[gentoo-user] How do kernel modules load automatically during boot?

[walt]

I just did a huge update on an older ~amd64 machine, and now only
three out of a dozen or so kernel modules load during bootup, which
leaves a lot of hardware in an unusable state.

What mystifies me is how those three kernel modules managed to get
loaded while all the others didn't. (It's the same three modules every
time, BTW.)

I grepped through /etc for the names of those modules, thinking maybe
I listed them in modprobe.conf.d sometime in the past, but no, those
modules are not listed anywhere in /etc, nor are any of the others
listed either.

I can modprobe all of the unloaded modules manually after bootup,
which shows that all of the modules exist and load without error,
so the problem lies elsewhere, but where?

Any clues would be most welcome.