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

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

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@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@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.

> 
> > +
> >  	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;
> >  

-- 
Ville Syrjälä
Intel
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