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[96.230.116.151]) by smtp.gmail.com with ESMTPSA id z80sm2099905qkb.2.2016.11.21.21.30.19 (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Mon, 21 Nov 2016 21:30:19 -0800 (PST) From: Len Brown To: linux-acpi@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Len Brown Subject: [PATCH 1/1] ACPI: Document _OSI and _REV for Linux BIOS writers Date: Tue, 22 Nov 2016 00:29:46 -0500 Message-Id: X-Mailer: git-send-email 2.11.0.rc1 Reply-To: Len Brown Organization: Intel Open Source Technology Center Sender: linux-acpi-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP From: Len Brown 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 --- Documentation/acpi/osi.txt | 187 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 187 insertions(+) create mode 100644 Documentation/acpi/osi.txt diff --git a/Documentation/acpi/osi.txt b/Documentation/acpi/osi.txt new file mode 100644 index 000000000000..f9a0352f1d28 --- /dev/null +++ b/Documentation/acpi/osi.txt @@ -0,0 +1,187 @@ +ACPI _OSI and _REV methods +-------------------------- + +The ACPI _OSI method, "Operating System Interfaces", +informs the ACPI BIOS about 'interfaces' supported by the Operating System. + +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 mis-used. + +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. +Many of those systems were not only tested with just Windows, they were +tested with just one specific version of Windows. Even though the BIOS +uses _OSI to query what version of Windows is running, only a single +path through the BIOS has every been tested. +For that reason, Linux must continue to claim compatibility with +all versions of Windows. To do otherwise exposes BIOS bugs on this +large group of machines. + +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. + +Then there are systems designed and tested to run Linux. +Often an OEM's ACPI BIOS team wants to support a unique feature +in Linux, to do this, 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. + +For this to work, a patch must be sent 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= 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. _OS returned "Microsoft Windows" on Windows 98 and SE. +It returned "Microsoft WindowsME:Millenium Edition" for Win-ME, +and for Windows NT it returned "Microsoft Windows NT". After that, +_OS was abandoned, though modern BIOS continue to contain +an ancient idiom looking for "Microsoft Windows NT" -- just +in case time travel becomes possible? + +On a platform tasked with running multiple versions of Windows, +the BIOS could use _OS to enable/disable devices that and OS +might support, or enable/disable quirks or bug work-arounds +necessary to make the platform compatible with a pre-existing OS. + +If a BIOS invokes _OS today, Linux will return +"Microsoft Windows NT". That is the *only* possible choice, +as doing otherwise can steer the BIOS down an un-tested path. + +_OS had fundamental problems. First, the BIOS needed to know the name +of every 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. + +_OSI is born, and immediately mis-used +-------------------------------------- + +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 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 mis-use +in example code using _OSI("Windows 2001"). + +The Windows BIOS/OS ecosystem continues to mis-use this method 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. + +The Linux community made things worse by returning TRUE +to _OSI("Linux"). Doing so is even worse than the Windows mis-use +of _OSI, as "Linux" contains no version information. +_OSI("Linux") led to some BIOS' malfunctioning due to BIOS writer's +using it in un-tested 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 that needed it, and added a BIOS_BUG warning +for all BIOS's that invoke it. No BIOS ever 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. + +BIOS for Linux and Windows +-------------------------- + +For some platforms, a BIOS that wants to properly run Windows +and Linux needs to know which is running. This is a valid requirement, +and it may be, for example, due to specific hardware that is supported by one +OS is incompatible with the same hardware support in the other OS. +Examples may be sound or storage devices with different hardware modes. +The BIOS can handle such quirks, only if it can reliably identify +what software is running. + +Do not use _REV +--------------- + +Some BIOS writers decided to use _REV to tell the difference +between Linux and Windows. _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 mis-used. 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. + +_OSI("Darwin") +-------------- + +No description of _OSI would be complete without mentioning _OSI("Darwin"). +This string is mis-used by Apple on Mac platforms, just as Linux +mis-used _OSI("Linux"). + +Linux did not originally return TRUE to _OSI("Darwin"), +on the assumption that on Apple hardware, Linux was +probably best pretending to be Windows, +rather than pretending to be OSX. + +But _OSI("Darwin") was required to make Thunderbolt work, +and so Linux changed to enable it by default. Making matters worse, +that change did not include the ability to disable _OSI("Darwin") +on the cmdline with acpi_OSI=!Darwin, and so the resulting +regressions had no workaround. + +The cmdline has since been fixed to enable !Darwin. +Today "Darwin" enables Thunderbolt, but significantly +hurts battery life, and so the default for _OSI("Darwin") +is still under debate. +