Re: [PATCH v4 4/4] drm/msm/dp: Add support for LTTPR handling

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

On 25-01-08 14:57:41, Abhinav Kumar wrote:
> 
> 
> On 1/8/2025 6:31 AM, 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.
> > 
> > According to the DisplayPort standard, LTTPRs have two operating
> > modes:
> >   - non-transparent - it replies to DPCD LTTPR field specific AUX
> >     requests, while passes through all other AUX requests
> >   - transparent - it passes through all AUX requests.
> > 
> > Switching between this two modes is done by the DPTX by issuing
> > an AUX write to the DPCD PHY_REPEATER_MODE register.
> > 
> > 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.
> > 
> > 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.
> > 
> > Tested-by: Johan Hovold <johan+linaro@kernel.org>
> > Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> > Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
> > Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
> > ---
> >   drivers/gpu/drm/msm/dp/dp_display.c | 17 +++++++++++++++++
> >   1 file changed, 17 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> > index 24dd37f1682bf5016bb0efbeb44489061deff060..ad09daa4c8ab5c0eb67890509b94e72820bab870 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_display.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> > @@ -107,6 +107,8 @@ struct msm_dp_display_private {
> >   	struct msm_dp_event event_list[DP_EVENT_Q_MAX];
> >   	spinlock_t event_lock;
> > +	u8 lttpr_caps[DP_LTTPR_COMMON_CAP_SIZE];
> > +
> 
> The reason downstream stored it panel is to read it first in dp_panel's
> read_sink_caps and call lttpr_init if drm_dp_lttpr_count() is non-zero.
> 
> But here it looks like  msm_dp_display_lttpr_init() internally handles this
> for us. So no need to store this?

Replied to Bjorn already about this and why I thought it was a good idea
to add it.

But will drop in the next version.

> 
> >   	bool wide_bus_supported;
> >   	struct msm_dp_audio *audio;
> > @@ -367,12 +369,27 @@ static int msm_dp_display_send_hpd_notification(struct msm_dp_display_private *d
> >   	return 0;
> >   }
> > +static void msm_dp_display_lttpr_init(struct msm_dp_display_private *dp)
> > +{
> > +	int rc;
> > +
> > +	if (drm_dp_read_lttpr_common_caps(dp->aux, dp->panel->dpcd,
> > +					  dp->lttpr_caps))
> > +		return;
> > +
> > +	rc = drm_dp_lttpr_init(dp->aux, drm_dp_lttpr_count(dp->lttpr_caps));
> > +	if (rc)
> > +		DRM_ERROR("failed to set LTTPRs transparency mode, rc=%d\n", rc);
> > +}
> > +
> >   static int msm_dp_display_process_hpd_high(struct msm_dp_display_private *dp)
> >   {
> >   	struct drm_connector *connector = dp->msm_dp_display.connector;
> >   	const struct drm_display_info *info = &connector->display_info;
> >   	int rc = 0;
> > +	msm_dp_display_lttpr_init(dp);
> > +
> 
> Can you pls move this call after msm_dp_panel_read_sink_caps()?
> 
> If msm_dp_panel_read_sink_caps() fails there is no need to call
> msm_dp_display_lttpr_init().

Sure.

> 
> 
> >   	rc = msm_dp_panel_read_sink_caps(dp->panel, connector);
> >   	if (rc)
> >   		goto end;
> > 

Thanks for reviewing!

Abel