Re: [PATCH RFC 4/4] drm/msm/dp: Add support for LTTPR handling

[Abel Vesa <abel.vesa@linaro.org>]

On 24-10-31 18:54:25, Dmitry Baryshkov wrote:
> On Thu, Oct 31, 2024 at 05:12:48PM +0200, Abel Vesa wrote:
> > Link Training Tunable PHY Repeaters (LTTPRs) are defined in DisplayPort
> > 1.4a specification. As the name suggests, these PHY repeaters are
> > capable of adjusting their output for link training purposes.
> > 
> > The msm DP driver is currently lacking any handling of LTTPRs.
> > This means that if at least one LTTPR is found between DPTX and DPRX,
> > the link training would fail if that LTTPR was not already configured
> > in transparent mode.
> 
> It might be nice to mention what is the transparent mode, especially for
> those who do not have the standard at hand.

Sorry for the late reply.

Will do in the next version.

> 
> > The section 3.6.6.1 from the DisplayPort v2.0 specification mandates
> > that before link training with the LTTPR is started, the DPTX may place
> > the LTTPR in non-transparent mode by first switching to transparent mode
> > and then to non-transparent mode. This operation seems to be needed only
> > on first link training and doesn't need to be done again until device is
> > unplugged.
> > 
> > It has been observed on a few X Elite-based platforms which have
> > such LTTPRs in their board design that the DPTX needs to follow the
> > procedure described above in order for the link training to be successful.
> > 
> > So add support for reading the LTTPR DPCD caps to figure out the number
> > of such LTTPRs first. Then, for platforms (or Type-C dongles) that have
> > at least one such an LTTPR, set its operation mode to transparent mode
> > first and then to non-transparent, just like the mentioned section of
> > the specification mandates.
> > 
> > Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
> > ---
> >  drivers/gpu/drm/msm/dp/dp_display.c | 25 +++++++++++++++++++++++++
> >  1 file changed, 25 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> > index f01980b0888a40b719d3958cb96c6341feada077..5d3d318d7b87ce3bf567d8b7435931d8e087f713 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_display.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> > @@ -107,6 +107,8 @@ struct dp_display_private {
> >  	struct dp_event event_list[DP_EVENT_Q_MAX];
> >  	spinlock_t event_lock;
> >  
> > +	u8 lttpr_caps[DP_LTTPR_COMMON_CAP_SIZE];
> > +
> >  	bool wide_bus_supported;
> >  
> >  	struct dp_audio *audio;
> > @@ -367,12 +369,35 @@ static int dp_display_send_hpd_notification(struct dp_display_private *dp,
> >  	return 0;
> >  }
> >  
> > +static void dp_display_lttpr_init(struct dp_display_private *dp)
> > +{
> > +	int lttpr_count;
> > +
> > +	if (drm_dp_read_lttpr_common_caps(dp->aux, dp->panel->dpcd,
> > +					  dp->lttpr_caps))
> > +		return;
> > +
> > +	lttpr_count = drm_dp_lttpr_count(dp->lttpr_caps);
> > +
> > +	if (lttpr_count) {
> > +		drm_dp_lttpr_set_transparent_mode(dp->aux, true);
> > +
> > +		if (lttpr_count > 0) {
> > +			if (drm_dp_lttpr_set_transparent_mode(dp->aux, false) != 1)
> > +				drm_dp_lttpr_set_transparent_mode(dp->aux, true);
> > +		}
> > +	}
> > +}
> > +
> >  static int dp_display_process_hpd_high(struct dp_display_private *dp)
> >  {
> >  	struct drm_connector *connector = dp->dp_display.connector;
> >  	const struct drm_display_info *info = &connector->display_info;
> >  	int rc = 0;
> >  
> > +	if (!dp->dp_display.is_edp)
> > +		dp_display_lttpr_init(dp);
> 
> Why is it limited to non-eDP cases only.

In case of eDP, I don't think that there will ever by a case that will
need an LTTPR in between the eDP PHY and the actual panel. It just
doesn't make sense.

IIUC, the LTTPRs, since are Training Tunnable capable, they help when
the physical link between the PHY and the sink can differ based on
different dongles and cables. This is obviously not applicable to eDP.

> 
> > +
> >  	rc = dp_panel_read_sink_caps(dp->panel, connector);
> >  	if (rc)
> >  		goto end;
> > 
> > -- 
> > 2.34.1
> > 
> 
> -- 
> With best wishes
> Dmitry