1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ========================== 3 ==========================
4 ACPI _OSI and _REV methods 4 ACPI _OSI and _REV methods
5 ========================== 5 ==========================
6 6
7 An ACPI BIOS can use the "Operating System Int 7 An ACPI BIOS can use the "Operating System Interfaces" method (_OSI)
8 to find out what the operating system supports 8 to find out what the operating system supports. Eg. If BIOS
9 AML code includes _OSI("XYZ"), the kernel's AM 9 AML code includes _OSI("XYZ"), the kernel's AML interpreter
10 can evaluate that method, look to see if it su 10 can evaluate that method, look to see if it supports 'XYZ'
11 and answer YES or NO to the BIOS. 11 and answer YES or NO to the BIOS.
12 12
13 The ACPI _REV method returns the "Revision of 13 The ACPI _REV method returns the "Revision of the ACPI specification
14 that OSPM supports" 14 that OSPM supports"
15 15
16 This document explains how and why the BIOS an 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 m 17 It also explains how and why they are widely misused.
18 18
19 How to use _OSI 19 How to use _OSI
20 =============== 20 ===============
21 21
22 Linux runs on two groups of machines -- those 22 Linux runs on two groups of machines -- those that are tested by the OEM
23 to be compatible with Linux, and those that we 23 to be compatible with Linux, and those that were never tested with Linux,
24 but where Linux was installed to replace the o 24 but where Linux was installed to replace the original OS (Windows or OSX).
25 25
26 The larger group is the systems tested to run 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 spec 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 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 actual 29 only a single path through the BIOS has actually been tested.
30 Experience shows that taking untested paths th 30 Experience shows that taking untested paths through the BIOS
31 exposes Linux to an entire category of BIOS bu 31 exposes Linux to an entire category of BIOS bugs.
32 For this reason, Linux _OSI defaults must cont 32 For this reason, Linux _OSI defaults must continue to claim compatibility
33 with all versions of Windows. 33 with all versions of Windows.
34 34
35 But Linux isn't actually compatible with Windo 35 But Linux isn't actually compatible with Windows, and the Linux community
36 has also been hurt with regressions when Linux 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 37 Windows to its list of _OSI strings. So it is possible that additional strings
38 will be more thoroughly vetted before shipping 38 will be more thoroughly vetted before shipping upstream in the future.
39 But it is likely that they will all eventually 39 But it is likely that they will all eventually be added.
40 40
41 What should an OEM do if they want to support 41 What should an OEM do if they want to support Linux and Windows
42 using the same BIOS image? Often they need to 42 using the same BIOS image? Often they need to do something different
43 for Linux to deal with how Linux is different 43 for Linux to deal with how Linux is different from Windows.
>> 44 Here the BIOS should ask exactly what it wants to know:
44 45
45 In this case, the OEM should create custom ASL <<
46 Linux kernel and changes to Linux kernel drive <<
47 ASL. The easiest way to accomplish this is to <<
48 method (_DSM) that is called from the Linux ke <<
49 <<
50 In the past the kernel used to support somethi <<
51 _OSI("Linux-OEM-my_interface_name") 46 _OSI("Linux-OEM-my_interface_name")
52 where 'OEM' is needed if this is an OEM-specif 47 where 'OEM' is needed if this is an OEM-specific hook,
53 and 'my_interface_name' describes the hook, wh 48 and 'my_interface_name' describes the hook, which could be a
54 quirk, a bug, or a bug-fix. 49 quirk, a bug, or a bug-fix.
55 50
56 However this was discovered to be abused by ot !! 51 In addition, the OEM should send a patch to upstream Linux
57 completely unrelated code on completely unrela !! 52 via the linux-acpi@vger.kernel.org mailing list. When that patch
58 an evaluation of all of its uses. This uncover !! 53 is checked into Linux, the OS will answer "YES" when the BIOS
59 for any of the original reasons. As such, the !! 54 on the OEM's system uses _OSI to ask if the interface is supported
60 any custom Linux-* strings by default. !! 55 by the OS. Linux distributors can back-port that patch for Linux
>> 56 pre-installs, and it will be included by all distributions that
>> 57 re-base to upstream. If the distribution can not update the kernel binary,
>> 58 they can also add an acpi_osi=Linux-OEM-my_interface_name
>> 59 cmdline parameter to the boot loader, as needed.
>> 60
>> 61 If the string refers to a feature where the upstream kernel
>> 62 eventually grows support, a patch should be sent to remove
>> 63 the string when that support is added to the kernel.
61 64
62 That was easy. Read on, to find out how to do 65 That was easy. Read on, to find out how to do it wrong.
63 66
64 Before _OSI, there was _OS 67 Before _OSI, there was _OS
65 ========================== 68 ==========================
66 69
67 ACPI 1.0 specified "_OS" as an 70 ACPI 1.0 specified "_OS" as an
68 "object that evaluates to a string that identi 71 "object that evaluates to a string that identifies the operating system."
