platform/wmi: Update driver development guide

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

        .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 */

  };

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.

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.

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

-------------------------------------

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.