188 lines
8.1 KiB
ReStructuredText
188 lines
8.1 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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==========================
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ACPI _OSI and _REV methods
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==========================
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An ACPI BIOS can use the "Operating System Interfaces" method (_OSI)
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to find out what the operating system supports. Eg. If BIOS
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AML code includes _OSI("XYZ"), the kernel's AML interpreter
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can evaluate that method, look to see if it supports 'XYZ'
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and answer YES or NO to the BIOS.
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The ACPI _REV method returns the "Revision of the ACPI specification
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that OSPM supports"
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This document explains how and why the BIOS and Linux should use these methods.
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It also explains how and why they are widely misused.
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How to use _OSI
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===============
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Linux runs on two groups of machines -- those that are tested by the OEM
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to be compatible with Linux, and those that were never tested with Linux,
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but where Linux was installed to replace the original OS (Windows or OSX).
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The larger group is the systems tested to run only Windows. Not only that,
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but many were tested to run with just one specific version of Windows.
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So even though the BIOS may use _OSI to query what version of Windows is running,
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only a single path through the BIOS has actually been tested.
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Experience shows that taking untested paths through the BIOS
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exposes Linux to an entire category of BIOS bugs.
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For this reason, Linux _OSI defaults must continue to claim compatibility
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with all versions of Windows.
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But Linux isn't actually compatible with Windows, and the Linux community
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has also been hurt with regressions when Linux adds the latest version of
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Windows to its list of _OSI strings. So it is possible that additional strings
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will be more thoroughly vetted before shipping upstream in the future.
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But it is likely that they will all eventually be added.
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What should an OEM do if they want to support Linux and Windows
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using the same BIOS image? Often they need to do something different
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for Linux to deal with how Linux is different from Windows.
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In this case, the OEM should create custom ASL to be executed by the
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Linux kernel and changes to Linux kernel drivers to execute this custom
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ASL. The easiest way to accomplish this is to introduce a device specific
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method (_DSM) that is called from the Linux kernel.
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In the past the kernel used to support something like:
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_OSI("Linux-OEM-my_interface_name")
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where 'OEM' is needed if this is an OEM-specific hook,
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and 'my_interface_name' describes the hook, which could be a
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quirk, a bug, or a bug-fix.
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However this was discovered to be abused by other BIOS vendors to change
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completely unrelated code on completely unrelated systems. This prompted
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an evaluation of all of it's uses. This uncovered that they aren't needed
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for any of the original reasons. As such, the kernel will not respond to
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any custom Linux-* strings by default.
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That was easy. Read on, to find out how to do it wrong.
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Before _OSI, there was _OS
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==========================
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ACPI 1.0 specified "_OS" as an
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"object that evaluates to a string that identifies the operating system."
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The ACPI BIOS flow would include an evaluation of _OS, and the AML
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interpreter in the kernel would return to it a string identifying the OS:
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Windows 98, SE: "Microsoft Windows"
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Windows ME: "Microsoft WindowsME:Millennium Edition"
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Windows NT: "Microsoft Windows NT"
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The idea was on a platform tasked with running multiple OS's,
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the BIOS could use _OS to enable devices that an OS
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might support, or enable quirks or bug workarounds
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necessary to make the platform compatible with that pre-existing OS.
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But _OS had fundamental problems. First, the BIOS needed to know the name
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of every possible version of the OS that would run on it, and needed to know
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all the quirks of those OS's. Certainly it would make more sense
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for the BIOS to ask *specific* things of the OS, such
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"do you support a specific interface", and thus in ACPI 3.0,
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_OSI was born to replace _OS.
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_OS was abandoned, though even today, many BIOS look for
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_OS "Microsoft Windows NT", though it seems somewhat far-fetched
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that anybody would install those old operating systems
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over what came with the machine.
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Linux answers "Microsoft Windows NT" to please that BIOS idiom.
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That is the *only* viable strategy, as that is what modern Windows does,
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and so doing otherwise could steer the BIOS down an untested path.
