What SHOULD go in the kernel

[Jim Reid]

Unix wizards,

I have some general questions concerning writing Unix device drivers:

Is there a rule of thumb of what should and should not be put in the
kernel?  To be more specific, is it better to make a device driver 
'lean-and-mean' or sophisticated, so that the user interface (the read(),
write(), ioctl()) is simpler?

What are the pros and cons of each?

Thanks for your time.

-- 

Jim Reid
Eastman Kodak Company
UUCP:   ...!rochester!kodak!ektools!jwr