driver interupt routine reentered -- why?

[Steve Nuchia]

It being spring break and all, I was hacking on my serial driver and
discovered something strange...

I had been having system lockups you see, and the only excuse I could
think of for them was that my interupt service routine was infinite
looping.  And the only excuse I could figure for that was an SPL botch.

I really don't think its an spl botch on my part, I think its something
the system is doing for me that I don't understand.  I added reentry
detection to the ISR and sure enough it is being reentered, so I'm
stumped.  For the time being I'm just returning if the "active"
flag is set.

The driver serves more than one interupt line, and the ISR is
called from the upper half, from a timeout, and on hardware
interupts.  It services all attached devices on each interupt,
since the devices have internal queues (NS16550As).

The upper-half call is inside an spl7, the timeout calls a
stub that calls the ISR and reschedules itself, inside an
spl7.  Anybody have any clues?  If somebody would like to
examine the source I'd be grateful, and happy to mail a copy.


The good news is that I squoze another 20% or so out
of the overhead, it has now demonstrated 3500 cps
aggregate instantaneous throughput, 2500 was my best previous,
and cpu utilization at high baud rates is way down -- I had
55-65% idle during the speed run.

The system is a 16 MHz non-cache 386 with Bell Tech sysVr3.0
(waiting for r4 before I spring for an upgrade).  The devices
are an internal modem with an intel 8250 chip and a digiboard
8-port dumb card with NS16550A's on it, two hooked to
telebit TB+s and 6 dangling.


I also hacked in a nap ioctl and an ioctl to return the
character count in the raw queue; needed them to get
"nbstime" working.  I now have my rc script call the
NIST time standard when I boot.  What a trip.  My mods
to nbstime also available.
-- 
Steve Nuchia	      South Coast Computing Services      (713) 964-2462
"You have no scars on your face, and you cannot handle pressure." - Billy Joel