MCR_LOOPBACK
uart_setreg(bas, REG_MCR, MCR_LOOPBACK | MCR_IE | MCR_DTR | MCR_RTS);
if (mcr & MCR_LOOPBACK) {
loopback = (sc->mcr & MCR_LOOPBACK) != 0;
(sc->mcr & MCR_LOOPBACK) != 0))