69 72
70 The ACPI BIOS flow would include an evaluation 73 The ACPI BIOS flow would include an evaluation of _OS, and the AML
71 interpreter in the kernel would return to it a 74 interpreter in the kernel would return to it a string identifying the OS:
72 75
73 Windows 98, SE: "Microsoft Windows" 76 Windows 98, SE: "Microsoft Windows"
74 Windows ME: "Microsoft WindowsME:Millennium Ed !! 77 Windows ME: "Microsoft WindowsME:Millenium Edition"
75 Windows NT: "Microsoft Windows NT" 78 Windows NT: "Microsoft Windows NT"
76 79
77 The idea was on a platform tasked with running 80 The idea was on a platform tasked with running multiple OS's,
78 the BIOS could use _OS to enable devices that 81 the BIOS could use _OS to enable devices that an OS
79 might support, or enable quirks or bug workaro 82 might support, or enable quirks or bug workarounds
80 necessary to make the platform compatible with 83 necessary to make the platform compatible with that pre-existing OS.
81 84
82 But _OS had fundamental problems. First, the 85 But _OS had fundamental problems. First, the BIOS needed to know the name
83 of every possible version of the OS that would 86 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 wo 87 all the quirks of those OS's. Certainly it would make more sense
85 for the BIOS to ask *specific* things of the O 88 for the BIOS to ask *specific* things of the OS, such
86 "do you support a specific interface", and thu 89 "do you support a specific interface", and thus in ACPI 3.0,
87 _OSI was born to replace _OS. 90 _OSI was born to replace _OS.
88 91
89 _OS was abandoned, though even today, many BIO 92 _OS was abandoned, though even today, many BIOS look for
90 _OS "Microsoft Windows NT", though it seems so 93 _OS "Microsoft Windows NT", though it seems somewhat far-fetched
91 that anybody would install those old operating 94 that anybody would install those old operating systems
92 over what came with the machine. 95 over what came with the machine.
93 96
94 Linux answers "Microsoft Windows NT" to please 97 Linux answers "Microsoft Windows NT" to please that BIOS idiom.
95 That is the *only* viable strategy, as that is 98 That is the *only* viable strategy, as that is what modern Windows does,
96 and so doing otherwise could steer the BIOS do 99 and so doing otherwise could steer the BIOS down an untested path.
97 100
98 _OSI is born, and immediately misused 101 _OSI is born, and immediately misused
99 ===================================== 102 =====================================
100 103
101 With _OSI, the *BIOS* provides the string desc 104 With _OSI, the *BIOS* provides the string describing an interface,
102 and asks the OS: "YES/NO, are you compatible w 105 and asks the OS: "YES/NO, are you compatible with this interface?"
103 106
104 eg. _OSI("3.0 Thermal Model") would return TRU 107 eg. _OSI("3.0 Thermal Model") would return TRUE if the OS knows how
105 to deal with the thermal extensions made to th 108 to deal with the thermal extensions made to the ACPI 3.0 specification.
106 An old OS that doesn't know about those extens 109 An old OS that doesn't know about those extensions would answer FALSE,
107 and a new OS may be able to return TRUE. 110 and a new OS may be able to return TRUE.
108 111
109 For an OS-specific interface, the ACPI spec sa 112 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 113 were to agree on a string of the form such as "Windows-interface_name".
111 114
112 But two bad things happened. First, the Windo 115 But two bad things happened. First, the Windows ecosystem used _OSI
113 not as designed, but as a direct replacement f 116 not as designed, but as a direct replacement for _OS -- identifying
114 the OS version, rather than an OS supported in 117 the OS version, rather than an OS supported interface. Indeed, right
115 from the start, the ACPI 3.0 spec itself codif 118 from the start, the ACPI 3.0 spec itself codified this misuse
116 in example code using _OSI("Windows 2001"). 119 in example code using _OSI("Windows 2001").
117 120
118 This misuse was adopted and continues today. 121 This misuse was adopted and continues today.
119 122
120 Linux had no choice but to also return TRUE to 123 Linux had no choice but to also return TRUE to _OSI("Windows 2001")
121 and its successors. To do otherwise would vir 124 and its successors. To do otherwise would virtually guarantee breaking
122 a BIOS that has been tested only with that _OS 125 a BIOS that has been tested only with that _OSI returning TRUE.
123 126
124 This strategy is problematic, as Linux is neve 127 This strategy is problematic, as Linux is never completely compatible with
125 the latest version of Windows, and sometimes i 128 the latest version of Windows, and sometimes it takes more than a year
126 to iron out incompatibilities. 129 to iron out incompatibilities.
127 130
128 Not to be out-done, the Linux community made t 131 Not to be out-done, the Linux community made things worse by returning TRUE
129 to _OSI("Linux"). Doing so is even worse than 132 to _OSI("Linux"). Doing so is even worse than the Windows misuse
130 of _OSI, as "Linux" does not even contain any 133 of _OSI, as "Linux" does not even contain any version information.
