Re: [PATCH 5/5] drm/i915/dp: Ignore max_requested_bpc if its too low for DSC

[Jani Nikula <jani.nikula@intel.com>]

On Wed, 13 Dec 2023, Ankit Nautiyal <ankit.k.nautiyal@intel.com> wrote:
> At the moment, while choosing the input bpc for DSC, we take into
> account the max_requested_bpc property. This creates a problem, if the
> max_requested_bpc is lower than the minimum bpc required by source with
> DSC.
>
> So consider max_requested_bpc if its within the limits that we can
> support with DSC.
>
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index e8aa2f469142..0014aa5ea652 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -1638,6 +1638,16 @@ int intel_dp_dsc_max_src_input_bpc(struct drm_i915_private *i915)
>  	return 12;
>  }
>  
> +static int
> +intel_dp_dsc_limit_max_bpc(int max_requested_bpc, int src_dsc_max_bpc, int src_dsc_min_bpc)
> +{
> +	/* Consider max_requested_bpc only if src can support it with DSC */
> +	if (max_requested_bpc >= src_dsc_min_bpc)
> +		return min(src_dsc_max_bpc, max_requested_bpc);
> +
> +	return src_dsc_max_bpc;
> +}
> +
>  int intel_dp_dsc_compute_max_bpp(const struct intel_connector *connector,
>  				 u8 max_req_bpc)
>  {
> @@ -1651,7 +1661,8 @@ int intel_dp_dsc_compute_max_bpp(const struct intel_connector *connector,
>  	if (!dsc_max_bpc)
>  		return dsc_max_bpc;
>  
> -	dsc_max_bpc = min(dsc_max_bpc, max_req_bpc);
> +	dsc_max_bpc = intel_dp_dsc_limit_max_bpc(max_req_bpc, dsc_max_bpc,
> +						 intel_dp_dsc_min_src_input_bpc(i915));

Somehow that doesn't read so well.

I think something like this would improve clarity:

	dsc_min_bpc = intel_dp_dsc_min_src_input_bpc(i915);
	dsc_max_bpc = intel_dp_dsc_max_src_input_bpc(i915);

	max_req_bpc = clamp(max_req_bpc, dsc_min_bpc, dsc_max_bpc);

i.e. clamp the request to reasonable limits. That's more like regular
input checking. That should be done *everywhere* when
conn_state->max_requested_bpc is used.

Then min(dsc_max_bpc, max_req_bpc) is no longer needed because
max_req_bpc <= dsc_max_bpc is guaranteed.

IOW this becomes:

	dsc_max_bpc = clamp(max_req_bpc, dsc_min_bpc, dsc_max_bpc);

BR,
Jani.

>  
>  	num_bpc = drm_dp_dsc_sink_supported_input_bpcs(connector->dp.dsc_dpcd,
>  						       dsc_bpc);
> @@ -2039,8 +2050,11 @@ bool is_dsc_pipe_bpp_sufficient(struct drm_i915_private *i915,
>  {
>  	int dsc_max_bpc, dsc_min_bpc, dsc_max_pipe_bpp, dsc_min_pipe_bpp;
>  
> -	dsc_max_bpc = min(intel_dp_dsc_max_src_input_bpc(i915), conn_state->max_requested_bpc);
>  	dsc_min_bpc = intel_dp_dsc_min_src_input_bpc(i915);
> +	dsc_max_bpc = intel_dp_dsc_max_src_input_bpc(i915);
> +
> +	dsc_max_bpc = intel_dp_dsc_limit_max_bpc(conn_state->max_requested_bpc,
> +						 dsc_max_bpc, dsc_min_bpc);
>  
>  	dsc_max_pipe_bpp = min(dsc_max_bpc * 3, limits->pipe.max_bpp);
>  	dsc_min_pipe_bpp = max(dsc_min_bpc * 3, limits->pipe.min_bpp);
> @@ -2100,14 +2114,14 @@ static int intel_dp_dsc_compute_pipe_bpp(struct intel_dp *intel_dp,
>  		}
>  	}
>  
> +	dsc_min_bpc = intel_dp_dsc_min_src_input_bpc(i915);
>  	dsc_max_bpc = intel_dp_dsc_max_src_input_bpc(i915);
>  	if (!dsc_max_bpc)
>  		return -EINVAL;
>  
> -	dsc_max_bpc = min(dsc_max_bpc, max_req_bpc);
> +	dsc_max_bpc = intel_dp_dsc_limit_max_bpc(max_req_bpc, dsc_max_bpc, dsc_min_bpc);
>  	dsc_max_bpp = min(dsc_max_bpc * 3, limits->pipe.max_bpp);
>  
> -	dsc_min_bpc = intel_dp_dsc_min_src_input_bpc(i915);
>  	dsc_min_bpp = max(dsc_min_bpc * 3, limits->pipe.min_bpp);
>  
>  	/*

-- 
Jani Nikula, Intel