path: root/Documentation/firmware-guide
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authorChangbin Du <>2019-04-25 01:52:51 +0800
committerRafael J. Wysocki <>2019-04-25 23:07:20 +0200
commiteea7803278619d1a3bda6306cd8470497cd2c5e3 (patch)
tree8ad22a9de9e892ec6b85a9a3c53d342abe26825b /Documentation/firmware-guide
parentb6dff0e153e919b62e2a1c90fd6e5a4ba922f99b (diff)
Documentation: ACPI: move method-customizing.txt to firmware-guide/acpi and convert to reST
This converts the plain text documentation to reStructuredText format and adds it to Sphinx TOC tree. No essential content change. Signed-off-by: Changbin Du <> Reviewed-by: Mauro Carvalho Chehab <> Signed-off-by: Rafael J. Wysocki <>
Diffstat (limited to 'Documentation/firmware-guide')
2 files changed, 91 insertions, 1 deletions
diff --git a/Documentation/firmware-guide/acpi/index.rst b/Documentation/firmware-guide/acpi/index.rst
index 61d67763851b..d1d069b26bbc 100644
--- a/Documentation/firmware-guide/acpi/index.rst
+++ b/Documentation/firmware-guide/acpi/index.rst
@@ -10,5 +10,6 @@ ACPI Support
+ method-customizing
- gpio-properties
+ gpio-properties \ No newline at end of file
diff --git a/Documentation/firmware-guide/acpi/method-customizing.rst b/Documentation/firmware-guide/acpi/method-customizing.rst
new file mode 100644
index 000000000000..de3ebcaed4cf
--- /dev/null
+++ b/Documentation/firmware-guide/acpi/method-customizing.rst
@@ -0,0 +1,89 @@
+.. SPDX-License-Identifier: GPL-2.0
+Linux ACPI Custom Control Method How To
+:Author: Zhang Rui <>
+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).