bk.c is the old version that didn't work with 2.9.  tty_bk.c is the new
version stolen from 4.3 that doesn't work with 2.10.  What you probably
want to do is figure out how to make bk.c work, and then fold the changes
into tty_bk.c. And then remember that it loses characters after every
carriage return.

It's clear that you don't want to use the buffer cache.  Maybe throw
1K of data space per line at the problem?  It's got to be fast.

berknet_driver is a copy of 2.9's driver that was supposedly fixed.  It
may have all of the mapped out stuff you need, so you might be able
to fold its fixes into the 4.3 one.

Enjoy.