[PATCH 04/20] drm/i915/dp: Sanitize intel_dp_get_link_train_fallback_values()

[Imre Deak]

On Wed, May 15, 2024 at 04:05:32PM +0300, Jani Nikula wrote:
> On Tue, 14 May 2024, Imre Deak <imre.deak at intel.com> wrote:
> > Reduce the indentation in intel_dp_get_link_train_fallback_values() by
> > adding separate helpers to reduce the link rate and lane count. Also
> > simplify things by passing crtc_state to the function.
> >
> > This also prepares for later patches in the patchset adding a limitation
> > on how the link params are reduced.
> 
> Currently we have these rigid reductions of rate, lane and bpp. I don't
> think we want to lock in to this policy, but rather make it more
> flexible. So I think this patch is counter productive.
> 
> My grand plan I never got around to was to create a list/array of
> possible sets of link parameters that you could trivially sort, filter,
> iterate, and remove parameters from. (And I don't mean a statically
> written one, but a dynamically generated list.)
> 
> In that scheme, you'd get the fallback values by advancing one element
> in the data structure. And you know when you've exhausted all fallbacks
> when you're at the end. None of this complicated "if we've finished x,
> then try y, then z".

Yes, having a different fallback order than the current one makes sense
to me both BW utilization wise and because the standard describes it (as
an alternative?). I don't think this patch makes it more difficult to
impplement that, but the change does improve the readability of the
current code.

> And then you could sort the list based on bandwidth, and go from
> uncompressed uhbr to compressed non-uhbr to whatever.
> 
> 
> BR,
> Jani.
> 
> 
> 
> >
> > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > ---
> >  .../drm/i915/display/intel_dp_link_training.c | 82 ++++++++++++-------
> >  1 file changed, 51 insertions(+), 31 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > index 1b4694b46cea7..1ea4aaf9592f1 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > @@ -1109,11 +1109,37 @@ static bool intel_dp_can_link_train_fallback_for_edp(struct intel_dp *intel_dp,
> >  	return true;
> >  }
> >  
> > +static int reduce_link_rate(struct intel_dp *intel_dp, int current_rate)
> > +{
> > +	int rate_index;
> > +	int new_rate;
> > +
> > +	rate_index = intel_dp_rate_index(intel_dp->common_rates,
> > +					 intel_dp->num_common_rates,
> > +					 current_rate);
> > +
> > +	if (rate_index <= 0)
> > +		return -1;
> > +
> > +	new_rate = intel_dp_common_rate(intel_dp, rate_index - 1);
> > +
> > +	return new_rate;
> > +}
> > +
> > +static int reduce_lane_count(struct intel_dp *intel_dp, int current_lane_count)
> > +{
> > +	if (current_lane_count > 1)
> > +		return current_lane_count >> 1;
> > +
> > +	return -1;
> > +}
> > +
> >  static int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
> > -						   int link_rate, u8 lane_count)
> > +						   const struct intel_crtc_state *crtc_state)
> >  {
> >  	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> > -	int index;
> > +	int new_link_rate;
> > +	int new_lane_count;
> >  
> >  	/*
> >  	 * TODO: Enable fallback on MST links once MST link compute can handle
> > @@ -1131,36 +1157,32 @@ static int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
> >  		return 0;
> >  	}
> >  
> > -	index = intel_dp_rate_index(intel_dp->common_rates,
> > -				    intel_dp->num_common_rates,
> > -				    link_rate);
> > -	if (index > 0) {
> > -		if (intel_dp_is_edp(intel_dp) &&
> > -		    !intel_dp_can_link_train_fallback_for_edp(intel_dp,
> > -							      intel_dp_common_rate(intel_dp, index - 1),
> > -							      lane_count)) {
> > -			drm_dbg_kms(&i915->drm,
> > -				    "Retrying Link training for eDP with same parameters\n");
> > -			return 0;
> > -		}
> > -		intel_dp->link_train.max_rate = intel_dp_common_rate(intel_dp, index - 1);
> > -		intel_dp->link_train.max_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,
> > -							      intel_dp_max_common_rate(intel_dp),
> > -							      lane_count >> 1)) {
> > -			drm_dbg_kms(&i915->drm,
> > -				    "Retrying Link training for eDP with same parameters\n");
> > -			return 0;
> > -		}
> > -		intel_dp->link_train.max_rate = intel_dp_max_common_rate(intel_dp);
> > -		intel_dp->link_train.max_lane_count = lane_count >> 1;
> > -	} else {
> > +	new_lane_count = crtc_state->lane_count;
> > +	new_link_rate = reduce_link_rate(intel_dp, crtc_state->port_clock);
> > +	if (new_link_rate < 0) {
> > +		new_lane_count = reduce_lane_count(intel_dp, crtc_state->lane_count);
> > +		new_link_rate = intel_dp_max_common_rate(intel_dp);
> > +	}
> > +
> > +	if (new_lane_count < 0) {
> >  		drm_err(&i915->drm, "Link Training Unsuccessful\n");
> >  		return -1;
> >  	}
> >  
> > +	if (intel_dp_is_edp(intel_dp) &&
> > +	    !intel_dp_can_link_train_fallback_for_edp(intel_dp, new_link_rate, new_lane_count)) {
> > +		drm_dbg_kms(&i915->drm,
> > +			    "Retrying Link training for eDP with same parameters\n");
> > +		return 0;
> > +	}
> > +
> > +	drm_dbg_kms(&i915->drm, "Reducing link parameters from %dx%d to %dx%d\n",
> > +		    crtc_state->port_clock, crtc_state->lane_count,
> > +		    new_link_rate, new_lane_count);
> > +
> > +	intel_dp->link_train.max_rate = new_link_rate;
> > +	intel_dp->link_train.max_lane_count = new_lane_count;
> > +
> >  	return 0;
> >  }
> >  
> > @@ -1178,9 +1200,7 @@ static void intel_dp_schedule_fallback_link_training(struct intel_dp *intel_dp,
> >  		lt_dbg(intel_dp, DP_PHY_DPRX,
> >  		       "Link Training failed with HOBL active, not enabling it from now on\n");
> >  		intel_dp->hobl_failed = true;
> > -	} else if (intel_dp_get_link_train_fallback_values(intel_dp,
> > -							   crtc_state->port_clock,
> > -							   crtc_state->lane_count)) {
> > +	} else if (intel_dp_get_link_train_fallback_values(intel_dp, crtc_state)) {
> >  		return;
> >  	}
> 
> -- 
> Jani Nikula, Intel