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-Linux ACPI Custom Control Method How To
-Written by Zhang Rui <email@example.com>
-Linux supports customizing ACPI control methods at runtime.
-Users can use this to
-1. override an existing method which may not work correctly,
- or just for debugging purposes.
-2. insert a completely new method in order to create a missing
- method such as _OFF, _ON, _STA, _INI, etc.
-For these cases, it is far simpler to dynamically install a single
-control method rather than override the entire DSDT, because kernel
-rebuild/reboot is not needed and test result can be got in minutes.
-Note: Only ACPI METHOD can be overridden, any other object types like
- "Device", "OperationRegion", are not recognized.
-Note: The same ACPI control method can be overridden for many times,
- and it's always the latest one that used by Linux/kernel.
-Note: To get the ACPI debug object output (Store (AAAA, Debug)),
- please run "echo 1 > /sys/module/acpi/parameters/aml_debug_output".
-1. override an existing method
- a) get the ACPI table via ACPI sysfs I/F. e.g. to get the DSDT,
- just run "cat /sys/firmware/acpi/tables/DSDT > /tmp/dsdt.dat"
- b) disassemble the table by running "iasl -d dsdt.dat".
- c) rewrite the ASL code of the method and save it in a new file,
- d) package the new file (psr.asl) to an ACPI table format.
- Here is an example of a customized \_SB._AC._PSR method,
- DefinitionBlock ("", "SSDT", 1, "", "", 0x20080715)
- External (ACON)
- Method (\_SB_.AC._PSR, 0, NotSerialized)
- Store ("In AC _PSR", Debug)
- Return (ACON)
- Note that the full pathname of the method in ACPI namespace
- should be used.
- And remember to use "External" to declare external objects.
- e) assemble the file to generate the AML code of the method.
- e.g. "iasl psr.asl" (psr.aml is generated as a result)
- f) mount debugfs by "mount -t debugfs none /sys/kernel/debug"
- g) override the old method via the debugfs by running
- "cat /tmp/psr.aml > /sys/kernel/debug/acpi/custom_method"
-2. insert a new method
- This is easier than overriding an existing method.
- We just need to create the ASL code of the method we want to
- insert and then follow the step c) ~ g) in section 1.
-3. undo your changes
- The "undo" operation is not supported for a new inserted method
- right now, i.e. we can not remove a method currently.
- For an overrided method, in order to undo your changes, please
- save a copy of the method original ASL code in step c) section 1,
- and redo step c) ~ g) to override the method with the original one.
-Note: We can use a kernel with multiple custom ACPI method running,
- But each individual write to debugfs can implement a SINGLE
- method override. i.e. if we want to insert/override multiple
- ACPI methods, we need to redo step c) ~ g) for multiple times.
-Note: Be aware that root can mis-use this driver to modify arbitrary
- memory and gain additional rights, if root's privileges got
- restricted (for example if root is not allowed to load additional
- modules after boot).