Re: [RFC PATCH v2 7/9] drm/amd/display: Use GAMCOR for first TF if CSC is used

[Harry Wentland <harry.wentland@amd.com>]

On 2026-04-21 17:08, Alex Hung wrote:
> 
> 
> On 3/30/26 09:34, Harry Wentland wrote:
>> For subsampled formats we need to use GAMCOR instead of
>> the DEGAM block. The color module can create a LUT for
>> that if we set map_user_ramp to true. So do that when
>> we have subsampled formats.
>>
>> Co-developed by Claude Sonnet 4.5.
> 
> This should be updated to "Assisted-by Claude:claude-Sonnet-4.5"
> 
>>
>> Signed-off-by: Harry Wentland <harry.wentland@amd.com>
>> ---
>>   .../amd/display/amdgpu_dm/amdgpu_dm_color.c   | 34 ++++++++++++++++++-
>>   1 file changed, 33 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
>> index d5b4190e635c..6403dfe4ee10 100644
>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
>> @@ -1471,6 +1471,8 @@ __set_colorop_in_tf_1d_curve(struct dc_plane_state *dc_plane_state,
>>       struct dc_transfer_func *tf = &dc_plane_state->in_transfer_func;
>>       struct drm_colorop *colorop = colorop_state->colorop;
>>       struct drm_device *drm = colorop->dev;
>> +    struct dc_color_caps *color_caps = NULL;
>> +    bool is_subsampled_format;
>>         if (colorop->type != DRM_COLOROP_1D_CURVE)
>>           return -EINVAL;
>> @@ -1486,9 +1488,39 @@ __set_colorop_in_tf_1d_curve(struct dc_plane_state *dc_plane_state,
>>         drm_dbg(drm, "Degamma colorop with ID: %d\n", colorop->base.id);
>>   -    tf->type = TF_TYPE_PREDEFINED;
> 
> 
> Setting tf-type is moved to the below if-else branch, but both set tf->type = TF_TYPE_PREDEFINED. Is this done intentionally to improve readability or for other purposes?
> 

Good catch. I should've noticed this senseless change
by the LLM. I'll also clean up the comments for v3.

Harry

>> +    /* Check if format requires post-scale color processing (subsampled formats) */
>> +    is_subsampled_format = (dc_plane_state->format >= SURFACE_PIXEL_FORMAT_VIDEO_BEGIN &&
>> +                dc_plane_state->format < SURFACE_PIXEL_FORMAT_SUBSAMPLE_END);
>> +
>>       tf->tf = amdgpu_colorop_tf_to_dc_tf(colorop_state->curve_1d_type);
>>   +    if (is_subsampled_format) {
>> +        /*
>> +         * For subsampled formats (P010, NV12), we need color processing
>> +         * to happen AFTER scaling (to expand UV channels first).
>> +         * Convert predefined TF to PWL so DC will use GAMCOR (post-scale)
>> +         * instead of PRE_DEGAM (pre-scale).
>> +         *
>> +         * IMPORTANT: We must pass map_user_ramp=true to force PWL conversion.
>> +         * Without it, mod_color_calculate_degamma_params() returns early for
>> +         * SRGB/Linear TFs without converting to TF_TYPE_DISTRIBUTED_POINTS.
>> +         */
>> +        tf->type = TF_TYPE_PREDEFINED;
>> +
>> +        if (dc_plane_state->ctx && dc_plane_state->ctx->dc)
>> +            color_caps = &dc_plane_state->ctx->dc->caps.color;
>> +
>> +        if (!mod_color_calculate_degamma_params(color_caps, tf, NULL, true)) {
>> +            drm_err(drm, "Failed to calculate degamma params for subsampled format\n");
>> +            return -EINVAL;
>> +        }
>> +
>> +        /* mod_color_calculate_degamma_params sets tf->type to TF_TYPE_DISTRIBUTED_POINTS */
>> +    } else {
>> +        /* For non-subsampled formats (RGB, XR30), use predefined ROM LUT (PRE_DEGAM) */
>> +        tf->type = TF_TYPE_PREDEFINED;
>> +    }
>> +
>>       return 0;
>>   }
>>   
>