DP_CEC_TUNNELING_CONTROL
err = drm_dp_dpcd_write_byte(aux, DP_CEC_TUNNELING_CONTROL, val);
err = drm_dp_dpcd_read_byte(aux, DP_CEC_TUNNELING_CONTROL, &val);
err = drm_dp_dpcd_write_byte(aux, DP_CEC_TUNNELING_CONTROL, val);