MBOX_TXDONE_BY_POLL
chan->txdone_method = MBOX_TXDONE_BY_POLL;
if (chan->txdone_method == MBOX_TXDONE_BY_POLL && cl->knows_txdone)
txdone = MBOX_TXDONE_BY_POLL;
if (txdone == MBOX_TXDONE_BY_POLL) {
if (!err && (chan->txdone_method & MBOX_TXDONE_BY_POLL)) {