sys/dev/acpica/acpi.c rev 1.196 a while ago:
Grab Giant around calls to DEVICE_SUSPEND/RESUME in
acpi_SetSleepState().
If we are resuming non-MPSAFE drivers, they need Giant held for them.
This may fix some obscure suspend/resume problems. It has fixed keyrate
setting problems that were triggered by cardbus (MPSAFE) changing the
ordering for syscons resume (non-MPSAFE). Also, add some asserts that
Giant is held in our suspend/resume and shutdown methods.
Submitted by: Marko Zec
760b9605ef