DP_DP13_DPCD_REV
ret = drm_dp_dpcd_readb(&port->aux, DP_DP13_DPCD_REV, &dpcd_rev);
DP_DP13_DPCD_REV,
ret = drm_dp_dpcd_read_data(aux, DP_DP13_DPCD_REV, &dpcd_ext,