USABLE_RX_BD
sc->free_rx_bd = USABLE_RX_BD;
sc->max_rx_bd = USABLE_RX_BD;
DBRUNIF(1, sc->rx_low_watermark = USABLE_RX_BD);
DBRUNIF((sc->free_rx_bd == USABLE_RX_BD), sc->rx_empty_count++);