131 _OSI("Linux") led to some BIOS' malfunctioning 134 _OSI("Linux") led to some BIOS' malfunctioning due to BIOS writer's
132 using it in untested BIOS flows. But some OEM 135 using it in untested BIOS flows. But some OEM's used _OSI("Linux")
133 in tested flows to support real Linux features 136 in tested flows to support real Linux features. In 2009, Linux
134 removed _OSI("Linux"), and added a cmdline par 137 removed _OSI("Linux"), and added a cmdline parameter to restore it
135 for legacy systems still needed it. Further a 138 for legacy systems still needed it. Further a BIOS_BUG warning prints
136 for all BIOS's that invoke it. 139 for all BIOS's that invoke it.
137 140
138 No BIOS should use _OSI("Linux"). 141 No BIOS should use _OSI("Linux").
139 142
140 The result is a strategy for Linux to maximize 143 The result is a strategy for Linux to maximize compatibility with
141 ACPI BIOS that are tested on Windows machines. 144 ACPI BIOS that are tested on Windows machines. There is a real risk
142 of over-stating that compatibility; but the al 145 of over-stating that compatibility; but the alternative has often been
143 catastrophic failure resulting from the BIOS t 146 catastrophic failure resulting from the BIOS taking paths that
144 were never validated under *any* OS. 147 were never validated under *any* OS.
145 148
146 Do not use _REV 149 Do not use _REV
147 =============== 150 ===============
148 151
149 Since _OSI("Linux") went away, some BIOS write 152 Since _OSI("Linux") went away, some BIOS writers used _REV
150 to support Linux and Windows differences in th 153 to support Linux and Windows differences in the same BIOS.
151 154
152 _REV was defined in ACPI 1.0 to return the ver 155 _REV was defined in ACPI 1.0 to return the version of ACPI
153 supported by the OS and the OS AML interpreter 156 supported by the OS and the OS AML interpreter.
154 157
155 Modern Windows returns _REV = 2. Linux used A 158 Modern Windows returns _REV = 2. Linux used ACPI_CA_SUPPORT_LEVEL,
156 which would increment, based on the version of 159 which would increment, based on the version of the spec supported.
157 160
158 Unfortunately, _REV was also misused. eg. som 161 Unfortunately, _REV was also misused. eg. some BIOS would check
159 for _REV = 3, and do something for Linux, but 162 for _REV = 3, and do something for Linux, but when Linux returned
160 _REV = 4, that support broke. 163 _REV = 4, that support broke.
161 164
162 In response to this problem, Linux returns _RE 165 In response to this problem, Linux returns _REV = 2 always,
163 from mid-2015 onward. The ACPI specification 166 from mid-2015 onward. The ACPI specification will also be updated
164 to reflect that _REV is deprecated, and always 167 to reflect that _REV is deprecated, and always returns 2.
165 168
166 Apple Mac and _OSI("Darwin") 169 Apple Mac and _OSI("Darwin")
167 ============================ 170 ============================
168 171
169 On Apple's Mac platforms, the ACPI BIOS invoke 172 On Apple's Mac platforms, the ACPI BIOS invokes _OSI("Darwin")
170 to determine if the machine is running Apple O 173 to determine if the machine is running Apple OSX.
171 174
172 Like Linux's _OSI("*Windows*") strategy, Linux 175 Like Linux's _OSI("*Windows*") strategy, Linux defaults to
173 answering YES to _OSI("Darwin") to enable full 176 answering YES to _OSI("Darwin") to enable full access
174 to the hardware and validated BIOS paths seen 177 to the hardware and validated BIOS paths seen by OSX.
175 Just like on Windows-tested platforms, this st 178 Just like on Windows-tested platforms, this strategy has risks.
176 179
177 Starting in Linux-3.18, the kernel answered YE 180 Starting in Linux-3.18, the kernel answered YES to _OSI("Darwin")
178 for the purpose of enabling Mac Thunderbolt su 181 for the purpose of enabling Mac Thunderbolt support. Further,
179 if the kernel noticed _OSI("Darwin") being inv 182 if the kernel noticed _OSI("Darwin") being invoked, it additionally
180 disabled all _OSI("*Windows*") to keep poorly 183 disabled all _OSI("*Windows*") to keep poorly written Mac BIOS
181 from going down untested combinations of paths 184 from going down untested combinations of paths.
182 185
183 The Linux-3.18 change in default caused power 186 The Linux-3.18 change in default caused power regressions on Mac
184 laptops, and the 3.18 implementation did not a 187 laptops, and the 3.18 implementation did not allow changing
185 the default via cmdline "acpi_osi=!Darwin". L 188 the default via cmdline "acpi_osi=!Darwin". Linux-4.7 fixed
186 the ability to use acpi_osi=!Darwin as a worka 189 the ability to use acpi_osi=!Darwin as a workaround, and
187 we hope to see Mac Thunderbolt power managemen 190 we hope to see Mac Thunderbolt power management support in Linux-4.11.
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