vsbus doc?

I'm finding myself wanting to backport support for the 3100/80 from 5.2
and/or 4.0.1 to 1.4T.

To some extent, I can just copy code.  But I'd like to understand it,
if only to make sure I get it integrated correctly.

The major piece I'm aware of at the moment that I don't understand is
the hardware backing (what 4.0.1 and 5.2 call) vsbus.  Clearly
_someone_ understood it at some point, or the code wouldn't exist.  So
what I'm asking is, does anyone know of any documentation on that
hardware, documentation suitable for someone writing OS-level code for
it?

I haven't gone searching because I don't know what to search for.  I've
done a little searching for 3100/80 stuff, but the false positives
always flood the search results to the point of uselessness, so I
clearly haven't found the right search terms.

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