Re: [PATCH 4/9] drm/i915/dp: Restructure the sink/output format selection

[Nicolas Frattaroli <nicolas.frattaroli@collabora.com>]

On Tuesday, 31 March 2026 01:53:34 Central European Summer Time Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> Restructure intel_dp_compute_output_format() to resemble the new
> intel_hdmi_compute_output_formats().
> 
> Again, we basically have two main code paths:
> - YCbCr 4:2:0 only modes
> - everything else including YCbCr 4:2:0 also modes
> 
> Take the exact same approach with the DP code, making the
> format selection much less convoluted.
> 
> Cc: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 98 +++++++++++++++++--------
>  1 file changed, 69 insertions(+), 29 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 4955bd8b11d7..230b45acde29 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -1371,6 +1371,28 @@ intel_dp_mode_valid_downstream(struct intel_connector *connector,
>  	return MODE_OK;
>  }
>  
> +static enum drm_mode_status
> +intel_dp_sink_format_valid(struct intel_connector *connector,
> +			   const struct drm_display_mode *mode,
> +			   enum intel_output_format sink_format)
> +{
> +	const struct drm_display_info *info = &connector->base.display_info;
> +
> +	switch (sink_format) {
> +	case INTEL_OUTPUT_FORMAT_YCBCR420:
> +		if (!connector->base.ycbcr_420_allowed ||
> +		    !drm_mode_is_420(info, mode))
> +			return MODE_NO_420;
> +
> +		return MODE_OK;
> +	case INTEL_OUTPUT_FORMAT_RGB:
> +		return MODE_OK;
> +	default:
> +		MISSING_CASE(sink_format);
> +		return MODE_BAD;
> +	}
> +}
> +

I think here we'll want another
---
diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index e23162fc3f8b..a1dc089c54f5 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -1373,6 +1373,11 @@ intel_dp_sink_format_valid(struct intel_connector *connector,
 
                return MODE_OK;
        case INTEL_OUTPUT_FORMAT_RGB:
+               return MODE_OK;
+       case INTEL_OUTPUT_FORMAT_YCBCR444:
+               if (!(info->color_formats & BIT(DRM_OUTPUT_COLOR_FORMAT_YCBCR444)))
+                       return MODE_BAD;
+
                return MODE_OK;
        default:
                MISSING_CASE(sink_format);
---

though this time, no bpc related changes. With that fixed, I get
YCbCr444 at 10bpc as well as 8bpc. Can't test 4:2:0 for what appears
to be unrelated userspace reasons, though the KMS property's enum
value is exposed properly.

>  int intel_dp_max_hdisplay_per_pipe(struct intel_display *display)
>  {
>  	return DISPLAY_VER(display) >= 30 ? 6144 : 5120;
> @@ -3330,41 +3352,59 @@ static int
>  intel_dp_compute_output_format(struct intel_encoder *encoder,
>  			       struct intel_crtc_state *crtc_state,
>  			       struct drm_connector_state *conn_state,
> -			       bool respect_downstream_limits)
> +			       bool respect_downstream_limits,
> +			       enum intel_output_format sink_format)
>  {
> -	struct intel_display *display = to_intel_display(encoder);
>  	struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
>  	struct intel_connector *connector = intel_dp->attached_connector;
> -	const struct drm_display_info *info = &connector->base.display_info;
>  	const struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
> -	bool ycbcr_420_only;
> -	int ret;
>  
> -	ycbcr_420_only = drm_mode_is_420_only(info, adjusted_mode);
> -
> -	if (ycbcr_420_only && !connector->base.ycbcr_420_allowed) {
> -		drm_dbg_kms(display->drm,
> -			    "YCbCr 4:2:0 mode but YCbCr 4:2:0 output not possible. Falling back to RGB.\n");
> -		crtc_state->sink_format = INTEL_OUTPUT_FORMAT_RGB;
> -	} else {
> -		crtc_state->sink_format = intel_dp_sink_format(connector, adjusted_mode);
> -	}
> +	if (intel_dp_sink_format_valid(connector, adjusted_mode,
> +				       sink_format) != MODE_OK)
> +		return -EINVAL;
>  
> +	crtc_state->sink_format = sink_format;
>  	crtc_state->output_format = intel_dp_output_format(connector, crtc_state->sink_format);
>  
> -	ret = intel_dp_compute_link_config(encoder, crtc_state, conn_state,
> -					   respect_downstream_limits);
> -	if (ret) {
> -		if (crtc_state->sink_format == INTEL_OUTPUT_FORMAT_YCBCR420 ||
> -		    !connector->base.ycbcr_420_allowed ||
> -		    !drm_mode_is_420_also(info, adjusted_mode))
> -			return ret;
> -
> -		crtc_state->sink_format = INTEL_OUTPUT_FORMAT_YCBCR420;
> -		crtc_state->output_format = intel_dp_output_format(connector,
> -								   crtc_state->sink_format);
> -		ret = intel_dp_compute_link_config(encoder, crtc_state, conn_state,
> -						   respect_downstream_limits);
> +	return intel_dp_compute_link_config(encoder, crtc_state, conn_state,
> +					    respect_downstream_limits);

