Documentation: ACPI: move osi.txt to firmware-guide/acpi and convert to reST

This converts the plain text documentation to reStructuredText format
and adds it to Sphinx TOC tree.

No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 0 insertions, 187 deletions

diff --git a/Documentation/acpi/osi.txt b/Documentation/acpi/osi.txt
deleted file mode 100644
index 50cde0ceb9b0..000000000000
--- a/Documentation/acpi/osi.txt
+++ /dev/null
@@ -1,187 +0,0 @@
-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
-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.