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

[Manasi Navare]

On Fri, May 15, 2020 at 10:59:57PM +0300, Ville Syrjälä wrote:
> 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 at 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 at 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.

Okay great yes confirmed from that table and looks good to me

Reviewed-by: Manasi Navare <manasi.d.navare at intel.com>

Manasi

> 
> -- 
> Ville Syrjälä
> Intel