Re: [Intel-gfx] [PATCH] drm/i915/display/dp: Add fallback on LT failure for DP2.0

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

On Tue, 20 Jun 2023, Arun R Murthy <arun.r.murthy@intel.com> wrote:
> For DP2.0 as per the Spec on LT failure we need to reduce the lane count
> if the lane count is not equal to 1. If lane count is 1 then need to
> retry with reducing the link rate.
>
> Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 54ddc953e5bc..2b12ca45596d 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -657,8 +657,20 @@ int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
>  				    "Retrying Link training for eDP with same parameters\n");
>  			return 0;
>  		}
> -		intel_dp->max_link_rate = intel_dp_common_rate(intel_dp, index - 1);
> -		intel_dp->max_link_lane_count = lane_count;
> +		if (intel_dp->dpcd[DP_MAIN_LINK_CHANNEL_CODING] & DP_CAP_ANSI_128B132B) {

This doesn't mean we're actually using UHBR. And we don't get here for
UHBR because it's always mst. And this doesn't handle mst (it
should). There are a lot of gaps here.

The whole thing has been discussed before, and my previous idea was that
we'd generate a list of {link rate, link lane count} pairs up front, in
the correct order, and just have one an index to that list for the
fallback. Because the logic here is complicated to follow.

Eventually we'd need to be able to switch between UHBR and non-UHBR in
the fallback too, which we aren't capable of doing at the moment.


BR,
Jani.


> +			if (lane_count > 1) {
> +				/* Reduce the lane count */
> +				intel_dp->max_link_lane_count = lane_count >> 1;
> +				intel_dp->max_link_rate = intel_dp_common_rate(intel_dp, index);
> +			} else {
> +				/* Reduce the link rate */
> +				intel_dp->max_link_rate = intel_dp_common_rate(intel_dp, index - 1);
> +				intel_dp->max_link_lane_count = intel_dp_max_common_lane_count(intel_dp);
> +			}
> +		} else {
> +			intel_dp->max_link_rate = intel_dp_common_rate(intel_dp, index - 1);
> +			intel_dp->max_link_lane_count = lane_count;
> +		}
>  	} else if (lane_count > 1) {
>  		if (intel_dp_is_edp(intel_dp) &&
>  		    !intel_dp_can_link_train_fallback_for_edp(intel_dp,
> @@ -675,6 +687,10 @@ int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
>  		return -1;
>  	}
>  
> +	drm_dbg_kms(&i915->drm,
> +		    "Retrying Link training with link rate %d and lane count %d\n",
> +		    intel_dp->max_link_rate, intel_dp->max_link_lane_count);
> +
>  	return 0;
>  }

-- 
Jani Nikula, Intel Open Source Graphics Center