diff --git a/sys/dev/drm/amd/amdgpu/atombios_dp.c b/sys/dev/drm/amd/amdgpu/atombios_dp.c
--- a/sys/dev/drm/amd/amdgpu/atombios_dp.c
+++ b/sys/dev/drm/amd/amdgpu/atombios_dp.c
@@ -262,6 +262,8 @@
 	static const unsigned link_rates[3] = { 162000, 270000, 540000 };
 	unsigned max_link_rate = drm_dp_max_link_rate(dpcd);
 	unsigned max_lane_num = drm_dp_max_lane_count(dpcd);
+	if (max_lane_num > 4)
+		max_lane_num = 4; /* DP spec max; train_set is fixed-size 4 */
 	unsigned lane_num, i, max_pix_clock;
 
 	if (amdgpu_connector_encoder_get_dp_bridge_encoder_id(connector) ==