[PATCH] drm/i915/display: Fix RGB limited range handling for DP

[Ville Syrjälä]

On Tue, Jul 01, 2025 at 01:47:56PM +0530, Uma Shankar wrote:
> RGB limited range should be selected only if explicitly asked by
> userspace by the broadcast RGB property with LIMITED_RANGE. This
> is mostly enabled in case of CEA modes.
> 
> Display port by default uses Full Range, fixed the same. This will help
> set correct MSA information for colorimetry. Fixes a CTS issue wrt
> colorimetry.

If the CTS is expecting that then it's not following the actual DP spec.
Originally the spec required limited range output for cea modes, but
later version relaxed it to "should" and also notes that full range may
also be used. Presumably it was realized that existing implementations
had no consitency here and so both behaviours must be allowed.

> 
> Signed-off-by: Uma Shankar <uma.shankar at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 21 +++++++--------------
>  1 file changed, 7 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index f48912f308df..8758b9d60d5e 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -2708,8 +2708,6 @@ bool intel_dp_limited_color_range(const struct intel_crtc_state *crtc_state,
>  {
>  	const struct intel_digital_connector_state *intel_conn_state =
>  		to_intel_digital_connector_state(conn_state);
> -	const struct drm_display_mode *adjusted_mode =
> -		&crtc_state->hw.adjusted_mode;
>  
>  	/*
>  	 * Our YCbCr output is always limited range.
> @@ -2721,18 +2719,13 @@ bool intel_dp_limited_color_range(const struct intel_crtc_state *crtc_state,
>  	if (crtc_state->output_format != INTEL_OUTPUT_FORMAT_RGB)
>  		return false;
>  
> -	if (intel_conn_state->broadcast_rgb == INTEL_BROADCAST_RGB_AUTO) {
> -		/*
> -		 * See:
> -		 * CEA-861-E - 5.1 Default Encoding Parameters
> -		 * VESA DisplayPort Ver.1.2a - 5.1.1.1 Video Colorimetry
> -		 */
> -		return crtc_state->pipe_bpp != 18 &&
> -			drm_default_rgb_quant_range(adjusted_mode) ==
> -			HDMI_QUANTIZATION_RANGE_LIMITED;
> -	} else {
> -		return intel_conn_state->broadcast_rgb ==
> -			INTEL_BROADCAST_RGB_LIMITED;
> +	switch (intel_conn_state->broadcast_rgb) {
> +	case INTEL_BROADCAST_RGB_LIMITED:
> +		return true;
> +	case INTEL_BROADCAST_RGB_FULL:
> +	case INTEL_BROADCAST_RGB_AUTO:
> +	default:
> +		return false;
>  	}
>  }
>  
> -- 
> 2.42.0

-- 
Ville Syrjälä
Intel