drm/amd/display: fix link training regression for 1 or 2 lane

[why]
We have a regression that cause maximize lane settings to use
uninitialized data from unused lanes.
This will cause link training to fail for 1 or 2 lanes because the lane
adjust is populated incorrectly sometimes.

v2: fix build without CONFIG_DRM_AMD_DC_DCN (Alex)
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -108,6 +108,9 @@ static struct dc_link_settings get_common_supported_link_settings(
 		struct dc_link_settings link_setting_b);
 static void maximize_lane_settings(const struct link_training_settings *lt_settings,
 		struct dc_lane_settings lane_settings[LANE_COUNT_DP_MAX]);
+static void override_lane_settings(const struct link_training_settings *lt_settings,
+		struct dc_lane_settings lane_settings[LANE_COUNT_DP_MAX]);
+
 static uint32_t get_cr_training_aux_rd_interval(struct dc_link *link,
 		const struct dc_link_settings *link_settings)
 {
@@ -734,15 +737,13 @@ void dp_decide_lane_settings(
 		}
 #endif
 	}
-
-	/* we find the maximum of the requested settings across all lanes*/
-	/* and set this maximum for all lanes*/
 	dp_hw_to_dpcd_lane_settings(lt_settings, hw_lane_settings, dpcd_lane_settings);
 
 	if (lt_settings->disallow_per_lane_settings) {
 		/* we find the maximum of the requested settings across all lanes*/
 		/* and set this maximum for all lanes*/
 		maximize_lane_settings(lt_settings, hw_lane_settings);
+		override_lane_settings(lt_settings, hw_lane_settings);
 
 		if (lt_settings->always_match_dpcd_with_hw_lane_settings)
 			dp_hw_to_dpcd_lane_settings(lt_settings, hw_lane_settings, dpcd_lane_settings);
@@ -833,6 +834,34 @@ static void maximize_lane_settings(const struct link_training_settings *lt_setti
 	}
 }
 
+static void override_lane_settings(const struct link_training_settings *lt_settings,
+		struct dc_lane_settings lane_settings[LANE_COUNT_DP_MAX])
+{
+	uint32_t lane;
+
+	if (lt_settings->voltage_swing == NULL &&
+	    lt_settings->pre_emphasis == NULL &&
+#if defined(CONFIG_DRM_AMD_DC_DCN)
+	    lt_settings->ffe_preset == NULL &&
+#endif
+	    lt_settings->post_cursor2 == NULL)
+
+		return;
+
+	for (lane = 1; lane < LANE_COUNT_DP_MAX; lane++) {
+		if (lt_settings->voltage_swing)
+			lane_settings[lane].VOLTAGE_SWING = *lt_settings->voltage_swing;
+		if (lt_settings->pre_emphasis)
+			lane_settings[lane].PRE_EMPHASIS = *lt_settings->pre_emphasis;
+		if (lt_settings->post_cursor2)
+			lane_settings[lane].POST_CURSOR2 = *lt_settings->post_cursor2;
+#if defined(CONFIG_DRM_AMD_DC_DCN)
+		if (lt_settings->ffe_preset)
+			lane_settings[lane].FFE_PRESET = *lt_settings->ffe_preset;
+#endif
+	}
+}
+
 enum dc_status dp_get_lane_status_and_lane_adjust(
 	struct dc_link *link,
 	const struct link_training_settings *link_training_setting,