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1 | Linux ACPI Custom Control Method How To | |
2 | ======================================= | |
3 | ||
4 | Written by Zhang Rui <rui.zhang@intel.com> | |
5 | ||
6 | ||
7 | Linux supports customizing ACPI control methods at runtime. | |
8 | ||
9 | Users can use this to | |
10 | 1. override an existing method which may not work correctly, | |
11 | or just for debugging purposes. | |
12 | 2. insert a completely new method in order to create a missing | |
13 | method such as _OFF, _ON, _STA, _INI, etc. | |
14 | For these cases, it is far simpler to dynamically install a single | |
15 | control method rather than override the entire DSDT, because kernel | |
16 | rebuild/reboot is not needed and test result can be got in minutes. | |
17 | ||
18 | Note: Only ACPI METHOD can be overridden, any other object types like | |
19 | "Device", "OperationRegion", are not recognized. | |
20 | Note: The same ACPI control method can be overridden for many times, | |
21 | and it's always the latest one that used by Linux/kernel. | |
22 | Note: To get the ACPI debug object output (Store (AAAA, Debug)), | |
23 | please run "echo 1 > /sys/module/acpi/parameters/aml_debug_output". | |
24 | ||
25 | 1. override an existing method | |
26 | a) get the ACPI table via ACPI sysfs I/F. e.g. to get the DSDT, | |
27 | just run "cat /sys/firmware/acpi/tables/DSDT > /tmp/dsdt.dat" | |
28 | b) disassemble the table by running "iasl -d dsdt.dat". | |
29 | c) rewrite the ASL code of the method and save it in a new file, | |
30 | d) package the new file (psr.asl) to an ACPI table format. | |
31 | Here is an example of a customized \_SB._AC._PSR method, | |
32 | ||
33 | DefinitionBlock ("", "SSDT", 1, "", "", 0x20080715) | |
34 | { | |
35 | External (ACON) | |
36 | ||
37 | Method (\_SB_.AC._PSR, 0, NotSerialized) | |
38 | { | |
39 | Store ("In AC _PSR", Debug) | |
40 | Return (ACON) | |
41 | } | |
42 | } | |
43 | Note that the full pathname of the method in ACPI namespace | |
44 | should be used. | |
45 | And remember to use "External" to declare external objects. | |
46 | e) assemble the file to generate the AML code of the method. | |
47 | e.g. "iasl psr.asl" (psr.aml is generated as a result) | |
48 | f) mount debugfs by "mount -t debugfs none /sys/kernel/debug" | |
49 | g) override the old method via the debugfs by running | |
50 | "cat /tmp/psr.aml > /sys/kernel/debug/acpi/custom_method" | |
51 | ||
52 | 2. insert a new method | |
53 | This is easier than overriding an existing method. | |
54 | We just need to create the ASL code of the method we want to | |
55 | insert and then follow the step c) ~ g) in section 1. | |
56 | ||
57 | 3. undo your changes | |
58 | The "undo" operation is not supported for a new inserted method | |
59 | right now, i.e. we can not remove a method currently. | |
60 | For an overrided method, in order to undo your changes, please | |
61 | save a copy of the method original ASL code in step c) section 1, | |
62 | and redo step c) ~ g) to override the method with the original one. | |
63 | ||
64 | ||
65 | Note: We can use a kernel with multiple custom ACPI method running, | |
66 | But each individual write to debugfs can implement a SINGLE | |
67 | method override. i.e. if we want to insert/override multiple | |
68 | ACPI methods, we need to redo step c) ~ g) for multiple times. | |
69 | ||
70 | Note: Be aware that root can mis-use this driver to modify arbitrary | |
71 | memory and gain additional rights, if root's privileges got | |
72 | restricted (for example if root is not allowed to load additional | |
73 | modules after boot). |