Documentation/firmware-guide/acpi/method-customizing.rst

.. 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).