Well, usually such a protocol is by means of read(2) and write(2) to a character device provided by your driver: userspace reads some data, does the fp ops, and writes it back. If you want realtime you'll have to use a realtime thread with the appropriate priority.
> unfortunately this only works for MMX not for real fpu (due to exception
> handling uglies)
concludes it's not multiplatform at all... For that reasen I (maybe) want some
"protocol" for communication with US, where I can easily compute it.