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

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

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?

> +	/* 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;
>   }
>