Re: [PATCH] drm/i915: make edp optimize config

["Atwood, Matthew S" <matthew.s.atwood@intel.com>]

On Fri, 2018-03-09 at 14:05 +0200, Jani Nikula wrote:
> On Thu, 08 Mar 2018, matthew.s.atwood@intel.com wrote:
> > 
> > From: Matt Atwood <matthew.s.atwood@intel.com>
> > 
> > Previously it was assumed that eDP panels would advertise the
> > lowest link
> > rate required for their singular mode to function. With the
> > introduction
> > of more advanced features there are advantages to a panel
> > advertising a
> > higher rate then it needs for a its given mode. For panels that
> > did, the
> > driver previously used a higher rate then necessary for that mode.
> > 
> > Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 10 ----------
> >  1 file changed, 10 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c
> > b/drivers/gpu/drm/i915/intel_dp.c
> > index a2eeede..aa6d77d 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -1758,16 +1758,6 @@ intel_dp_compute_config(struct intel_encoder
> > *encoder,
> >  				      dev_priv->vbt.edp.bpp);
> >  			bpp = dev_priv->vbt.edp.bpp;
> >  		}
> > -
> > -		/*
> > -		 * Use the maximum clock and number of lanes the
> > eDP panel
> > -		 * advertizes being capable of. The panels are
> > generally
> > -		 * designed to support only a single clock and
> > lane
> > -		 * configuration, and typically these values
> > correspond to the
> > -		 * native resolution of the panel.
> > -		 */
> > -		min_lane_count = max_lane_count;
> > -		min_clock = max_clock;
> Please see my reply to Manasi's identical patch [1]. If we apply this
> as-is, it will regress and will be reverted.
> 
> BR,
> Jani.
> 
> 
> [1] http://patchwork.freedesktop.org/patch/msgid/1520579339-14745-1-
> git-send-email-manasi.d.navare@intel.com
to consolidate some of the information the bug that's referenced in
Manasi's patch is https://bugs.freedesktop.org/show_bug.cgi?id=73539. I
understand this bug as the following some panels may advertise a mode
that requires less then MAX_LANE_COUNT, however those panels would fail
if less lanes were used.

When this bug was filed the compute config inner for loop iterated on
rate and the outer iterated on lanes; this is now flipped. It *should*
be the case that the lowest frequency with the max amount of lanes is
preferred. Given the opposite behavior of the alogorithm to select I
dont think we'd come across this again. Even if I'm wrong we could
still min_lane_count = max_lane count and let the clock optimize
itself.

I guess my question is, is there also a bug where if MAX_RATE wasnt
used we saw a panel fail as well?

Matt
> 
> 
> > 
> >  	}
> >  
> >  	for (; bpp >= 6*3; bpp -= 2*3) {
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