[Intel-gfx] [PATCH 3/4] drm/i915: Cache sink_count for eDP

[Souza, Jose]

On Tue, 2018-10-09 at 16:27 +0300, Ville Syrjälä wrote:
> On Mon, Oct 08, 2018 at 05:54:17PM -0700, Dhinakaran Pandiyan wrote:
> > On Mon, 2018-10-08 at 17:35 -0700, Souza, Jose wrote:
> > > On Mon, 2018-10-08 at 17:19 -0700, Dhinakaran Pandiyan wrote:
> > > > On Fri, 2018-10-05 at 16:35 -0700, José Roberto de Souza wrote:
> > > > > For eDP panels all the DPCD and EDID data is cached when
> > > > > initializing
> > > > > the eDP connector so in futher detection it do not call
> > > > > intel_dp_detect_dpcd() for eDP.
> > > > > The problem is on the first short pulse interruption it calls
> > > > > intel_dp_get_dpcd() for eDP and DP and it will read and set
> > > > > the
> > > > > sink
> > > > > count, causing a mismatch between old sink count and the new
> > > > > one
> > > > > triggering a full detection without needed.
> > > > > 
> > > > > Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
> > > > > Signed-off-by: José Roberto de Souza <jose.souza at intel.com>
> > > > > ---
> > > > >  drivers/gpu/drm/i915/intel_dp.c | 5 +++++
> > > > >  1 file changed, 5 insertions(+)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/i915/intel_dp.c
> > > > > b/drivers/gpu/drm/i915/intel_dp.c
> > > > > index 19f0c3f59cbe..4a1c31ec9065 100644
> > > > > --- a/drivers/gpu/drm/i915/intel_dp.c
> > > > > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > > > > @@ -3926,6 +3926,7 @@ intel_edp_init_dpcd(struct intel_dp
> > > > > *intel_dp)
> > > > >  {
> > > > >  	struct drm_i915_private *dev_priv =
> > > > >  		to_i915(dp_to_dig_port(intel_dp)-
> > > > > >base.base.dev);
> > > > > +	u8 val;
> > > > >  
> > > > >  	/* this function is meant to be called only once */
> > > > >  	WARN_ON(intel_dp->dpcd[DP_DPCD_REV] != 0);
> > > > > @@ -3997,6 +3998,10 @@ intel_edp_init_dpcd(struct intel_dp
> > > > > *intel_dp)
> > > > >  
> > > > >  	intel_dp_set_common_rates(intel_dp);
> > > > >  
> > > > > +	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_SINK_COUNT,
> > > > > &val) <=
> > > > > 0)
> > > > > +		return false;
> > > > > +	intel_dp->sink_count = DP_GET_SINK_COUNT(val);
> > > > 
> > > > Is this even relevant for eDPs? Seems unnecessary to read or
> > > > compare
> > > > sink count for eDP. I'd suggest skipping DP_SINK_COUNT checks
> > > > for
> > > > eDP.
> > > 
> > > I'm not sure as DP specs for DP_SINK_COUNT says:
> > > 
> > > The Sink device shall add one more if it has a local Rendering
> > > Function.
> > > 
> > > and eDP spec do not redefine or alter this, so I guess is more
> > > safe
> > > also read for eDP too.
> > > 
> > 
> > We already special case eDP in several places, for example, don't
> > update link rates from the short pulse handler etc. And also don't
> > support hotplug, I don't see a point.
> 
> IIRC some conformance test or something required that we read this.
> I guess what we could do is still read it but just not update
> intel_dp->sink_count. We already seem to have a special case which
> ignores a zero sink_count on eDP. Might as well extend that a bit
> I suppose.

Okay, I will skip the comparison for eDP.

> 
> In general I think special cases are bad, so IMO we should try
> hard not add more unless really necessary. In this case it seems
> the special case is warranted. Unfortunately commit 1034ce707b57
> ("drm/i915: Fixing eDP detection on certain platforms") failed to add
> a comment explaining why. I'd appreciate if someone could add that
> comment now so that we don't forget this in the future.

Sure, I will add this comment.

>