UMSR_DSR
com->c_msr = UMSR_DSR | UMSR_CTS | UMSR_DCD;
msr = com->c_msr & ~(UMSR_DCD | UMSR_DSR | UMSR_RI);
msr |= UMSR_DSR;
if (ISSET(combits, UMSR_DSR))
sc->sc_msr |= UMSR_DSR;
sc->sc_msr |= UMSR_DSR;
sc->sc_msr |= UMSR_DSR;
*msr |= ISSET(mdmsts, SLSA_MDMSTS_DSR) ? UMSR_DSR : 0;
SET(sc->sc_msr, UMSR_DSR);