Is there any standard way for kernel code to access a device driver?

I have several ideas how one might do this, but none of them 
seems to stand out as being the "right way".

I have a standard character mode driver that appears in the file 
system (in the "dev" directory) to enable it to be accessed from 
user space. I ALSO need to access the same driver from kernel 
space.

More specifically, I want a network pseudo driver to be able to 
access a driver for a custom ISDN card. I also want the custom 
ISDN card to be accessible from use space.

I have had a look at how the line-discipline story works with ttys, 
but this is not quite the same thing.

Thanks for any help.

Dave