2.8bsd tty drivers bi-directional

[utzoo!decvax!ucbvax!dove at mit-dspg]

From: dove at mit-dspg at mit-mc
Tony Drogaris and I have succeeded in
modifying our local tty device drivers
(dh dz dhdm) to allow modem lines to be used
for both incoming and outgoing calls without
having to kill the init that normally waits
for the incoming carrier.  It works by using
the 128 bit in the minor device number to indicate
which direction is currently being used.
Outgoing attempts to open when the line is
in use ingoing fail with the EBUSY error.
Ingoing attempts to open when the line
is going out sleep until the line is closed
and the carrier dropped, then until the carrier comes on again as usual.

This stuff is fairly new, but seems to work
and in the process some screwy code in our
dz got fixed.  This code will be made available
to the bsd distrib people for future releases.