drm/i915/dp: Don't switch the LTTPR mode on an active link

Switching to transparent mode leads to a loss of link synchronization,
so prevent doing this on an active link. This happened at least on an
Intel N100 system / DELL UD22 dock, the LTTPR residing either on the
host or the dock. To fix the issue, keep the current mode on an active
link, adjusting the LTTPR count accordingly (resetting it to 0 in
transparent mode).

v2: Adjust code comment during link training about reiniting the LTTPRs.
   (Ville)

Fixes: 7b2a4ab8 ("drm/i915: Switch to LTTPR transparent mode link training")
Reported-and-tested-by: Gareth Yu <gareth.yu@intel.com>
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/10902
Cc: <stable@vger.kernel.org> # v5.15+
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240708190029.271247-3-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_dp_link_training.c

@@ -117,10 +117,24 @@ intel_dp_set_lttpr_transparent_mode(struct intel_dp *intel_dp, bool enable)
 	return drm_dp_dpcd_write(&intel_dp->aux, DP_PHY_REPEATER_MODE, &val, 1) == 1;
 }
 
-static int intel_dp_init_lttpr(struct intel_dp *intel_dp, const u8 dpcd[DP_RECEIVER_CAP_SIZE])
+static bool intel_dp_lttpr_transparent_mode_enabled(struct intel_dp *intel_dp)
+{
+	return intel_dp->lttpr_common_caps[DP_PHY_REPEATER_MODE -
+					   DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV] ==
+		DP_PHY_REPEATER_MODE_TRANSPARENT;
+}
+
+/*
+ * Read the LTTPR common capabilities and switch the LTTPR PHYs to
+ * non-transparent mode if this is supported. Preserve the
+ * transparent/non-transparent mode on an active link.
+ *
+ * Return the number of detected LTTPRs in non-transparent mode or 0 if the
+ * LTTPRs are in transparent mode or the detection failed.
+ */
+static int intel_dp_init_lttpr_phys(struct intel_dp *intel_dp, const u8 dpcd[DP_RECEIVER_CAP_SIZE])
 {
 	int lttpr_count;
-	int i;
 
 	if (!intel_dp_read_lttpr_common_caps(intel_dp, dpcd))
 		return 0;
@@ -134,6 +148,19 @@ static int intel_dp_init_lttpr(struct intel_dp *intel_dp, const u8 dpcd[DP_RECEI
 	if (lttpr_count == 0)
 		return 0;
 
+	/*
+	 * Don't change the mode on an active link, to prevent a loss of link
+	 * synchronization. See DP Standard v2.0 3.6.7. about the LTTPR
+	 * resetting its internal state when the mode is changed from
+	 * non-transparent to transparent.
+	 */
+	if (intel_dp->link_trained) {
+		if (lttpr_count < 0 || intel_dp_lttpr_transparent_mode_enabled(intel_dp))
+			goto out_reset_lttpr_count;
+
+		return lttpr_count;
+	}
+
 	/*
 	 * See DP Standard v2.0 3.6.6.1. about the explicit disabling of
 	 * non-transparent mode and the disable->enable non-transparent mode
@@ -154,11 +181,25 @@ static int intel_dp_init_lttpr(struct intel_dp *intel_dp, const u8 dpcd[DP_RECEI
 		       "Switching to LTTPR non-transparent LT mode failed, fall-back to transparent mode\n");
 
 		intel_dp_set_lttpr_transparent_mode(intel_dp, true);
-		intel_dp_reset_lttpr_count(intel_dp);
 
-		return 0;
+		goto out_reset_lttpr_count;
 	}
 
+	return lttpr_count;
+
+out_reset_lttpr_count:
+	intel_dp_reset_lttpr_count(intel_dp);
+
+	return 0;
+}
+
+static int intel_dp_init_lttpr(struct intel_dp *intel_dp, const u8 dpcd[DP_RECEIVER_CAP_SIZE])
+{
+	int lttpr_count;
+	int i;
+
+	lttpr_count = intel_dp_init_lttpr_phys(intel_dp, dpcd);
+
 	for (i = 0; i < lttpr_count; i++)
 		intel_dp_read_lttpr_phy_caps(intel_dp, dpcd, DP_PHY_LTTPR(i));
 
@@ -1482,10 +1523,10 @@ void intel_dp_start_link_train(struct intel_atomic_state *state,
 	struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
 	struct intel_encoder *encoder = &dig_port->base;
 	bool passed;
-
 	/*
-	 * TODO: Reiniting LTTPRs here won't be needed once proper connector
-	 * HW state readout is added.
+	 * Reinit the LTTPRs here to ensure that they are switched to
+	 * non-transparent mode. During an earlier LTTPR detection this
+	 * could've been prevented by an active link.
 	 */
 	int lttpr_count = intel_dp_init_lttpr_and_dprx_caps(intel_dp);