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

[Manasi Navare <manasi.d.navare@intel.com>]

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?

Manasi

>  	}
>  }
>  
> @@ -68,15 +68,15 @@ void intel_dp_get_adjust_train(struct intel_dp *intel_dp,
>  			p = this_p;
>  	}
>  
> -	voltage_max = intel_dp->voltage_max(intel_dp);
> -	if (v >= voltage_max)
> -		v = voltage_max | DP_TRAIN_MAX_SWING_REACHED;
> -
> -	preemph_max = min(intel_dp->preemph_max(intel_dp),
> -			  dp_pre_emphasis_max(v));
> +	preemph_max = intel_dp->preemph_max(intel_dp);
>  	if (p >= preemph_max)
>  		p = preemph_max | DP_TRAIN_MAX_PRE_EMPHASIS_REACHED;
>  
> +	voltage_max = min(intel_dp->voltage_max(intel_dp),
> +			  dp_voltage_max(p));
> +	if (v >= voltage_max)
> +		v = voltage_max | DP_TRAIN_MAX_SWING_REACHED;
> +
>  	for (lane = 0; lane < 4; lane++)
>  		intel_dp->train_set[lane] = v | p;
>  }
> -- 
> 2.26.2
> 
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