asmc: Convert driver to CTLFLAG_MPSAFE

[guest-seuros]

Summary

Replace CTLFLAG_NEEDGIANT with CTLFLAG_MPSAFE for all sysctls.
The driver already uses spin mutexes (sc->sc_mtx) for hardware
access protection and does not require the Giant lock.

This improves scalability by allowing concurrent sysctl access
without Giant serialization.

[markj]

Some of these handlers perform read-modify-writes. The spin mutex serializes hardware accesses but not higher-level operations, as in asmc_mb_sysctl_fanmanual() for instance. Is it ok to leave them unserialized?

[guest-seuros]

We could wrap it with a sleep mutex, but concurrent fan control from multiple threads is not a realistic scenario.
The worst case is a lost update, not a crash.

My approach is to upgrade to MPSAFE, stress test on real hardware, then ship.
That's also why I'm not submitting upgrades for every drivers, I only upgrade what I can actually test on hardware I have.

A follow-up sweep can remove unnecessary locking where it makes no sense.

[adrian]

I had a chat with seuros on discord about it. I think its fine as an intermediary step, but I do think it'll be worth revisiting the driver locking later and pushing more stuff into functionality locks rather than per device IO locks.

(I do need to get a couple reference devices though, my macbook air devices don't have fans :-) )

[adrian]

yeah, looking at asmc again with spin mutex use i really don't want to clean that up whilst removing GIANT here.
it should be ok-ish for now.

[adrian]

let's land this and then figure out how to expand the spin mutexes a bit to cover features a bit cleaner.
(And then see i they really NEED to be spin mutexes..)