ACPI: Document _OSI and _REV for Linux BIOS writers

Based on a recent session at the Linux Plumber's Conference,
we need to be more clear about how a BIOS should use _OSI
to properly support Linux.
Signed-off-by: Len Brown <len.brown@intel.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/acpi/osi.txt

+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
+AML code includes _OSI("XYZ"), the kernel's AML interpreter
+can evaluate that method, look to see if it supports 'XYZ'
+and answer YES or NO to the BIOS.
+
+The ACPI _REV method returns the "Revision of the ACPI specification
+that OSPM supports"
+
+This document explains how and why the BIOS and Linux should use these methods.
+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,
+but where Linux was installed to replace the original OS (Windows or OSX).
+
+The larger group is the systems tested to run only Windows.  Not only that,
+but many were tested to run with just one specific version of Windows.
+So even though the BIOS may use _OSI to query what version of Windows is running,
+only a single path through the BIOS has actually been tested.
+Experience shows that taking untested paths through the BIOS
+exposes Linux to an entire category of BIOS bugs.
+For this reason, Linux _OSI defaults must continue to claim compatibility
+with all versions of Windows.
+
+But Linux isn't actually compatible with Windows, and the Linux community
+has also been hurt with regressions when Linux adds the latest version of
+Windows to its list of _OSI strings.  So it is possible that additional strings
+will be more thoroughly vetted before shipping upstream in the future.
+But it is likely that they will all eventually be added.
+
+What should an OEM do if they want to support Linux and Windows
+using the same BIOS image?  Often they need to do something different
+for Linux to deal with how Linux is different from Windows.
+Here the BIOS should ask exactly what it wants to know:
+
+_OSI("Linux-OEM-my_interface_name")
+where 'OEM' is needed if this is an OEM-specific hook,
+and 'my_interface_name' describes the hook, which could be a
+quirk, a bug, or a bug-fix.
+
+In addition, the OEM should send a patch to upstream Linux
+via the linux-acpi@vger.kernel.org mailing list.  When that patch
+is checked into Linux, the OS will answer "YES" when the BIOS
+on the OEM's system uses _OSI to ask if the interface is supported
+by the OS.  Linux distributors can back-port that patch for Linux
+pre-installs, and it will be included by all distributions that
+re-base to upstream.  If the distribution can not update the kernel binary,
+they can also add an acpi_osi=Linux-OEM-my_interface_name
+cmdline parameter to the boot loader, as needed.
+
+If the string refers to a feature where the upstream kernel
+eventually grows support, a patch should be sent to remove
+the string when that support is added to the kernel.
+
+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."
+
+The ACPI BIOS flow would include an evaluation of _OS, and the AML
+interpreter in the kernel would return to it a string identifying the OS:
+
+Windows 98, SE: "Microsoft Windows"
+Windows ME: "Microsoft WindowsME:Millenium Edition"
+Windows NT: "Microsoft Windows NT"
+
+The idea was on a platform tasked with running multiple OS's,
+the BIOS could use _OS to enable devices that an OS
+might support, or enable quirks or bug workarounds
+necessary to make the platform compatible with that pre-existing OS.
+
+But _OS had fundamental problems.  First, the BIOS needed to know the name
+of every possible version of the OS that would run on it, and needed to know
+all the quirks of those OS's.  Certainly it would make more sense
+for the BIOS to ask *specific* things of the OS, such
+"do you support a specific interface", and thus in ACPI 3.0,
+_OSI was born to replace _OS.
+
+_OS was abandoned, though even today, many BIOS look for
+_OS "Microsoft Windows NT", though it seems somewhat far-fetched
+that anybody would install those old operating systems
+over what came with the machine.
+
+Linux answers "Microsoft Windows NT" to please that BIOS idiom.
+That is the *only* viable strategy, as that is what modern Windows does,
+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?"
+
+eg. _OSI("3.0 Thermal Model") would return TRUE if the OS knows how
+to deal with the thermal extensions made to the ACPI 3.0 specification.
+An old OS that doesn't know about those extensions would answer FALSE,
+and a new OS may be able to return TRUE.
+
+For an OS-specific interface, the ACPI spec said that the BIOS and the OS
+were to agree on a string of the form such as "Windows-interface_name".
+
+But two bad things happened.  First, the Windows ecosystem used _OSI
+not as designed, but as a direct replacement for _OS -- identifying
+the OS version, rather than an OS supported interface.  Indeed, right
+from the start, the ACPI 3.0 spec itself codified this misuse
+in example code using _OSI("Windows 2001").
+
+This misuse was adopted and continues today.
+
+Linux had no choice but to also return TRUE to _OSI("Windows 2001")
+and its successors.  To do otherwise would virtually guarantee breaking
+a BIOS that has been tested only with that _OSI returning TRUE.
+
+This strategy is problematic, as Linux is never completely compatible with
+the latest version of Windows, and sometimes it takes more than a year
+to iron out incompatibilities.
+
+Not to be out-done, the Linux community made things worse by returning TRUE
+to _OSI("Linux").  Doing so is even worse than the Windows misuse
+of _OSI, as "Linux" does not even contain any version information.
+_OSI("Linux") led to some BIOS' malfunctioning due to BIOS writer's
+using it in untested BIOS flows.  But some OEM's used _OSI("Linux")
+in tested flows to support real Linux features.  In 2009, Linux
+removed _OSI("Linux"), and added a cmdline parameter to restore it
+for legacy systems still needed it.  Further a BIOS_BUG warning prints
+for all BIOS's that invoke it.
+
+No BIOS should use _OSI("Linux").
+
+The result is a strategy for Linux to maximize compatibility with
+ACPI BIOS that are tested on Windows machines.  There is a real risk
+of over-stating that compatibility; but the alternative has often been
+catastrophic failure resulting from the BIOS taking paths that
+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.
+
+_REV was defined in ACPI 1.0 to return the version of ACPI
+supported by the OS and the OS AML interpreter.
+
+Modern Windows returns _REV = 2.  Linux used ACPI_CA_SUPPORT_LEVEL,
+which would increment, based on the version of the spec supported.
+
+Unfortunately, _REV was also misused.  eg. some BIOS would check
+for _REV = 3, and do something for Linux, but when Linux returned
+_REV = 4, that support broke.
+
+In response to this problem, Linux returns _REV = 2 always,
+from mid-2015 onward.  The ACPI specification will also be updated
+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.
+
+Like Linux's _OSI("*Windows*") strategy, Linux defaults to
+answering YES to _OSI("Darwin") to enable full access
+to the hardware and validated BIOS paths seen by OSX.
+Just like on Windows-tested platforms, this strategy has risks.
+
+Starting in Linux-3.18, the kernel answered YES to _OSI("Darwin")
+for the purpose of enabling Mac Thunderbolt support.  Further,
+if the kernel noticed _OSI("Darwin") being invoked, it additionally
+disabled all _OSI("*Windows*") to keep poorly written Mac BIOS
+from going down untested combinations of paths.
+
+The Linux-3.18 change in default caused power regressions on Mac
+laptops, and the 3.18 implementation did not allow changing
+the default via cmdline "acpi_osi=!Darwin".  Linux-4.7 fixed
+the ability to use acpi_osi=!Darwin as a workaround, and
+we hope to see Mac Thunderbolt power management support in Linux-4.11.