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+Linux ACPI Custom Control Method How To
+Written by 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.
+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).