My (limited) understanding is that _DSM is used to turn the card on/off and
switch the mux on relevant laptops between the different GPU's. And the ACPI
vbios method was introduced for optimus laptops -- those checks have always
been there. I'm fairly sure they can be dropped.

Just to confirm -- dropping the checks from
nouveau_acpi.c:nouveau_acpi_rom_supported (and no other changes) makes the
VBIOS get loaded from ACPI, without errors.

For the kworker thing -- see what's going on -- 'echo w > /proc/sysrq-trigger'
(iirc; perhaps a different sysrq would be helpful... see if it's in nouveau or
not. even if it is, it may not be related to this specific issue.)