Loadable device drivers...in V.4?

[Tim W Smith]

I wish vendors would do this.  It's not even hard.  I worked out
how it could be done a long time ago (when I worked at Callan
Data Systems), but didn't try it.  When I worked at Interactive,
I got to try it.  It only requires a couple of relatively simple
kernel changes.

First, you need a system call that will allocate kernel memory
for the driver.  Second, you need a system call that will install
an interrupt handler for the driver.

The rest you do with a program.  The basic things that must be done
to load a driver are these:

	Assuming the driver object file is named Driver.o, get
	a list of undefined external references in Driver.o.
	Use nm or ld to do this for you.

	Look up each of these in the kernel symbol table.  You  must
	have a kernel symbol table, of course, for this to work.

	Make a file that defines these externals as absolute references
	to the values obtained from the kernel symbol table.
	I don't remember if I did this by making as assembly file
	that declared them this way, or some sort if input file
	for ld.  It's been a while since I did this.

	The program (which must be running as root) now uses the
	system call for memory allocation to allocate memory in
	the kernel for the driver.  This system call returns the
	base address in kernel space of the allocated memory.

	Use ld and the file created two steps ago and the return value
	from the memory allocation system call to create a .o file
	that has all external references resolved and is set up
	to be loaded at the address of the kernel memory allocated
	in the previous step.

	The driver loader program now opens /dev/kmem and copies
	the driver into the memory allocated a couple of steps ago.
	
	The driver loader then adds entries for the driver open, close,
	read, etc., routines into the appropriate kernel table (bdevsw,
	cdevsw, or whatever).

	The driver load program uses the second new system call to
	tell the kernel what interrupt the driver uses, and where
	the interrupt routine is at.

The ISC version of this could load drivers and streams modules.

Facilities were also included to unload drivers.

The main reason it was never released by ISC was that they were
interested in this as a way to make the kernel smaller on the
boot floppy.  At the time, there were limits in the boot program
on the size of a kernel that could be loaded, and if all the drivers
they wanted were in the kernel, it would exceed those limits.

However, someone fixed the boot program, so they did not need the
loadable driver stuff.  I was in the process of leaving at the time,
so I don't know what ever happened to it after that.

I wish they had pursued it.  I've developed a couple of drivers since
then, and would have really appreciated being able to test minor changes
without having to rebuild the kernel.  In my experience, there is a long
period at the end of a driver project where the thing works well enough
that it is not crashing the kernel, but it needs various tweeks.  This
stage would go much faster with a way to load drivers without rebooting
the system.

						Tim Smith