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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 | ||
23 | 1. override an existing method | |
24 | a) get the ACPI table via ACPI sysfs I/F. e.g. to get the DSDT, | |
25 | just run "cat /sys/firmware/acpi/tables/DSDT > /tmp/dsdt.dat" | |
26 | b) disassemble the table by running "iasl -d dsdt.dat". | |
27 | c) rewrite the ASL code of the method and save it in a new file, | |
28 | d) package the new file (psr.asl) to an ACPI table format. | |
29 | Here is an example of a customized \_SB._AC._PSR method, | |
30 | ||
31 | DefinitionBlock ("", "SSDT", 1, "", "", 0x20080715) | |
32 | { | |
33 | External (ACON) | |
34 | ||
35 | Method (\_SB_.AC._PSR, 0, NotSerialized) | |
36 | { | |
37 | Store ("In AC _PSR", Debug) | |
38 | Return (ACON) | |
39 | } | |
40 | } | |
41 | Note that the full pathname of the method in ACPI namespace | |
42 | should be used. | |
43 | And remember to use "External" to declare external objects. | |
44 | e) assemble the file to generate the AML code of the method. | |
45 | e.g. "iasl psr.asl" (psr.aml is generated as a result) | |
46 | f) mount debugfs by "mount -t debugfs none /sys/kernel/debug" | |
47 | g) override the old method via the debugfs by running | |
48 | "cat /tmp/psr.aml > /sys/kernel/debug/acpi/custom_method" | |
49 | ||
50 | 2. insert a new method | |
51 | This is easier than overriding an existing method. | |
52 | We just need to create the ASL code of the method we want to | |
53 | insert and then follow the step c) ~ g) in section 1. | |
54 | ||
55 | 3. undo your changes | |
56 | The "undo" operation is not supported for a new inserted method | |
57 | right now, i.e. we can not remove a method currently. | |
58 | For an overrided method, in order to undo your changes, please | |
59 | save a copy of the method original ASL code in step c) section 1, | |
60 | and redo step c) ~ g) to override the method with the original one. | |
61 | ||
62 | ||
63 | Note: We can use a kernel with multiple custom ACPI method running, | |
64 | But each individual write to debugfs can implement a SINGLE | |
65 | method override. i.e. if we want to insert/override multiple | |
66 | ACPI methods, we need to redo step c) ~ g) for multiple times. |