API stability is not guaranteed, though it's stable enough that the likes of Nvidia can write installers for their proprietary drivers that recompile a bit of interface glue between kernel and blob.
Given a particular install, yes, but it will change independent of kernel version based on e.g. build flags and even conceivably compiler chosen, etc.
> it's stable enough that the likes of Nvidia can write installers for their proprietary drivers that recompile a bit of interface glue between kernel and blob
Well...they certainly can. It also can infuriate me beyond all reason...
Making a stable API that covers the entire kernel would be a monumental task on the other hand.