90 lines
3.3 KiB
ReStructuredText
90 lines
3.3 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=======================================
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Linux ACPI Custom Control Method How To
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=======================================
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:Author: Zhang Rui <rui.zhang@intel.com>
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Linux supports customizing ACPI control methods at runtime.
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Users can use this to:
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1. override an existing method which may not work correctly,
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or just for debugging purposes.
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2. insert a completely new method in order to create a missing
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method such as _OFF, _ON, _STA, _INI, etc.
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For these cases, it is far simpler to dynamically install a single
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control method rather than override the entire DSDT, because kernel
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rebuild/reboot is not needed and test result can be got in minutes.
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.. note::
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- Only ACPI METHOD can be overridden, any other object types like
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"Device", "OperationRegion", are not recognized. Methods
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declared inside scope operators are also not supported.
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- The same ACPI control method can be overridden for many times,
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and it's always the latest one that used by Linux/kernel.
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- To get the ACPI debug object output (Store (AAAA, Debug)),
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please run::
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echo 1 > /sys/module/acpi/parameters/aml_debug_output
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1. override an existing method
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==============================
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a) get the ACPI table via ACPI sysfs I/F. e.g. to get the DSDT,
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just run "cat /sys/firmware/acpi/tables/DSDT > /tmp/dsdt.dat"
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b) disassemble the table by running "iasl -d dsdt.dat".
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c) rewrite the ASL code of the method and save it in a new file,
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d) package the new file (psr.asl) to an ACPI table format.
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Here is an example of a customized \_SB._AC._PSR method::
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DefinitionBlock ("", "SSDT", 1, "", "", 0x20080715)
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{
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Method (\_SB_.AC._PSR, 0, NotSerialized)
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{
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Store ("In AC _PSR", Debug)
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Return (ACON)
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}
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}
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Note that the full pathname of the method in ACPI namespace
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should be used.
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e) assemble the file to generate the AML code of the method.
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e.g. "iasl -vw 6084 psr.asl" (psr.aml is generated as a result)
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If parameter "-vw 6084" is not supported by your iASL compiler,
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please try a newer version.
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f) mount debugfs by "mount -t debugfs none /sys/kernel/debug"
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g) override the old method via the debugfs by running
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"cat /tmp/psr.aml > /sys/kernel/debug/acpi/custom_method"
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2. insert a new method
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======================
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This is easier than overriding an existing method.
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We just need to create the ASL code of the method we want to
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insert and then follow the step c) ~ g) in section 1.
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3. undo your changes
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====================
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The "undo" operation is not supported for a new inserted method
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right now, i.e. we can not remove a method currently.
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For an overridden method, in order to undo your changes, please
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save a copy of the method original ASL code in step c) section 1,
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and redo step c) ~ g) to override the method with the original one.
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.. note:: We can use a kernel with multiple custom ACPI method running,
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But each individual write to debugfs can implement a SINGLE
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method override. i.e. if we want to insert/override multiple
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ACPI methods, we need to redo step c) ~ g) for multiple times.
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.. note:: Be aware that root can mis-use this driver to modify arbitrary
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memory and gain additional rights, if root's privileges got
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restricted (for example if root is not allowed to load additional
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modules after boot).
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