GET_TXBUF
tbp = GET_TXBUF(dp, head);
tbp = GET_TXBUF(dp, head);
tbp = GET_TXBUF(dp, head);
tbp = GET_TXBUF(dp, new_tail);
tbp = GET_TXBUF(dp, sn);
tbp_head = GET_TXBUF(dp, head);
tbp_tail = GET_TXBUF(dp, tail - 1);
tbp = GET_TXBUF(dp, head + avail - 1);
tbp = GET_TXBUF(dp, head);
tbp = GET_TXBUF(dp, dp->tx_active_head);