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>

6 files changed, 139 insertions, 0 deletions

diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index ff9aa1afdc62..1e16a40da3ba 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -113,6 +113,7 @@ available subsections can be seen below.
    xillybus
    zorro
    hte/index
+   wmi
 
 .. only::  subproject and html
 
diff --git a/Documentation/driver-api/wmi.rst b/Documentation/driver-api/wmi.rst
new file mode 100644
index 000000000000..6ca58c8249e5
--- /dev/null
+++ b/Documentation/driver-api/wmi.rst
@@ -0,0 +1,21 @@
+.. 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:
diff --git a/Documentation/subsystem-apis.rst b/Documentation/subsystem-apis.rst
index b51f38527e14..69f5e4d53bad 100644
--- a/Documentation/subsystem-apis.rst
+++ b/Documentation/subsystem-apis.rst
@@ -57,3 +57,4 @@ needed).
    scheduler/index
    mhi/index
    peci/index
+   wmi/index
diff --git a/Documentation/wmi/acpi-interface.rst b/Documentation/wmi/acpi-interface.rst
new file mode 100644
index 000000000000..d31af0ed9c08
--- /dev/null
+++ b/Documentation/wmi/acpi-interface.rst
@@ -0,0 +1,96 @@
+.. 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.
diff --git a/Documentation/wmi/index.rst b/Documentation/wmi/index.rst
new file mode 100644
index 000000000000..b29933a86380
--- /dev/null
+++ b/Documentation/wmi/index.rst
@@ -0,0 +1,18 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+=============
+WMI Subsystem
+=============
+
+.. toctree::
+   :maxdepth: 1
+
+   acpi-interface
+
+.. only::  subproject and html
+
+
+   Indices
+   =======
+
+   * :ref:`genindex`
diff --git a/MAINTAINERS b/MAINTAINERS
index 7e0b87d5aa2e..46ad203dad12 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -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