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-.. SPDX-License-Identifier: GPL-2.0
-
-=======================================
-Linux ACPI Custom Control Method How To
-=======================================
-
-:Author: Zhang Rui <rui.zhang@intel.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. Methods
- declared inside scope operators are also not supported.
-
- - The same ACPI control method can be overridden for many times,
- and it's always the latest one that used by Linux/kernel.
-
- - 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)
- {
- 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.
-e) assemble the file to generate the AML code of the method.
- e.g. "iasl -vw 6084 psr.asl" (psr.aml is generated as a result)
- If parameter "-vw 6084" is not supported by your iASL compiler,
- please try a newer version.
-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 overridden 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).