TX_RING_MASK
sc->cur_tx = (sc->cur_tx + 1) & TX_RING_MASK;
desc->next = vtophys(sc->tx_desc + ((i+1) & TX_RING_MASK));
sc->cur_tx = (sc->cur_tx + 1) & TX_RING_MASK;
sc->dirty_tx = (sc->dirty_tx + 1) & TX_RING_MASK;