Re: [Intel-gfx] [PATCH 01/17] drm/dp: add DP 2.0 UHBR link rate and bw code conversions

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Aug 18, 2021 at 09:10:36PM +0300, Jani Nikula wrote:
> The bw code equals link_rate / 0.27 Gbps only for 8b/10b link
> rates. Handle DP 2.0 UHBR rates as special cases, though this is not
> pretty.

Ugh. So if I'm reading the spec right the behaviour of this
register now changes dynamically depending on the state of
some other bit in another register?

> 
> Cc: dri-devel@lists.freedesktop.org
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 6d0f2c447f3b..9b2a2961fca8 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -207,15 +207,33 @@ EXPORT_SYMBOL(drm_dp_lttpr_link_train_channel_eq_delay);
>  
>  u8 drm_dp_link_rate_to_bw_code(int link_rate)
>  {
> -	/* Spec says link_bw = link_rate / 0.27Gbps */
> -	return link_rate / 27000;
> +	switch (link_rate) {
> +	case 1000000:
> +		return DP_LINK_BW_10;
> +	case 1350000:
> +		return DP_LINK_BW_13_5;
> +	case 2000000:
> +		return DP_LINK_BW_20;
> +	default:
> +		/* Spec says link_bw = link_rate / 0.27Gbps */
> +		return link_rate / 27000;
> +	}
>  }
>  EXPORT_SYMBOL(drm_dp_link_rate_to_bw_code);
>  
>  int drm_dp_bw_code_to_link_rate(u8 link_bw)
>  {
> -	/* Spec says link_rate = link_bw * 0.27Gbps */
> -	return link_bw * 27000;
> +	switch (link_bw) {
> +	case DP_LINK_BW_10:
> +		return 1000000;
> +	case DP_LINK_BW_13_5:
> +		return 1350000;
> +	case DP_LINK_BW_20:
> +		return 2000000;
> +	default:
> +		/* Spec says link_rate = link_bw * 0.27Gbps */
> +		return link_bw * 27000;
> +	}
>  }
>  EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);
>  
> -- 
> 2.20.1

-- 
Ville Syrjälä
Intel