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_OSI is born, and immediately misused
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=====================================
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With _OSI, the *BIOS* provides the string describing an interface,
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and asks the OS: "YES/NO, are you compatible with this interface?"
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eg. _OSI("3.0 Thermal Model") would return TRUE if the OS knows how
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to deal with the thermal extensions made to the ACPI 3.0 specification.
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An old OS that doesn't know about those extensions would answer FALSE,
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and a new OS may be able to return TRUE.
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For an OS-specific interface, the ACPI spec said that the BIOS and the OS
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were to agree on a string of the form such as "Windows-interface_name".
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But two bad things happened. First, the Windows ecosystem used _OSI
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not as designed, but as a direct replacement for _OS -- identifying
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the OS version, rather than an OS supported interface. Indeed, right
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from the start, the ACPI 3.0 spec itself codified this misuse
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in example code using _OSI("Windows 2001").
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This misuse was adopted and continues today.
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Linux had no choice but to also return TRUE to _OSI("Windows 2001")
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and its successors. To do otherwise would virtually guarantee breaking
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a BIOS that has been tested only with that _OSI returning TRUE.
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This strategy is problematic, as Linux is never completely compatible with
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the latest version of Windows, and sometimes it takes more than a year
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to iron out incompatibilities.
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Not to be out-done, the Linux community made things worse by returning TRUE
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to _OSI("Linux"). Doing so is even worse than the Windows misuse
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of _OSI, as "Linux" does not even contain any version information.
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_OSI("Linux") led to some BIOS' malfunctioning due to BIOS writer's
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using it in untested BIOS flows. But some OEM's used _OSI("Linux")
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in tested flows to support real Linux features. In 2009, Linux
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removed _OSI("Linux"), and added a cmdline parameter to restore it
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for legacy systems still needed it. Further a BIOS_BUG warning prints
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for all BIOS's that invoke it.
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No BIOS should use _OSI("Linux").
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The result is a strategy for Linux to maximize compatibility with
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ACPI BIOS that are tested on Windows machines. There is a real risk
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of over-stating that compatibility; but the alternative has often been
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catastrophic failure resulting from the BIOS taking paths that
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were never validated under *any* OS.
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Do not use _REV
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===============
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Since _OSI("Linux") went away, some BIOS writers used _REV
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to support Linux and Windows differences in the same BIOS.
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_REV was defined in ACPI 1.0 to return the version of ACPI
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supported by the OS and the OS AML interpreter.
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Modern Windows returns _REV = 2. Linux used ACPI_CA_SUPPORT_LEVEL,
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which would increment, based on the version of the spec supported.
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Unfortunately, _REV was also misused. eg. some BIOS would check
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for _REV = 3, and do something for Linux, but when Linux returned
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_REV = 4, that support broke.
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In response to this problem, Linux returns _REV = 2 always,
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from mid-2015 onward. The ACPI specification will also be updated
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to reflect that _REV is deprecated, and always returns 2.
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Apple Mac and _OSI("Darwin")
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============================
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On Apple's Mac platforms, the ACPI BIOS invokes _OSI("Darwin")
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to determine if the machine is running Apple OSX.
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Like Linux's _OSI("*Windows*") strategy, Linux defaults to
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answering YES to _OSI("Darwin") to enable full access
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to the hardware and validated BIOS paths seen by OSX.
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Just like on Windows-tested platforms, this strategy has risks.
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Starting in Linux-3.18, the kernel answered YES to _OSI("Darwin")
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for the purpose of enabling Mac Thunderbolt support. Further,
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if the kernel noticed _OSI("Darwin") being invoked, it additionally
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disabled all _OSI("*Windows*") to keep poorly written Mac BIOS
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from going down untested combinations of paths.
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The Linux-3.18 change in default caused power regressions on Mac
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laptops, and the 3.18 implementation did not allow changing
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the default via cmdline "acpi_osi=!Darwin". Linux-4.7 fixed
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the ability to use acpi_osi=!Darwin as a workaround, and
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we hope to see Mac Thunderbolt power management support in Linux-4.11.
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