On Fri, 02 Sep 2022, Ville Syrjala <ville.syrjala@xxxxxxxxxxxxxxx> wrote: > From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > > A lot of modern laptops use the Parade PS8461E MUX for eDP > switching. The MUX can operate in jitter cleaning mode or > redriver mode, the first one resulting in higher link > quality. The jitter cleaning mode needs to know the link > rate used and the MUX achieves this by snooping the > LINK_BW_SET, LINK_RATE_SELECT and SUPPORTED_LINK_RATES > DPCD accesses. > > When the MUX is powered down (seems this can happen whenever > the display is turned off) it loses track of the snooped > link rates so when we do the LINK_RATE_SELECT write it no > longer knowns which link rate we're selecting, and thus it > falls back to the lower quality redriver mode. This results > in unstable high link rates (eg. usually 8.1Gbps link rate > no longer works correctly). > > In order to avoid all that let's re-snoop SUPPORTED_LINK_RATES > from the sink at the start of every link training. > > Unfortunately we don't have a way to detect the presence of > the MUX. It looks like the set of laptops equipped with this > MUX is fairly large and contains devices from multiple > manufacturers. It may also still be growing with new models. > So a quirk doesn't seem like a very easily maintainable > option, thus we shall attempt to do this unconditionally on > all machines that use LINK_RATE_SELECT. Hopefully this extra > DPCD read doesn't cause issues for any unaffected machine. > If that turns out to be the case we'll need to convert this > into a quirk in the future. > > Cc: stable@xxxxxxxxxxxxxxx > Cc: Jason A. Donenfeld <Jason@xxxxxxxxx> > Cc: Ankit Nautiyal <ankit.k.nautiyal@xxxxxxxxx> > Cc: Jani Nikula <jani.nikula@xxxxxxxxx> > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6205 > Signed-off-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> Reviewed-by: Jani Nikula <jani.nikula@xxxxxxxxx> > --- > .../drm/i915/display/intel_dp_link_training.c | 22 +++++++++++++++++++ > 1 file changed, 22 insertions(+) > > 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 9feaf1a589f3..d213d8ad1ea5 100644 > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c > @@ -671,6 +671,28 @@ intel_dp_prepare_link_train(struct intel_dp *intel_dp, > intel_dp_compute_rate(intel_dp, crtc_state->port_clock, > &link_bw, &rate_select); > > + /* > + * WaEdpLinkRateDataReload > + * > + * Parade PS8461E MUX (used on varius TGL+ laptops) needs > + * to snoop the link rates reported by the sink when we > + * use LINK_RATE_SET in order to operate in jitter cleaning > + * mode (as opposed to redriver mode). Unfortunately it > + * loses track of the snooped link rates when powered down, > + * so we need to make it re-snoop often. Without this high > + * link rates are not stable. > + */ > + if (!link_bw) { > + struct intel_connector *connector = intel_dp->attached_connector; > + __le16 sink_rates[DP_MAX_SUPPORTED_RATES]; > + > + drm_dbg_kms(&i915->drm, "[CONNECTOR:%d:%s] Reloading eDP link rates\n", > + connector->base.base.id, connector->base.name); > + > + drm_dp_dpcd_read(&intel_dp->aux, DP_SUPPORTED_LINK_RATES, > + sink_rates, sizeof(sink_rates)); > + } > + > if (link_bw) > drm_dbg_kms(&i915->drm, > "[ENCODER:%d:%s] Using LINK_BW_SET value %02x\n", -- Jani Nikula, Intel Open Source Graphics Center
