drm/amd/display: Consider dp cable id only when data is non zero

Cable ID is a DP2 feature to identify max certified link rate that
a cable can carry. The cable identification method requires both
cable and display hardware support. Since the specs comes late, it is
anticipated that the first round of DP2 cables and displays may not
be fully compatible to reliably return cable ID data. Therefore the
decision of our cable id policy is that if the cable can return non
zero cable id data, we will take cable's link rate capability into
account. However if we get zero data, the cable link rate capability
is considered inconclusive. In this case, we will not take cable's
capability into account to avoid of over limiting hardware capability
from users. The max overall link rate capability is still determined
after actual dp pre-training. Cable id is considered as an auxiliary
method of determining max link bandwidth capability.
Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@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/core/dc_link_dp.c

@@ -3020,7 +3020,7 @@ static enum dc_link_rate get_lttpr_max_link_rate(struct dc_link *link)
 
 static enum dc_link_rate get_cable_max_link_rate(struct dc_link *link)
 {
-	enum dc_link_rate cable_max_link_rate = LINK_RATE_HIGH3;
+	enum dc_link_rate cable_max_link_rate = LINK_RATE_UNKNOWN;
 
 	if (link->dpcd_caps.cable_id.bits.UHBR10_20_CAPABILITY & DP_UHBR20)
 		cable_max_link_rate = LINK_RATE_UHBR20;
@@ -3083,15 +3083,29 @@ struct dc_link_settings dp_get_max_link_cap(struct dc_link *link)
 		max_link_cap.link_spread =
 				link->reported_link_cap.link_spread;
 
-	/* Lower link settings based on cable attributes */
+	/* 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
+	 * cable and display hardware support. Since the specs comes late, it is
+	 * anticipated that the first round of DP2 cables and displays may not
+	 * be fully compatible to reliably return cable ID data. Therefore the
+	 * decision of our cable id policy is that if the cable can return non
+	 * zero cable id data, we will take cable's link rate capability into
+	 * account. However if we get zero data, the cable link rate capability
+	 * is considered inconclusive. In this case, we will not take cable's
+	 * capability into account to avoid of over limiting hardware capability
+	 * from users. The max overall link rate capability is still determined
+	 * after actual dp pre-training. Cable id is considered as an auxiliary
+	 * method of determining max link bandwidth capability.
+	 */
 	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;
 
-	/*
-	 * account for lttpr repeaters cap
+	/* account for lttpr repeaters cap
 	 * notes: repeaters do not snoop in the DPRX Capabilities addresses (3.6.3).
 	 */
 	if (dp_is_lttpr_present(link)) {