+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
ACPI _OSI and _REV methods
---------------------------
+==========================
An ACPI BIOS can use the "Operating System Interfaces" method (_OSI)
to find out what the operating system supports. Eg. If BIOS
It also explains how and why they are widely misused.
How to use _OSI
----------------
+===============
Linux runs on two groups of machines -- those that are tested by the OEM
to be compatible with Linux, and those that were never tested with Linux,
That was easy. Read on, to find out how to do it wrong.
Before _OSI, there was _OS
---------------------------
+==========================
ACPI 1.0 specified "_OS" as an
"object that evaluates to a string that identifies the operating system."
and so doing otherwise could steer the BIOS down an untested path.
_OSI is born, and immediately misused
---------------------------------------
+=====================================
With _OSI, the *BIOS* provides the string describing an interface,
and asks the OS: "YES/NO, are you compatible with this interface?"
were never validated under *any* OS.
Do not use _REV
----------------
+===============
Since _OSI("Linux") went away, some BIOS writers used _REV
to support Linux and Windows differences in the same BIOS.
to reflect that _REV is deprecated, and always returns 2.
Apple Mac and _OSI("Darwin")
-----------------------------
+============================
On Apple's Mac platforms, the ACPI BIOS invokes _OSI("Darwin")
to determine if the machine is running Apple OSX.