drm/msm/dp: Add support for LTTPR handling

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 these 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>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250203-drm-dp-msm-add-lttpr-transparent-mode-set-v5-4-c865d0e56d6e@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

Author: abelvesa

drivers/gpu/drm/msm/dp/dp_display.c

@@ -367,6 +367,19 @@ 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)
+{
+	u8 lttpr_caps[DP_LTTPR_COMMON_CAP_SIZE];
+	int rc;
+
+	if (drm_dp_read_lttpr_common_caps(dp->aux, dp->panel->dpcd, lttpr_caps))
+		return;
+
+	rc = drm_dp_lttpr_init(dp->aux, drm_dp_lttpr_count(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;
@@ -377,6 +390,8 @@ static int msm_dp_display_process_hpd_high(struct msm_dp_display_private *dp)
 	if (rc)
 		goto end;
 
+	msm_dp_display_lttpr_init(dp);
+
 	msm_dp_link_process_request(dp->link);
 
 	if (!dp->msm_dp_display.is_edp)