Re: [Intel-gfx] [PATCH 4/6] drm/i915/dp: Ensure sink/link max lane count values are always valid

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Oct 18, 2021 at 06:13:19PM +0300, Imre Deak wrote:
> On Mon, Oct 18, 2021 at 06:04:18PM +0300, Ville Syrjälä wrote:
> > On Mon, Oct 18, 2021 at 12:41:52PM +0300, Imre Deak wrote:
> > > Print an error if the DPCD sink max lane count is invalid and fix it up.
> > > 
> > > While at it also add an assert that the link max lane count (derived
> > > from intel_dp_max_common_lane_count(), potentially reduced by the LT
> > > fallback logic) value is also valid.
> > > 
> > > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > > ---
> > >  .../drm/i915/display/intel_display_types.h    |  2 +
> > >  drivers/gpu/drm/i915/display/intel_dp.c       | 44 ++++++++++++++++++-
> > >  2 files changed, 44 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h b/drivers/gpu/drm/i915/display/intel_display_types.h
> > > index 39e11eaec1a3f..1e42bf901263c 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_display_types.h
> > > +++ b/drivers/gpu/drm/i915/display/intel_display_types.h
> > > @@ -1563,6 +1563,8 @@ struct intel_dp {
> > >  	int num_sink_rates;
> > >  	int sink_rates[DP_MAX_SUPPORTED_RATES];
> > >  	bool use_rate_select;
> > > +	/* Max sink lane count as reported by DP_MAX_LANE_COUNT */
> > > +	int max_sink_lane_count;
> > >  	/* intersection of source and sink rates */
> > >  	int num_common_rates;
> > >  	int common_rates[DP_MAX_SUPPORTED_RATES];
> > > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> > > index 1935eb49f9574..f7711779df132 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > > @@ -197,6 +197,35 @@ static void intel_dp_set_sink_rates(struct intel_dp *intel_dp)
> > >  	intel_dp->num_sink_rates = i;
> > >  }
> > >  
> > > +static void intel_dp_set_default_max_sink_lane_count(struct intel_dp *intel_dp)
> > > +{
> > > +	intel_dp->max_sink_lane_count = 1;
> > > +}
> > > +
> > > +static void intel_dp_set_max_sink_lane_count(struct intel_dp *intel_dp)
> > > +{
> > > +	struct intel_connector *connector = intel_dp->attached_connector;
> > > +	struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
> > > +	struct intel_encoder *encoder = &intel_dig_port->base;
> > > +
> > > +	intel_dp->max_sink_lane_count = drm_dp_max_lane_count(intel_dp->dpcd);
> > > +
> > > +	switch (intel_dp->max_sink_lane_count) {
> > > +	case 1:
> > > +	case 2:
> > > +	case 4:
> > > +		return;
> > > +	}
> > > +
> > > +	drm_err(&dp_to_i915(intel_dp)->drm,
> > > +		"[CONNECTOR:%d:%s][ENCODER:%d:%s] Invalid DPCD max lane count (%d), using default\n",
> > > +		connector->base.base.id, connector->base.name,
> > > +		encoder->base.base.id, encoder->base.name,
> > > +		intel_dp->max_sink_lane_count);
> > > +
> > > +	intel_dp_set_default_max_sink_lane_count(intel_dp);
> > > +}
> > > +
> > >  /* Get length of rates array potentially limited by max_rate. */
> > >  static int intel_dp_rate_limit_len(const int *rates, int len, int max_rate)
> > >  {
> > > @@ -230,7 +259,7 @@ static int intel_dp_max_common_lane_count(struct intel_dp *intel_dp)
> > >  {
> > >  	struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
> > >  	int source_max = dig_port->max_lanes;
> > > -	int sink_max = drm_dp_max_lane_count(intel_dp->dpcd);
> > > +	int sink_max = intel_dp->max_sink_lane_count;
> > >  	int fia_max = intel_tc_port_fia_max_lane_count(dig_port);
> > >  	int lttpr_max = drm_dp_lttpr_max_lane_count(intel_dp->lttpr_common_caps);
> > >  
> > > @@ -242,7 +271,15 @@ static int intel_dp_max_common_lane_count(struct intel_dp *intel_dp)
> > >  
> > >  int intel_dp_max_lane_count(struct intel_dp *intel_dp)
> > >  {
> > > -	return intel_dp->max_link_lane_count;
> > > +	switch (intel_dp->max_link_lane_count) {
> > > +	case 1:
> > > +	case 2:
> > > +	case 4:
> > > +		return intel_dp->max_link_lane_count;
> > > +	default:
> > > +		MISSING_CASE(intel_dp->max_link_lane_count);
> > > +		return 1;
> > > +	}
> > >  }
> > 
> > I guess this is just a second level sanity check. I was wondering it
> > gets confusing and people start thinking this can actually happen,
> > but I suppose the MISSING_CASE() should be indication enough that it
> > in fact should not happen.
> 
> Yes it shouldn't happen. Given that we don't consider the FIA reg value
> external, but I think that's a reasonable assumption.
> 
> > Series looks sane to me:
> > Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> Thanks.
> 
> > BTW there's now some kms_bw thing in igt which seems to forcing
> > DP connectors on left and right, and thus triggering a bunch of 
> > WARNs.
> 
> Yes, noticed. This series should fix the WARN, however the modeset will
> still fail, due to using the minimum link_rate/lane_count set by default
> in this patchset. But due to the LT failing and fallback reducing the
> link params it would anyway fail after the first modeset. I wonder what
> would be a good solution if the above kind of use case is important
> enough (I at least use this for debugging). Maybe a virtual loopback
> connector which could handle LT (in kernel perhaps) and even any AUX
> traffic (in userspace)? Anyway that's far beyond the scope of this
> patchset.

Yeah, a virtual DP sink with DPCD and the works is something
I've occasionally pondered. But it would take some actual work.

-- 
Ville Syrjälä
Intel