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platform/wmi: Update driver development guide

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New WMI drivers should use the new buffer-based WMI API instead of
the deprecated ACPI-based API. Update the driver development guide
to recommend the buffer-based API to driver developers and explain
the purpose of struct wmi_buffer.

Also update the ACPI interface documentation to describe the
conversion rules for converting ACPI objects into WMI buffers.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://patch.msgid.link/20260116204116.4030-10-W_Armin@gmx.de
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
This commit is contained in:
Armin Wolf 2026-01-16 21:41:16 +01:00 • committed by Ilpo Järvinen
parent 926a266575
commit 2177a02246
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2 changed files with 121 additions and 23 deletions
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68
Documentation/wmi/acpi-interface.rst
View file

@ -104,3 +104,71 @@ holding the notification ID of the event. This method should be evaluated every
time an ACPI notification is received, since some ACPI implementations use a
queue to store WMI event data items. This queue will overflow after a couple
of WMI events are received without retrieving the associated WMI event data.
Conversion rules for ACPI data types
------------------------------------
Consumers of the ACPI-WMI interface use binary buffers to exchange data with the WMI driver core,
with the internal structure of the buffer being only know to the consumers. The WMI driver core is
thus responsible for converting the data inside the buffer into an appropriate ACPI data type for
consumption by the ACPI firmware. Additionally, any data returned by the various ACPI methods needs
to be converted back into a binary buffer.
The layout of said buffers is defined by the MOF description of the WMI method or data block in
question [1]_:
=============== ======================================================================= =========
Data Type Layout Alignment
=============== ======================================================================= =========
``string`` Starts with an unsigned 16-bit little endian integer specifying 2 bytes
the length of the string data in bytes, followed by the string data
encoded as UTF-16LE with **optional** NULL termination and padding.
Keep in mind that some firmware implementations might depend on the
terminating NULL character to be present. Also the padding should
always be performed with NULL characters.
``boolean`` Single byte where 0 means ``false`` and nonzero means ``true``. 1 byte
``sint8`` Signed 8-bit integer. 1 byte
``uint8`` Unsigned 8-bit integer. 1 byte
``sint16`` Signed 16-bit little endian integer. 2 bytes
``uint16`` Unsigned 16-bit little endian integer. 2 bytes
``sint32`` Signed 32-bit little endian integer. 4 bytes
``uint32`` Unsigned 32-bit little endian integer. 4 bytes
``sint64`` Signed 64-bit little endian integer. 8 bytes
``uint64`` Unsigned 64-bit little endian integer. 8 bytes
``datetime`` A fixed-length 25-character UTF-16LE string with the format 2 bytes
*yyyymmddhhmmss.mmmmmmsutc* where *yyyy* is the 4-digit year, *mm* is
the 2-digit month, *dd* is the 2-digit day, *hh* is the 2-digit hour
based on a 24-hour clock, *mm* is the 2-digit minute, *ss* is the
2-digit second, *mmmmmm* is the 6-digit microsecond, *s* is a plus or
minus character depending on whether *utc* is a positive or negative
offset from UTC (or a colon if the date is an interval). Unpopulated
fields should be filled with asterisks.
=============== ======================================================================= =========
Arrays should be aligned based on the alignment of their base type, while objects should be
aligned based on the largest alignment of an element inside them.
All buffers returned by the WMI driver core are 8-byte aligned. When converting ACPI data types
into such buffers the following conversion rules apply:
=============== ============================================================
ACPI Data Type Converted into
=============== ============================================================
Buffer Copied as-is.
Integer Converted into a ``uint32``.
String Converted into a ``string`` with a terminating NULL character
to match the behavior the of the Windows driver.
Package Each element inside the package is converted with alignment
of the resulting data types being respected. Nested packages
are not allowed.
=============== ============================================================
The Windows driver does attempt to handle nested packages, but this results in internal data
structures (``_ACPI_METHOD_ARGUMENT_V1``) erroneously being copied into the resulting buffer.
ACPI firmware implementations should thus not return nested packages from ACPI methods
associated with the ACPI-WMI interface.
References
==========
.. [1] https://learn.microsoft.com/en-us/windows-hardware/drivers/kernel/driver-defined-wmi-data-items

