nexttxd
if (WARN_ON(nexttxd(di, txout) == di->txin))
txout = nexttxd(di, txout);
if (di->dma.txavail == 0 || nexttxd(di, di->txout) == di->txin)
for (i = start; i != end && !txp; i = nexttxd(di, i)) {
i = nexttxd(di, i);