DP_GUID
ret = drm_dp_dpcd_read(mgr->aux, DP_GUID, buf, sizeof(buf));
ret = drm_dp_dpcd_write(mgr->aux, DP_GUID, buf, sizeof(buf));
ret = drm_dp_dpcd_write_data(aux, DP_GUID, buf, sizeof(buf));
ret = drm_dp_dpcd_read_data(mgr->aux, DP_GUID, buf, sizeof(buf));