader <robert.mader@collabora.com>]

Tested-by: Robert Mader <robert.mader@collabora.com>

On 22.07.26 15:46, Harry Wentland wrote:
> The ROM early-return checks for PQ, Gamma 2.2, and HLG in
> mod_color_calculate_degamma_params() do not consider the map_user_ramp
> parameter. When map_user_ramp is true (indicating a subsampled surface
> that requires post-scaler degamma via GAMCOR), the function still takes
> the ROM path, programming PRE_DEGAM instead.
>
> For subsampled formats the chroma channels must be upsampled by the
> scaler before degamma is applied. PRE_DEGAM sits before the scaler in
> the pipeline (CNVC domain), while GAMCOR sits after it (CM domain).
> Using the ROM for subsampled surfaces means degamma is applied to
> subsampled chroma before upsampling, producing incorrect results.
>
> Fix by gating the dcn_arch ROM capability checks on !map_user_ramp,
> forcing PQ/Gamma22/HLG through the GAMCOR LUT path when the surface
> is subsampled.
>
> Fixes: a8bf71649088 ("drm/amd/display: Internal refactoring to abstract color caps")
> Cc: stable@vger.kernel.org
> Assisted-by: Copilot:claude-opus-4.6
> Signed-off-by: Harry Wentland <harry.wentland@amd.com>
> ---
>   drivers/gpu/drm/amd/display/modules/color/color_gamma.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/amd/display/modules/color/color_gamma.c b/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
> index 03d88e78165d..5786ef5787ac 100644
> --- a/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
> +++ b/drivers/gpu/drm/amd/display/modules/color/color_gamma.c
> @@ -1696,7 +1696,8 @@ bool mod_color_calculate_degamma_params(struct dc_color_caps *dc_caps,
>   			return true;
>   
>   		if (dc_caps != NULL &&
> -			dc_caps->dpp.dcn_arch == 1) {
> +			dc_caps->dpp.dcn_arch == 1 &&
> +			!map_user_ramp) {
>   
>   			if (input_tf->tf == TRANSFER_FUNCTION_PQ &&
>   					dc_caps->dpp.dgam_rom_caps.pq == 1)

-- 
Robert Mader
Consultant Software Developer

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