Excerpts from netbsd: 1-Sep-98 Re: Serial console hangs Andreas
Gustafsson@arane (3379)

> In other words, the solution is simply to remove the 'bus_space_read'
> at the end of 'com_common_putc' which clears the interrupts (as
> suggested in PR #4263).

Another method would be to call the interrupt handler explicitely
from comcnpoll(0). (That's what the keyboard controller driver
(dev/isa/pckbc.c) does.)
It would need a slight rearrangement of comintr() as it checks
the interrupt bit at the beginning.
This might behave better in the case where a serial console is
used but not found in autoconfiguration - the interrupt would
stick there forever otherwise.

best regards
Matthias