1 .. SPDX-License-Identifier: GPL-2.0
2
3 ==========================
4 ACPI _OSI and _REV methods
5 ==========================
6
7 An ACPI BIOS can use the "Operating System Interfaces" method (_OSI)
8 to find out what the operating system supports. Eg. If BIOS
9 AML code includes _OSI("XYZ"), the kernel's AML interpreter
10 can evaluate that method, look to see if it supports 'XYZ'
11 and answer YES or NO to the BIOS.
12
13 The ACPI _REV method returns the "Revision of the ACPI specification
14 that OSPM supports"
15
16 This document explains how and why the BIOS and Linux should use these methods.
17 It also explains how and why they are widely misused.
18
19 How to use _OSI
20 ===============
21
22 Linux runs on two groups of machines -- those that are tested by the OEM
23 to be compatible with Linux, and those that were never tested with Linux,
24 but where Linux was installed to replace the original OS (Windows or OSX).
25
26 The larger group is the systems tested to run only Windows. Not only that,
27 but many were tested to run with just one specific version of Windows.
28 So even though the BIOS may use _OSI to query what version of Windows is running,
29 only a single path through the BIOS has actually been tested.
30 Experience shows that taking untested paths through the BIOS
31 exposes Linux to an entire category of BIOS bugs.
32 For this reason, Linux _OSI defaults must continue to claim compatibility
33 with all versions of Windows.
34
35 But Linux isn't actually compatible with Windows, and the Linux community
36 has also been hurt with regressions when Linux adds the latest version of
37 Windows to its list of _OSI strings. So it is possible that additional strings
38 will be more thoroughly vetted before shipping upstream in the future.
39 But it is likely that they will all eventually be added.
40
41 What should an OEM do if they want to support Linux and Windows
42 using the same BIOS image? Often they need to do something different
43 for Linux to deal with how Linux is different from Windows.
44
45 In this case, the OEM should create custom ASL to be executed by the
46 Linux kernel and changes to Linux kernel drivers to execute this custom
47 ASL. The easiest way to accomplish this is to introduce a device specific
48 method (_DSM) that is called from the Linux kernel.
49
50 In the past the kernel used to support something like:
51 _OSI("Linux-OEM-my_interface_name")
52 where 'OEM' is needed if this is an OEM-specific hook,
53 and 'my_interface_name' describes the hook, which could be a
54 quirk, a bug, or a bug-fix.
55
56 However this was discovered to be abused by other BIOS vendors to change
57 completely unrelated code on completely unrelated systems. This prompted
58 an evaluation of all of its uses. This uncovered that they aren't needed
59 for any of the original reasons. As such, the kernel will not respond to
60 any custom Linux-* strings by default.
61
62 That was easy. Read on, to find out how to do it wrong.
63
64 Before _OSI, there was _OS
65 ==========================
66
67 ACPI 1.0 specified "_OS" as an
68 "object that evaluates to a string that identifies the operating system."
69
70 The ACPI BIOS flow would include an evaluation of _OS, and the AML
71 interpreter in the kernel would return to it a string identifying the OS:
72
73 Windows 98, SE: "Microsoft Windows"
74 Windows ME: "Microsoft WindowsME:Millennium Edition"
75 Windows NT: "Microsoft Windows NT"
76
77 The idea was on a platform tasked with running multiple OS's,
78 the BIOS could use _OS to enable devices that an OS
79 might support, or enable quirks or bug workarounds
80 necessary to make the platform compatible with that pre-existing OS.
81
82 But _OS had fundamental problems. First, the BIOS needed to know the name
83 of every possible version of the OS that would run on it, and needed to know
84 all the quirks of those OS's. Certainly it would make more sense
85 for the BIOS to ask *specific* things of the OS, such
86 "do you support a specific interface", and thus in ACPI 3.0,
87 _OSI was born to replace _OS.
88
89 _OS was abandoned, though even today, many BIOS look for
90 _OS "Microsoft Windows NT", though it seems somewhat far-fetched
91 that anybody would install those old operating systems
92 over what came with the machine.
93
94 Linux answers "Microsoft Windows NT" to please that BIOS idiom.
95 That is the *only* viable strategy, as that is what modern Windows does,
96 and so doing otherwise could steer the BIOS down an untested path.
