platform/x86: wmi: Add documentation

Add documentation for the WMI subsystem. The documentation describes
both the ACPI WMI interface and the driver API for interacting with
the WMI driver core. The information regarding the ACPI interface
was retrieved from the Ubuntu kernel references and the Windows driver
samples available on GitHub. The documentation is supposed to help
driver developers writing WMI drivers, as many modern machines designed
to run Windows provide an ACPI WMI interface.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20230424222939.208137-4-W_Armin@gmx.de
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
This commit is contained in:
Armin Wolf 2023-04-25 00:29:38 +02:00 committed by Hans de Goede
parent d54bd4bc7b
commit e8f8b448c1
6 changed files with 139 additions and 0 deletions

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@ -113,6 +113,7 @@ available subsections can be seen below.
xillybus
zorro
hte/index
wmi
.. only:: subproject and html

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.. SPDX-License-Identifier: GPL-2.0-or-later
==============
WMI Driver API
==============
The WMI driver core supports a more modern bus-based interface for interacting
with WMI devices, and an older GUID-based interface. The latter interface is
considered to be deprecated, so new WMI drivers should generally avoid it since
it has some issues with multiple WMI devices and events sharing the same GUIDs
and/or notification IDs. The modern bus-based interface instead maps each
WMI device to a :c:type:`struct wmi_device <wmi_device>`, so it supports
WMI devices sharing GUIDs and/or notification IDs. Drivers can then register
a :c:type:`struct wmi_driver <wmi_driver>`, which will be bound to compatible
WMI devices by the driver core.
.. kernel-doc:: include/linux/wmi.h
:internal:
.. kernel-doc:: drivers/platform/x86/wmi.c
:export:

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scheduler/index
mhi/index
peci/index
wmi/index

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.. SPDX-License-Identifier: GPL-2.0-or-later
==================
ACPI WMI interface
==================
The ACPI WMI interface is a proprietary extension of the ACPI specification made
by Microsoft to allow hardware vendors to embed WMI (Windows Management Instrumentation)
objects inside their ACPI firmware. Typical functions implemented over ACPI WMI
are hotkey events on modern notebooks and configuration of BIOS options.
PNP0C14 ACPI device
-------------------
Discovery of WMI objects is handled by defining ACPI devices with a PNP ID
of ``PNP0C14``. These devices will contain a set of ACPI buffers and methods
used for mapping and execution of WMI methods and/or queries. If there exist
multiple of such devices, then each device is required to have a
unique ACPI UID.
_WDG buffer
-----------
The ``_WDG`` buffer is used to discover WMI objects and is required to be
static. Its internal structure consists of data blocks with a size of 20 bytes,
containing the following data:
======= =============== =====================================================
Offset Size (in bytes) Content
======= =============== =====================================================
0x00 16 128 bit Variant 2 object GUID.
0x10 2 2 character method ID or single byte notification ID.
0x12 1 Object instance count.
0x13 1 Object flags.
======= =============== =====================================================
The WMI object flags control whether the method or notification ID is used:
- 0x1: Data block usage is expensive and must be explicitly enabled/disabled.
- 0x2: Data block contains WMI methods.
- 0x4: Data block contains ASCIZ string.
- 0x8: Data block describes a WMI event, use notification ID instead
of method ID.
Each WMI object GUID can appear multiple times inside a system.
The method/notification ID is used to construct the ACPI method names used for
interacting with the WMI object.
WQxx ACPI methods
-----------------
If a data block does not contain WMI methods, then its content can be retrieved
by this required ACPI method. The last two characters of the ACPI method name
are the method ID of the data block to query. Their single parameter is an
integer describing the instance which should be queried. This parameter can be
omitted if the data block contains only a single instance.
WSxx ACPI methods
-----------------
Similar to the ``WQxx`` ACPI methods, except that it is optional and takes an
additional buffer as its second argument. The instance argument also cannot
be omitted.
WMxx ACPI methods
-----------------
Used for executing WMI methods associated with a data block. The last two
characters of the ACPI method name are the method ID of the data block
containing the WMI methods. Their first parameter is a integer describing the
instance which methods should be executed. The second parameter is an integer
describing the WMI method ID to execute, and the third parameter is a buffer
containing the WMI method parameters. If the data block is marked as containing
an ASCIZ string, then this buffer should contain an ASCIZ string. The ACPI
method will return the result of the executed WMI method.
WExx ACPI methods
-----------------
Used for optionally enabling/disabling WMI events, the last two characters of
the ACPI method are the notification ID of the data block describing the WMI
event as hexadecimal value. Their first parameter is an integer with a value
of 0 if the WMI event should be disabled, other values will enable
the WMI event.
WCxx ACPI methods
-----------------
Similar to the ``WExx`` ACPI methods, except that it controls data collection
instead of events and thus the last two characters of the ACPI method name are
the method ID of the data block to enable/disable.
_WED ACPI method
----------------
Used to retrieve additional WMI event data, its single parameter is a integer
holding the notification ID of the event.

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.. SPDX-License-Identifier: GPL-2.0-or-later
=============
WMI Subsystem
=============
.. toctree::
:maxdepth: 1
acpi-interface
.. only:: subproject and html
Indices
=======
* :ref:`genindex`

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@ -449,6 +449,8 @@ F: include/linux/acpi_viot.h
ACPI WMI DRIVER
L: platform-driver-x86@vger.kernel.org
S: Orphan
F: Documentation/driver-api/wmi.rst
F: Documentation/wmi/
F: drivers/platform/x86/wmi.c
F: include/uapi/linux/wmi.h