Re: [Intel-gfx] [PATCH 5/7] drm/i915: Reverse preemph vs. voltage swing preference

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Manasi Navare <manasi.d.navare@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH 5/7] drm/i915: Reverse preemph vs. voltage swing preference
Date: Fri, 15 May 2020 22:59:57 +0300	[thread overview]
Message-ID: <20200515195957.GO6112@intel.com> (raw)
In-Reply-To: <20200515191822.GB20478@intel.com>

On Fri, May 15, 2020 at 12:18:22PM -0700, Manasi Navare wrote:
> On Tue, May 12, 2020 at 08:41:43PM +0300, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > The DP spec says:
> > "When the combination of the requested pre-emphasis level and
> >  voltage swing exceeds the capability of a DPTX, the DPTX shall
> >  set the pre-emphasis level according to the request and use the
> >  highest voltage swing it can output with the given pre-emphasis level."
> > and
> > "When a DPTX reads a request beyond the limits of this Standard,
> >  the DPTX shall set the pre-emphasis level according to the request
> >  and set the highest voltage swing level it can output with the
> >  given pre-emphasis level. If a DPTX is requested for 9.5dB of
> >  pre-emphasis level (may be supported for a DPTX) and cannot support
> >  that level, it shall set the pre-emphasis level to the next
> >  highest level, 6dB."
> > 
> > Ie. we should first validate the pre-emphasis, and then select
> > the appropriate vswing for it.
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> So basically reverse the logic for selecting the vswing and pre emphasis
> 
> > ---
> >  .../drm/i915/display/intel_dp_link_training.c | 32 +++++++++----------
> >  1 file changed, 16 insertions(+), 16 deletions(-)
> > 
> > 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 171d9e842fc0..573f93779449 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > @@ -34,18 +34,18 @@ intel_dp_dump_link_status(const u8 link_status[DP_LINK_STATUS_SIZE])
> >  		      link_status[3], link_status[4], link_status[5]);
> >  }
> >  
> > -static u8 dp_pre_emphasis_max(u8 voltage_swing)
> > +static u8 dp_voltage_max(u8 preemph)
> >  {
> > -	switch (voltage_swing & DP_TRAIN_VOLTAGE_SWING_MASK) {
> > -	case DP_TRAIN_VOLTAGE_SWING_LEVEL_0:
> > -		return DP_TRAIN_PRE_EMPH_LEVEL_3;
> > -	case DP_TRAIN_VOLTAGE_SWING_LEVEL_1:
> > -		return DP_TRAIN_PRE_EMPH_LEVEL_2;
> > -	case DP_TRAIN_VOLTAGE_SWING_LEVEL_2:
> > -		return DP_TRAIN_PRE_EMPH_LEVEL_1;
> > -	case DP_TRAIN_VOLTAGE_SWING_LEVEL_3:
> > +	switch (preemph & DP_TRAIN_PRE_EMPHASIS_MASK) {
> > +	case DP_TRAIN_PRE_EMPH_LEVEL_0:
> > +		return DP_TRAIN_VOLTAGE_SWING_LEVEL_3;
> > +	case DP_TRAIN_PRE_EMPH_LEVEL_1:
> > +		return DP_TRAIN_VOLTAGE_SWING_LEVEL_2;
> > +	case DP_TRAIN_PRE_EMPH_LEVEL_2:
> > +		return DP_TRAIN_VOLTAGE_SWING_LEVEL_1;
> > +	case DP_TRAIN_PRE_EMPH_LEVEL_3:
> >  	default:
> > -		return DP_TRAIN_PRE_EMPH_LEVEL_0;
> > +		return DP_TRAIN_VOLTAGE_SWING_LEVEL_0;
> 
> These vswing levels for that specific pre emph level comes from the Bspec
> or from the DP spec? It wasnt clear to me how level3 of vswing was the max for pre emphasis level 0 and all others?

From DP 1.4 spec "Table 3-1: Allowed Vdiff_pre_pp and Pre-emphasis
Combinations"

Previosuly this was present in some semi-mangled way in each
platform's max preeph calculation. Now we just have one canonical
copy of it. Later on we could probably lift this into drm_dp_helper.

-- 
Ville Syrjälä
Intel
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