With the removal of intel_dp_sink_format in this function, I wonder
if we can get rid of it entirely now. It's only called in
intel_dp_mode_min_link_bpp_x16, which seems to be used for bandwidth
limitation calculations (where YCbCr444 vs RGB444 doesn't matter, so
we're fine in that regard).

Judging by the "min" in the function name, I assume it should be
using INTEL_OUTPUT_FORMAT_YCBCR420 in drm_mode_is_420_also cases,
whereas right now it only gets this from intel_dp_sink_format if
the mode is drm_mode_is_420_only. So I think removing
intel_dp_sink_format entirely as a follow-up, and folding a
corrected minimum bandwidth computation that uses 420 if
drm_mode_is_420 into intel_dp_mode_min_link_bpp_x16 would make
sense, unless I'm totally misunderstanding the code here.

> +}
> +
> +static int
> +intel_dp_compute_formats(struct intel_encoder *encoder,
> +			 struct intel_crtc_state *crtc_state,
> +			 struct drm_connector_state *conn_state,
> +			 bool respect_downstream_limits)
> +{
> +	struct intel_display *display = to_intel_display(encoder);
> +	struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
> +	struct intel_connector *connector = intel_dp->attached_connector;
> +	const struct drm_display_info *info = &connector->base.display_info;
> +	const struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
> +	int ret;
> +
> +	if (drm_mode_is_420_only(info, adjusted_mode)) {
> +		ret = intel_dp_compute_output_format(encoder, crtc_state, conn_state,
> +						     respect_downstream_limits,
> +						     INTEL_OUTPUT_FORMAT_YCBCR420);
> +
> +		if (ret) {
> +			drm_dbg_kms(display->drm,
> +				    "YCbCr 4:2:0 mode but YCbCr 4:2:0 output not possible. Falling back to RGB.\n");
> +
> +			ret = intel_dp_compute_output_format(encoder, crtc_state, conn_state,
> +							     respect_downstream_limits,
> +							     INTEL_OUTPUT_FORMAT_RGB);
> +		}
> +	} else {
> +		ret = intel_dp_compute_output_format(encoder, crtc_state, conn_state,
> +						     respect_downstream_limits,
> +						     INTEL_OUTPUT_FORMAT_RGB);
> +
> +		if (ret && drm_mode_is_420_also(info, adjusted_mode))
> +			ret = intel_dp_compute_output_format(encoder, crtc_state, conn_state,
> +							     respect_downstream_limits,
> +							     INTEL_OUTPUT_FORMAT_YCBCR420);
>  	}
>  
>  	return ret;
> @@ -3539,9 +3579,9 @@ intel_dp_compute_config(struct intel_encoder *encoder,
>  	 * Try to respect downstream TMDS clock limits first, if
>  	 * that fails assume the user might know something we don't.
>  	 */
> -	ret = intel_dp_compute_output_format(encoder, pipe_config, conn_state, true);
> +	ret = intel_dp_compute_formats(encoder, pipe_config, conn_state, true);
>  	if (ret)
> -		ret = intel_dp_compute_output_format(encoder, pipe_config, conn_state, false);
> +		ret = intel_dp_compute_formats(encoder, pipe_config, conn_state, false);
>  	if (ret)
>  		return ret;
>  
>