97
98 _OSI is born, and immediately misused
99 =====================================
100
101 With _OSI, the *BIOS* provides the string describing an interface,
102 and asks the OS: "YES/NO, are you compatible with this interface?"
103
104 eg. _OSI("3.0 Thermal Model") would return TRUE if the OS knows how
105 to deal with the thermal extensions made to the ACPI 3.0 specification.
106 An old OS that doesn't know about those extensions would answer FALSE,
107 and a new OS may be able to return TRUE.
108
109 For an OS-specific interface, the ACPI spec said that the BIOS and the OS
110 were to agree on a string of the form such as "Windows-interface_name".
111
112 But two bad things happened. First, the Windows ecosystem used _OSI
113 not as designed, but as a direct replacement for _OS -- identifying
114 the OS version, rather than an OS supported interface. Indeed, right
115 from the start, the ACPI 3.0 spec itself codified this misuse
116 in example code using _OSI("Windows 2001").
117
118 This misuse was adopted and continues today.
119
120 Linux had no choice but to also return TRUE to _OSI("Windows 2001")
121 and its successors. To do otherwise would virtually guarantee breaking
122 a BIOS that has been tested only with that _OSI returning TRUE.
123
124 This strategy is problematic, as Linux is never completely compatible with
125 the latest version of Windows, and sometimes it takes more than a year
126 to iron out incompatibilities.
127
128 Not to be out-done, the Linux community made things worse by returning TRUE
129 to _OSI("Linux"). Doing so is even worse than the Windows misuse
130 of _OSI, as "Linux" does not even contain any version information.
131 _OSI("Linux") led to some BIOS' malfunctioning due to BIOS writer's
132 using it in untested BIOS flows. But some OEM's used _OSI("Linux")
133 in tested flows to support real Linux features. In 2009, Linux
134 removed _OSI("Linux"), and added a cmdline parameter to restore it
135 for legacy systems still needed it. Further a BIOS_BUG warning prints
136 for all BIOS's that invoke it.
137
138 No BIOS should use _OSI("Linux").
139
140 The result is a strategy for Linux to maximize compatibility with
141 ACPI BIOS that are tested on Windows machines. There is a real risk
142 of over-stating that compatibility; but the alternative has often been
143 catastrophic failure resulting from the BIOS taking paths that
144 were never validated under *any* OS.
145
146 Do not use _REV
147 ===============
148
149 Since _OSI("Linux") went away, some BIOS writers used _REV
150 to support Linux and Windows differences in the same BIOS.
151
152 _REV was defined in ACPI 1.0 to return the version of ACPI
153 supported by the OS and the OS AML interpreter.
154
155 Modern Windows returns _REV = 2. Linux used ACPI_CA_SUPPORT_LEVEL,
156 which would increment, based on the version of the spec supported.
157
158 Unfortunately, _REV was also misused. eg. some BIOS would check
159 for _REV = 3, and do something for Linux, but when Linux returned
160 _REV = 4, that support broke.
161
162 In response to this problem, Linux returns _REV = 2 always,
163 from mid-2015 onward. The ACPI specification will also be updated
164 to reflect that _REV is deprecated, and always returns 2.
165
166 Apple Mac and _OSI("Darwin")
167 ============================
168
169 On Apple's Mac platforms, the ACPI BIOS invokes _OSI("Darwin")
170 to determine if the machine is running Apple OSX.
171
172 Like Linux's _OSI("*Windows*") strategy, Linux defaults to
173 answering YES to _OSI("Darwin") to enable full access
174 to the hardware and validated BIOS paths seen by OSX.
175 Just like on Windows-tested platforms, this strategy has risks.
176
177 Starting in Linux-3.18, the kernel answered YES to _OSI("Darwin")
178 for the purpose of enabling Mac Thunderbolt support. Further,
179 if the kernel noticed _OSI("Darwin") being invoked, it additionally
180 disabled all _OSI("*Windows*") to keep poorly written Mac BIOS
181 from going down untested combinations of paths.
182
183 The Linux-3.18 change in default caused power regressions on Mac
184 laptops, and the 3.18 implementation did not allow changing
185 the default via cmdline "acpi_osi=!Darwin". Linux-4.7 fixed
186 the ability to use acpi_osi=!Darwin as a workaround, and
187 we hope to see Mac Thunderbolt power management support in Linux-4.11.
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