76
Documentation/wmi/driver-development-guide.rst
View file

@ -70,7 +70,7 @@ to matching WMI devices using a struct wmi_device_id table:
.probe = foo_probe,
.remove = foo_remove, /* optional, devres is preferred */
.shutdown = foo_shutdown, /* optional, called during shutdown */
.notify = foo_notify, /* optional, for event handling */
.notify_new = foo_notify, /* optional, for event handling */
.no_notify_data = true, /* optional, enables events containing no additional data */
.no_singleton = true, /* required for new WMI drivers */
};
@ -90,9 +90,9 @@ the WMI device and put it in a well-known state for the WMI driver to pick up la
or kexec. Most WMI drivers need no special shutdown handling and can thus omit this callback.
Please note that new WMI drivers are required to be able to be instantiated multiple times,
and are forbidden from using any deprecated GUID-based WMI functions. This means that the
WMI driver should be prepared for the scenario that multiple matching WMI devices are present
on a given machine.
and are forbidden from using any deprecated GUID-based or ACPI-based WMI functions. This means
that the WMI driver should be prepared for the scenario that multiple matching WMI devices are
present on a given machine.
Because of this, WMI drivers should use the state container design pattern as described in
Documentation/driver-api/driver-model/design-patterns.rst.
@ -104,38 +104,37 @@ Documentation/driver-api/driver-model/design-patterns.rst.
WMI method drivers
------------------
WMI drivers can call WMI device methods using wmidev_evaluate_method(), the
structure of the ACPI buffer passed to this function is device-specific and usually
needs some tinkering to get right. Looking at the ACPI tables containing the WMI
device usually helps here. The method id and instance number passed to this function
are also device-specific, looking at the decoded Binary MOF is usually enough to
find the right values.
WMI drivers can call WMI device methods using wmidev_invoke_method(). For each WMI method
invocation the WMI driver needs to provide the instance number and the method ID, as well as
a buffer with the method arguments and optionally a buffer for the results.
The maximum instance number can be retrieved during runtime using wmidev_instance_count().
The layout of said buffers is device-specific and described by the Binary MOF data associated
with a given WMI device. Said Binary MOF data also describes the method ID of a given WMI method
with the ``WmiMethodId`` qualifier. WMI devices exposing WMI methods usually expose only a single
instance (instance number 0), but in theory can expose multiple instances as well. In such a case
the number of instances can be retrieved using wmidev_instance_count().
Take a look at drivers/platform/x86/inspur_platform_profile.c for an example WMI method driver.
Take a look at drivers/platform/x86/intel/wmi/thunderbolt.c for an example WMI method driver.
WMI data block drivers
----------------------
WMI drivers can query WMI device data blocks using wmidev_block_query(), the
structure of the returned ACPI object is again device-specific. Some WMI devices
also allow for setting data blocks using wmidev_block_set().
WMI drivers can query WMI data blocks using wmidev_query_block(), the layout of the returned
buffer is again device-specific and described by the Binary MOF data. Some WMI data blocks are
also writeable and can be set using wmidev_set_block(). The number of data block instances can
again be retrieved using wmidev_instance_count().
The maximum instance number can also be retrieved using wmidev_instance_count().
Take a look at drivers/platform/x86/intel/wmi/sbl-fw-update.c for an example
WMI data block driver.
Take a look at drivers/platform/x86/intel/wmi/sbl-fw-update.c for an example WMI data block driver.
WMI event drivers
-----------------
WMI drivers can receive WMI events via the notify() callback inside the struct wmi_driver.
WMI drivers can receive WMI events via the notify_new() callback inside the struct wmi_driver.
The WMI subsystem will then take care of setting up the WMI event accordingly. Please note that
the structure of the ACPI object passed to this callback is device-specific, and freeing the
ACPI object is being done by the WMI subsystem, not the driver.
the layout of the buffer passed to this callback is device-specific, and freeing of the buffer
is done by the WMI subsystem itself, not the driver.
The WMI driver core will take care that the notify() callback will only be called after
The WMI driver core will take care that the notify_new() callback will only be called after
the probe() callback has been called, and that no events are being received by the driver
right before and after calling its remove() or shutdown() callback.
@ -147,6 +146,36 @@ the ``no_notify_data`` flag inside struct wmi_driver should be set to ``true``.
Take a look at drivers/platform/x86/xiaomi-wmi.c for an example WMI event driver.
Exchanging data with the WMI driver core
----------------------------------------
WMI drivers can exchange data with the WMI driver core using struct wmi_buffer. The internal
structure of those buffers is device-specific and only known by the WMI driver. Because of this
the WMI driver itself is responsible for parsing and validating the data received from its
WMI device.
The structure of said buffers is described by the MOF data associated with the WMI device in
question. When such a buffer contains multiple data items it usually makes sense to define a
C structure and use it during parsing. Since the WMI driver core guarantees that all buffers
received from a WMI device are aligned on an 8-byte boundary, WMI drivers can simply perform
a cast between the WMI buffer data and this C structure.
This however should only be done after the size of the buffer was verified to be large enough
to hold the whole C structure. WMI drivers should reject undersized buffers as they are usually
sent by the WMI device to signal an internal error. Oversized buffers however should be accepted
to emulate the behavior of the Windows WMI implementation.
When defining a C structure for parsing WMI buffers the alignment of the data items should be
respected. This is especially important for 64-bit integers as those have different alignments
on 64-bit (8-byte alignment) and 32-bit (4-byte alignment) architectures. It is thus a good idea
to manually specify the alignment of such data items or mark the whole structure as packed when
appropriate. Integer data items in general are little-endian integers and should be marked as
such using ``__le64`` and friends. When parsing WMI string data items the struct wmi_string should
be used as WMI strings have a different layout than C strings.
See Documentation/wmi/acpi-interface.rst for more information regarding the binary format
of WMI data items.
Handling multiple WMI devices at once
-------------------------------------
@ -171,6 +200,7 @@ Things to avoid
When developing WMI drivers, there are a couple of things which should be avoided:
- usage of the deprecated GUID-based WMI interface which uses GUIDs instead of WMI device structs
- usage of the deprecated ACPI-based WMI interface which uses ACPI objects instead of plain buffers
- bypassing of the WMI subsystem when talking to WMI devices
- WMI drivers which cannot be instantiated multiple times.