Lodable Device Drivers for Sun (Vax?) Unix 4.2

[Lee Hasiuk]

I have posted to net.sources a package which will allow users of
Sun Unix 4.2 to develop device drivers without reloading and rebooting
their systems.  Although the code is Sun specific, the principles 
behind it should be applicable to almost any Unix implementation, and
I hope that some of you 'device driver types' out there find it useful
to port and distribute versions for other machines (or improvements for
the Sun version, which is by no means perfect).

The basic idea behind the package is to take advantage of the fact that
executable code may be placed into the data space of the kernel and run
in place.  The package consists of a device driver template, which accesses
its open, close, read, write, and ioctl routines indirectly through an
array whose name appears in the kernel's namelist.  Also in the driver
is an array into which a 'loadable' device driver may be loaded by writing 
to /dev/kmem.  The name of this array also appears in the namelist.  The
bulk of the code is a program which root runs in user mode which looks in the
namelist of /vmunix, finds the addresses of these arrays, ld's the driver .o 
file against /vmunix to resolve global symbols (such as calls to sleep and 
wakeup) producing an executable which runs at the starting address of the
large array.  The 'loader' program then copies the driver into the kernel,
and fills in the indirection array with the entry points of the loadable
driver.

Although the program will allow driver development while a system is on-line
and servicing other users, this practice is not recommended because
drivers with bugs frequently crash a system.  The major savings of using
loadable drivers comes from the reduction in the number of link/reboot cycles.
My feeling is that once you try loadable drivers, you'll never want to go
back to doing it the 'old way'.

Lee Hasiuk
{ucbvax, decvax, ihnp4}!trwrb!trwspp!spp2!hasiuk