DP_TEST_SINK_START
WARN_ON(!(buf & DP_TEST_SINK_START));
ret = drm_dp_dpcd_write_byte(aux, DP_TEST_SINK, buf | DP_TEST_SINK_START);
ret = drm_dp_dpcd_write_byte(aux, DP_TEST_SINK, buf & ~DP_TEST_SINK_START);