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

[Murthy, Arun R]

> -----Original Message-----
> From: Kandpal, Suraj <suraj.kandpal at intel.com>
> Sent: Tuesday, June 20, 2023 4:35 PM
> To: Murthy, Arun R <arun.r.murthy at intel.com>; intel-
> gfx at lists.freedesktop.org
> Subject: RE: [Intel-gfx] [PATCH] drm/i915/display/dp: Add fallback on LT
> failure for DP2.0
> 
> > 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.
> 
> Maybe adding the section where one can refer this in DP 2.0 spec can help.
> 
Ok Sure!

> >
> > Signed-off-by: Arun R Murthy <arun.r.murthy at 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;
> 
> NIT: Can we add a small comment here on what is being done.
> 
Comments are added below as to what each if condition is doing or is it something different that you are referring to?

> > +		if (intel_dp->dpcd[DP_MAIN_LINK_CHANNEL_CODING] &
> > DP_CAP_ANSI_128B132B) {
> > +			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;
> > +		}
> 
> Also I see a little issue here so previously what used to happen was we
> reduced the Link rate first and once we were at the lowest only then proceed
> to reduce the lane count so has this been reversed in DP2.0 spec.
> 
Yes here we reduce the lane count until 1 and then reduce the link rate.
Ref: Table 3-31 of DP2.1 Spec

Thanks and Regards,
Arun R Murthy
--------------------
> Regards,
> Suraj Kandpal
> 
> >  	} 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;
> >  }
> >
> > --
> > 2.25.1