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;
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)) {