[PATCH 2/2] drm/i915/dp: Set min_bpp limit to 30 in HDR mode

[Shankar, Uma]

> -----Original Message-----
> From: Borah, Chaitanya Kumar <chaitanya.kumar.borah at intel.com>
> Sent: Wednesday, July 30, 2025 11:25 AM
> To: intel-xe at lists.freedesktop.org; intel-gfx at lists.freedesktop.org
> Cc: Deak, Imre <imre.deak at intel.com>; ville.syrjala at linux.intel.com; Shankar,
> Uma <uma.shankar at intel.com>; Borah, Chaitanya Kumar
> <chaitanya.kumar.borah at intel.com>
> Subject: [PATCH 2/2] drm/i915/dp: Set min_bpp limit to 30 in HDR mode
> 
> Update intel_dp_compute_config_limits() to use a minimum of
> 30 bits per pixel when the connector is in HDR mode (specifically, when EOTF is
> SMPTE ST2084), aligning with HDR display requirements.
> 
> To support this, the function now takes a drm_connector_state instead of an
> intel_connector, and the required updates are made in all call sites, including MST
> handling.
> 
> This ensures sufficient bitdepth for HDR content to avoid banding.
> 
> If the required bandwidth for 30 bpp cannot be supported, the driver will either fall
> back to DSC or reject the mode during atomic check if DSC is not supported.

There will be some case where sink supports HDR but at a lower clock with same mode
and lower refresh rate. After driver rejects, since there is no reason for userspace to know,
it may end up not enabling HDR at all. Not sure, how many such real world sinks we have though.
This can be taken up later if something comes along like above.

For now, this Change Looks Good to me.
Reviewed-by: Uma Shankar <uma.shankar at intel.com>

> Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c     | 11 +++++++----
>  drivers/gpu/drm/i915/display/intel_dp.h     |  2 +-
>  drivers/gpu/drm/i915/display/intel_dp_mst.c | 11 +++++++----
>  3 files changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c
> b/drivers/gpu/drm/i915/display/intel_dp.c
> index 59d814abd3a9..49a3ff414dc4 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -2536,13 +2536,15 @@ intel_dp_dsc_compute_pipe_bpp_limits(struct
> intel_dp *intel_dp,
> 
>  bool
>  intel_dp_compute_config_limits(struct intel_dp *intel_dp,
> -			       struct intel_connector *connector,
> +			       struct drm_connector_state *conn_state,
>  			       struct intel_crtc_state *crtc_state,
>  			       bool respect_downstream_limits,
>  			       bool dsc,
>  			       struct link_config_limits *limits)  {
>  	bool is_mst = intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DP_MST);
> +	struct intel_connector *connector =
> +		to_intel_connector(conn_state->connector);
> 
>  	limits->min_rate = intel_dp_min_link_rate(intel_dp);
>  	limits->max_rate = intel_dp_max_link_rate(intel_dp); @@ -2552,7
> +2554,8 @@ intel_dp_compute_config_limits(struct intel_dp *intel_dp,
>  	limits->min_lane_count = intel_dp_min_lane_count(intel_dp);
>  	limits->max_lane_count = intel_dp_max_lane_count(intel_dp);
> 
> -	limits->pipe.min_bpp = intel_dp_min_bpp(crtc_state->output_format);
> +	limits->pipe.min_bpp = intel_dp_in_hdr_mode(conn_state) ? 30 :
> +				intel_dp_min_bpp(crtc_state->output_format);
>  	if (is_mst) {
>  		/*
>  		 * FIXME: If all the streams can't fit into the link with their @@ -
> 2651,7 +2654,7 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
>  	joiner_needs_dsc = intel_dp_joiner_needs_dsc(display,
> num_joined_pipes);
> 
>  	dsc_needed = joiner_needs_dsc || intel_dp->force_dsc_en ||
> -		     !intel_dp_compute_config_limits(intel_dp, connector,
> pipe_config,
> +		     !intel_dp_compute_config_limits(intel_dp, conn_state,
> +pipe_config,
>  						     respect_downstream_limits,
>  						     false,
>  						     &limits);
> @@ -2685,7 +2688,7 @@ intel_dp_compute_link_config(struct intel_encoder
> *encoder,
>  			    str_yes_no(ret), str_yes_no(joiner_needs_dsc),
>  			    str_yes_no(intel_dp->force_dsc_en));
> 
> -		if (!intel_dp_compute_config_limits(intel_dp, connector,
> pipe_config,
> +		if (!intel_dp_compute_config_limits(intel_dp, conn_state,
> +pipe_config,
>  						    respect_downstream_limits,
>  						    true,
>  						    &limits))
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.h
> b/drivers/gpu/drm/i915/display/intel_dp.h
> index 5def589e3c0e..f90cfd1dbbd0 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.h
> +++ b/drivers/gpu/drm/i915/display/intel_dp.h
> @@ -193,7 +193,7 @@ void intel_dp_wait_source_oui(struct intel_dp *intel_dp);
> int intel_dp_output_bpp(enum intel_output_format output_format, int bpp);
> 
>  bool intel_dp_compute_config_limits(struct intel_dp *intel_dp,
> -				    struct intel_connector *connector,
> +				    struct drm_connector_state *conn_state,
>  				    struct intel_crtc_state *crtc_state,
>  				    bool respect_downstream_limits,
>  				    bool dsc,
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index 74497c9a0554..352f7ef29c28 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -611,12 +611,15 @@ adjust_limits_for_dsc_hblank_expansion_quirk(struct
> intel_dp *intel_dp,
> 
>  static bool
>  mst_stream_compute_config_limits(struct intel_dp *intel_dp,
> -				 struct intel_connector *connector,
> +				 struct drm_connector_state *conn_state,
>  				 struct intel_crtc_state *crtc_state,
>  				 bool dsc,
>  				 struct link_config_limits *limits)  {
> -	if (!intel_dp_compute_config_limits(intel_dp, connector,
> +	struct intel_connector *connector =
> +		to_intel_connector(conn_state->connector);
> +
> +	if (!intel_dp_compute_config_limits(intel_dp, conn_state,
>  					    crtc_state, false, dsc,
>  					    limits))
>  		return false;
> @@ -665,7 +668,7 @@ static int mst_stream_compute_config(struct
> intel_encoder *encoder,
>  	joiner_needs_dsc = intel_dp_joiner_needs_dsc(display,
> num_joined_pipes);
> 
>  	dsc_needed = joiner_needs_dsc || intel_dp->force_dsc_en ||
> -		!mst_stream_compute_config_limits(intel_dp, connector,
> +		!mst_stream_compute_config_limits(intel_dp, conn_state,
>  						  pipe_config, false, &limits);
> 
>  	if (!dsc_needed) {
> @@ -691,7 +694,7 @@ static int mst_stream_compute_config(struct
> intel_encoder *encoder,
>  			    str_yes_no(intel_dp->force_dsc_en));
> 
> 
> -		if (!mst_stream_compute_config_limits(intel_dp, connector,
> +		if (!mst_stream_compute_config_limits(intel_dp, conn_state,
>  						      pipe_config, true,
>  						      &limits))
>  			return -EINVAL;
> --
> 2.25.1