[Intel-gfx] [PATCH] drm/i915: WARN if max vswing/pre-emphasis violates the DP spec

[Souza, Jose]

On Fri, 2020-07-10 at 15:02 +0300, Ville Syrjälä wrote:
> On Thu, Jul 09, 2020 at 08:14:05PM +0000, Souza, Jose wrote:
> > On Thu, 2020-07-09 at 17:58 +0300, Ville Syrjala wrote:
> > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > 
> > > According to the DP spec a DPTX must support vswing/pre-emphasis
> > > up to and including level 2. Level 3 is optional (actually DP 1.4a
> > > seems to make even level 3 mandatory for HBR2/3, while leaving it
> > > optional for RBR/HBR1).
> > > 
> > > WARN if out encoders' .voltage_max()/.preemph_max() return
> > > an illegal value.
> > > 
> > > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/display/intel_dp_link_training.c | 9 +++++++++
> > >  1 file changed, 9 insertions(+)
> > > 
> > > 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 2493142a70e9..a23ed7290843 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > > @@ -52,6 +52,7 @@ static u8 dp_voltage_max(u8 preemph)
> > >  void intel_dp_get_adjust_train(struct intel_dp *intel_dp,
> > >  			       const u8 link_status[DP_LINK_STATUS_SIZE])
> > >  {
> > > +	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> > >  	u8 v = 0;
> > >  	u8 p = 0;
> > >  	int lane;
> > > @@ -64,12 +65,20 @@ void intel_dp_get_adjust_train(struct intel_dp *intel_dp,
> > >  	}
> > >  
> > >  	preemph_max = intel_dp->preemph_max(intel_dp);
> > > +	drm_WARN_ON_ONCE(&i915->drm,
> > > +			 preemph_max != DP_TRAIN_PRE_EMPH_LEVEL_2 &&
> > > +			 preemph_max != DP_TRAIN_PRE_EMPH_LEVEL_3);
> > 
> > Okay matches but I guess the preemph_max function should receive as parameter the voltage level to it return the max preemph of that voltage level,
> > no?
> 
> That was the old way which didn't match the spec. With
> the new way those just return the overall maximum that
> we support. And the dp_voltage_max() call below fixes up
> any out of spec requests.

It still do not handle a table that do not have DP_TRAIN_VOLTAGE_SWING_LEVEL_0 | DP_TRAIN_PRE_EMPH_LEVEL_3, but in this case we would be using a wrong
entry.

The warnings LGTM.
Reviewed-by: José Roberto de Souza <jose.souza at intel.com>


> 
> > > +
> > >  	if (p >= preemph_max)
> > >  		p = preemph_max | DP_TRAIN_MAX_PRE_EMPHASIS_REACHED;
> > >  
> > >  	v = min(v, dp_voltage_max(p));
> > >  
> > >  	voltage_max = intel_dp->voltage_max(intel_dp);
> > > +	drm_WARN_ON_ONCE(&i915->drm,
> > > +			 voltage_max != DP_TRAIN_VOLTAGE_SWING_LEVEL_2 &&
> > > +			 voltage_max != DP_TRAIN_VOLTAGE_SWING_LEVEL_3);
> > > +
> > >  	if (v >= voltage_max)
> > >  		v = voltage_max | DP_TRAIN_MAX_SWING_REACHED;
> > >