device driver help

[Henry F Turner]

	I'm trying to modify a device driver that supposedly worked on
4.1bsd to work on 4.2bsd.  The device is a DEC VS11 display processor.
It gets its instructions and data from a "display file" in host (VAX)
memory via dma. Thus the driver handles it as an exclusive access device
and allows only one read or write, which merely sets up mapping the
display file from host memory to the device (on a unibus). Ioctl's are
are used to otherwise control/communicate with the device. The original
driver used vslock to lock the display file down and included only a
comment wondering whether or not it was necessary to unlock it at close
time.
	Well, I made what I thought were the obvious changes and got the
driver installed and working, but sure enough, every time we used the
device, we would lose memory equivalent to the size of the display file.
I added code to save the address and size in the softc structure and then
used that at close time to vsunlock it. That worked fine, all memory was
returned to the free list. That is, fine PROVIDED the user closed the
device before exiting! It seems exit "vrelse"es all a proc's memory and
THEN closef's the files. Calling vsunlock with the proc pte entry zeroed
out seems to do nasty things to the kernal. It crashes! I put in a check
for p_flags with SWEXIT. If set, I print a message about the memory we
just pissed away and skip the vsunlock. This makes the device usable with
a reasonably conscientious user population but its not a solution.
	I've tried to study the code to get a real understanding of how
the memory management works and what is really going on but the code gets
a bit opaque for my experience level. I've notice that among all the
drivers that came with the distribution, only one uses the vslock/unlock
calls (directly). There must be another way around this problem. If you
have any insights into this or have an ideas about where I might be
screwing up, I would much appreciate enlightenment. 

Thanks for your time,

Henry F Turner
Computer Science Department
Room 298 Coates Hall
Louisiana State University
Baton Rouge, LA  70803

Phone:	(504) 388-1495
CSNET:	turner at lsu
ARPA:	turner%lsu at csnet-relay