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

From: Imre Deak <imre.deak@intel.com>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH 4/6] drm/i915/dp: Ensure sink/link max lane count values are always valid
Date: Mon, 18 Oct 2021 18:13:19 +0300	[thread overview]
Message-ID: <20211018151319.GD1409539@ideak-desk.fi.intel.com> (raw)
In-Reply-To: <YW2M8h02rslxYBSO@intel.com>

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.

> >  /*
> > @@ -2600,6 +2637,7 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
> >  		intel_dp->use_rate_select = true;
> >  	else
> >  		intel_dp_set_sink_rates(intel_dp);
> > +	intel_dp_set_max_sink_lane_count(intel_dp);
> >  
> >  	intel_dp_set_common_rates(intel_dp);
> >  	intel_dp_reset_max_link_params(intel_dp);
> > @@ -2645,6 +2683,7 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
> >  				 drm_dp_is_branch(intel_dp->dpcd));
> >  
> >  		intel_dp_set_sink_rates(intel_dp);
> > +		intel_dp_set_max_sink_lane_count(intel_dp);
> >  		intel_dp_set_common_rates(intel_dp);
> >  	}
> >  
> > @@ -5011,6 +5050,7 @@ intel_dp_init_connector(struct intel_digital_port *dig_port,
> >  
> >  	intel_dp_set_source_rates(intel_dp);
> >  	intel_dp_set_default_sink_rates(intel_dp);
> > +	intel_dp_set_default_max_sink_lane_count(intel_dp);
> >  	intel_dp_set_common_rates(intel_dp);
> >  	intel_dp_reset_max_link_params(intel_dp);
> >  
> > -- 
> > 2.27.0
> 
> -- 
> Ville Syrjälä
> Intel