ACPI core does has notifiers that are used, but they don't work the same.
If you look at patch 4, you'll see amdgpu registers and unregisters using
both

acpi_install_notify_handler()
and
acpi_remove_notify_handler()

If we supported both ACPI notifications and non-ACPI notifications
all consumers would have to have support to register and use both types.

Why would you want to support ACPI notifications and non-ACPI
notifications? All you need is wbrf notification.

The new wbrf.c should implement wbrf_install_notify_handler() and
wbrf_remove_notify_handler().

As to where to put wbrf.c? I guess either drivers/base/ or
drivers/wbrf/. Maybe ask GregKH?

   Andrew