drm/amd/display: Check UHBR13.5 cap when determining max link cap

[WHY]
UHBR13.5 support is optional, even if UHBR20 is supported by the device.
If source supports max UHBR13.5 while sink, cable and LTTPR support
UHBR20 but not UHBR13.5, UHBR10 should be used as the max link cap.
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c

@@ -2062,7 +2062,7 @@ struct dc_link_settings dp_get_max_link_cap(struct dc_link *link)
 	enum dc_link_rate lttpr_max_link_rate;
 	enum dc_link_rate cable_max_link_rate;
 	struct link_encoder *link_enc = NULL;
-
+	bool is_uhbr13_5_supported = true;
 
 	link_enc = link_enc_cfg_get_link_enc(link);
 	ASSERT(link_enc);
@@ -2083,6 +2083,9 @@ struct dc_link_settings dp_get_max_link_cap(struct dc_link *link)
 		max_link_cap.link_spread =
 				link->reported_link_cap.link_spread;
 
+	if (!link->dpcd_caps.dp_128b_132b_supported_link_rates.bits.UHBR13_5)
+		is_uhbr13_5_supported = false;
+
 	/* Lower link settings based on cable attributes
 	 * Cable ID is a DP2 feature to identify max certified link rate that
 	 * a cable can carry. The cable identification method requires both
@@ -2101,9 +2104,13 @@ struct dc_link_settings dp_get_max_link_cap(struct dc_link *link)
 	cable_max_link_rate = get_cable_max_link_rate(link);
 
 	if (!link->dc->debug.ignore_cable_id &&
-			cable_max_link_rate != LINK_RATE_UNKNOWN &&
-			cable_max_link_rate < max_link_cap.link_rate)
-		max_link_cap.link_rate = cable_max_link_rate;
+			cable_max_link_rate != LINK_RATE_UNKNOWN) {
+		if (cable_max_link_rate < max_link_cap.link_rate)
+			max_link_cap.link_rate = cable_max_link_rate;
+
+		if (!link->dpcd_caps.cable_id.bits.UHBR13_5_CAPABILITY)
+			is_uhbr13_5_supported = false;
+	}
 
 	/* account for lttpr repeaters cap
 	 * notes: repeaters do not snoop in the DPRX Capabilities addresses (3.6.3).
@@ -2116,12 +2123,19 @@ struct dc_link_settings dp_get_max_link_cap(struct dc_link *link)
 		if (lttpr_max_link_rate < max_link_cap.link_rate)
 			max_link_cap.link_rate = lttpr_max_link_rate;
 
+		if (!link->dpcd_caps.lttpr_caps.supported_128b_132b_rates.bits.UHBR13_5)
+			is_uhbr13_5_supported = false;
+
 		DC_LOG_HW_LINK_TRAINING("%s\n Training with LTTPR,  max_lane count %d max_link rate %d \n",
 						__func__,
 						max_link_cap.lane_count,
 						max_link_cap.link_rate);
 	}
 
+	if (max_link_cap.link_rate == LINK_RATE_UHBR13_5 &&
+			!is_uhbr13_5_supported)
+		max_link_cap.link_rate = LINK_RATE_UHBR10;
+
 	if (link_dp_get_encoding_format(&max_link_cap) == DP_128b_132b_ENCODING &&
 			link->dc->debug.disable_uhbr)
 		max_link_cap.link_rate = LINK_RATE_HIGH3;