Re: [PATCH] drm/i915/dp: Add current maximum eDP link rate to sink_rate array.

[Alex Deucher <alexdeucher@gmail.com>]

On Thu, Jan 9, 2020 at 10:08 AM Mario Kleiner
<mario.kleiner.de@gmail.com> wrote:
>
> If the current eDP link rate, as read from hw, provides a
> higher bandwidth than the standard link rates, then add the
> current link rate to the link_rates array for consideration
> in future mode-sets.
>
> These initial current eDP link settings have been set up by
> firmware during boot, so they should work on the eDP panel.
> Therefore use them if the firmware thinks they are good and
> they provide higher link bandwidth, e.g., to enable higher
> resolutions / color depths.
>
> This fixes a problem found on the MacBookPro 2017 Retina panel:
>
> The panel reports 10 bpc color depth in its EDID, and the UEFI
> firmware chooses link settings at boot which support enough
> bandwidth for 10 bpc (324000 kbit/sec to be precise), but the
> DP_MAX_LINK_RATE dpcd register only reports 2.7 Gbps as possible,
> so intel_dp_set_sink_rates() would cap at that. This restricts
> achievable color depth to 8 bpc, not providing the full color
> depth of the panel. With this commit, we can use firmware setting
> and get the full 10 bpc advertised by the Retina panel.

Would it make more sense to just add a quirk for this particular
panel?  Would there be cases where the link was programmed wrong and
then we end up using that additional link speed as supported?

Alex

>
> Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 2f31d226c6eb..aa3e0b5108c6 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -4368,6 +4368,8 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
>  {
>         struct drm_i915_private *dev_priv =
>                 to_i915(dp_to_dig_port(intel_dp)->base.base.dev);
> +       int max_rate;
> +       u8 link_bw;
>
>         /* this function is meant to be called only once */
>         WARN_ON(intel_dp->dpcd[DP_DPCD_REV] != 0);
> @@ -4433,6 +4435,27 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
>         else
>                 intel_dp_set_sink_rates(intel_dp);
>
> +       /*
> +        * If the firmware programmed a rate higher than the standard sink rates
> +        * during boot, then add that rate as a valid sink rate, as fw knows
> +        * this is a good rate and we get extra bandwidth.
> +        *
> +        * Helps, e.g., on the Apple MacBookPro 2017 Retina panel, which is only
> +        * eDP 1.1, but supports the unusual rate of 324000 kHz at bootup, for
> +        * 10 bpc / 30 bit color depth.
> +        */
> +       if (!intel_dp->use_rate_select &&
> +           (drm_dp_dpcd_read(&intel_dp->aux, DP_LINK_BW_SET, &link_bw, 1) == 1) &&
> +           (link_bw > 0) && (intel_dp->num_sink_rates < DP_MAX_SUPPORTED_RATES)) {
> +               max_rate = drm_dp_bw_code_to_link_rate(link_bw);
> +               if (max_rate > intel_dp->sink_rates[intel_dp->num_sink_rates - 1]) {
> +                       intel_dp->sink_rates[intel_dp->num_sink_rates] = max_rate;
> +                       intel_dp->num_sink_rates++;
> +                       DRM_DEBUG_KMS("Adding max bandwidth eDP rate %d kHz.\n",
> +                                     max_rate);
> +               }
> +       }
> +
>         intel_dp_set_common_rates(intel_dp);
>
>         /* Read the eDP DSC DPCD registers */
> --
> 2.24.0
>
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