[PATCH] drm/i915: Do not explicilty enable FEC in DP_TP_CTL for UHBR rates

[Imre Deak]

On Thu, Aug 22, 2024 at 11:44:48AM +0530, Chaitanya Kumar Borah wrote:
> In case of UHBR rates, we do not need to explicitly enable FEC by writing
> to DP_TP_CTL register.
> For MST use-cases, intel_dp_mst_find_vcpi_slots_for_bpp() takes care of
> setting fec_enable to false. However, it gets overwritten in
> intel_dp_dsc_compute_config(). This change keeps fec_enable false across
> MST and SST use-cases for UHBR rates.
> 
> While at it, add a comment explaining why we don't enable FEC in eDP v1.5.
> 
> v2: Correct logic to cater to SST use-cases (Jani)
> 
> Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 49a37b996530..de30eaacd9b4 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -2298,9 +2298,15 @@ int intel_dp_dsc_compute_config(struct intel_dp *intel_dp,
>  		&pipe_config->hw.adjusted_mode;
>  	int ret;
>  
> +	/*
> +	 * Though eDP v1.5 supports FEC with DSC, unlike DP, it is optional.
> +	 * Since, FEC is a bandwidth overhead, continue to not enable it for
> +	 * eDP. Until, there is a good reason to do so.
> +	 */
>  	pipe_config->fec_enable = pipe_config->fec_enable ||
>  		(!intel_dp_is_edp(intel_dp) &&
> -		 intel_dp_supports_fec(intel_dp, connector, pipe_config));
> +		 intel_dp_supports_fec(intel_dp, connector, pipe_config) &&
> +		 !intel_dp_is_uhbr(pipe_config));

I think this should be only computed by the DP encoder specific handler
(in intel_dp_compute_link_config()) instead of recomputing here for the
DP MST case as well. Also intel_dp_supports_dsc() just happens to work
now, since it doesn't check for this flag on MST, so that may need to be
updated. Both of these can be done as a follow up, so for this patch:

Reviewed-by: Imre Deak <imre.deak at intel.com>

>  
>  	if (!intel_dp_supports_dsc(connector, pipe_config))
>  		return -EINVAL;
> -- 
> 2.25.1
>