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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).