What's an "MPSAFE" driver need to do?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I'm looking at writing driver code for 5.2.  A few manpages (notably
selinit(9) and other aliases for the same page) speak of "non-MPSAFE
drivers", but I have been unable to find what a driver has to do in
order to be MPSAFE - nor for that matter how it is that drivers are or
aren't marked as MPSAFE, though the latter I may be able to figure out
(mostly meaning, if I can find and identify the relevant kernel code).

Did I miss something, or is this just not documented?

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