9c01a318d2
It was based on the pty(4) driver which as a tty side an a non-tty side.
Nmdm(4) seems to have inherited two symmetric sides from pty but
unfortunately they are not quite ttys. Running a getty one one
side and tip on the other failed to produce NL->CRNL mapping for
instance.
Rip out the basically bogus cdevsw->{read,write} functions and rely
on ttyread() and ttywrite() which does the same thing.
Use taskqueue_swi_giant to run a task for either side to do what
needs to be done. (Direct calling is not an option as it leads to
recursion.) Trigger the task from the t_oproc and t_stop methods.
Default the ports to not ECHO. Since we neither rate limiting nor
emulation, two ports echoing each other is a really bad idea, which
can only be properly mitigated by rate limiting, rate emulation or
intelligent detection. Rate emulation would be a neat feature.
Ditch the modem-line emulation, if needed for some app, it needs
to be thought much more about how it interacts with the open/close
logic.