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platform/x86: wmi: Add driver development guide
Since 2010, an LWN article covering WMI drivers exists: https://lwn.net/Articles/391230/ Since the introduction of the modern bus-based interface and other userspace tooling (bmfdec, lswmi, ...), this article is outdated and causes people to still submit new WMI drivers using the deprecated GUID-based interface. Fix this by adding a short guide on how to develop WMI drivers using the modern bus-based interface. Signed-off-by: Armin Wolf <W_Armin@gmx.de> Link: https://lore.kernel.org/r/20240402143059.8456-4-W_Armin@gmx.de Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com> Reviewed-by: Hans de Goede <hdegoede@redhat.com> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
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Armin Wolf
Hans de Goede
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Documentation/wmi/driver-development-guide.rst
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Documentation/wmi/driver-development-guide.rst
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.. SPDX-License-Identifier: GPL-2.0-or-later
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============================
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WMI driver development guide
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============================
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The WMI subsystem provides a rich driver API for implementing WMI drivers,
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documented at Documentation/driver-api/wmi.rst. This document will serve
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as an introductory guide for WMI driver writers using this API. It is supposed
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to be a successor to the original LWN article [1]_ which deals with WMI drivers
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using the deprecated GUID-based WMI interface.
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Obtaining WMI device information
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--------------------------------
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Before developing an WMI driver, information about the WMI device in question
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must be obtained. The `lswmi <https://pypi.org/project/lswmi>`_ utility can be
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used to extract detailed WMI device information using the following command:
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::
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lswmi -V
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The resulting output will contain information about all WMI devices available on
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a given machine, plus some extra information.
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In order to find out more about the interface used to communicate with a WMI device,
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the `bmfdec <https://github.com/pali/bmfdec>`_ utilities can be used to decode
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the Binary MOF (Managed Object Format) information used to describe WMI devices.
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The ``wmi-bmof`` driver exposes this information to userspace, see
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Documentation/wmi/devices/wmi-bmof.rst.
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In order to retrieve the decoded Binary MOF information, use the following command (requires root):
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::
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./bmf2mof /sys/bus/wmi/devices/05901221-D566-11D1-B2F0-00A0C9062910[-X]/bmof
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Sometimes, looking at the disassembled ACPI tables used to describe the WMI device
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helps in understanding how the WMI device is supposed to work. The path of the ACPI
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method associated with a given WMI device can be retrieved using the ``lswmi`` utility
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as mentioned above.
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Basic WMI driver structure
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--------------------------
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The basic WMI driver is build around the struct wmi_driver, which is then bound
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to matching WMI devices using a struct wmi_device_id table:
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::
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static const struct wmi_device_id foo_id_table[] = {
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{ "936DA01F-9ABD-4D9D-80C7-02AF85C822A8", NULL },
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{ }
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};
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MODULE_DEVICE_TABLE(wmi, foo_id_table);
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static struct wmi_driver foo_driver = {
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.driver = {
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.name = "foo",
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.probe_type = PROBE_PREFER_ASYNCHRONOUS, /* recommended */
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.pm = pm_sleep_ptr(&foo_dev_pm_ops), /* optional */
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},
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.id_table = foo_id_table,
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.probe = foo_probe,
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.remove = foo_remove, /* optional, devres is preferred */
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.notify = foo_notify, /* optional, for event handling */
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.no_notify_data = true, /* optional, enables events containing no additional data */
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.no_singleton = true, /* required for new WMI drivers */
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};
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module_wmi_driver(foo_driver);
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The probe() callback is called when the WMI driver is bound to a matching WMI device. Allocating
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driver-specific data structures and initialising interfaces to other kernel subsystems should
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normally be done in this function.
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The remove() callback is then called when the WMI driver is unbound from a WMI device. In order
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to unregister interfaces to other kernel subsystems and release resources, devres should be used.
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This simplifies error handling during probe and often allows to omit this callback entirely, see
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Documentation/driver-api/driver-model/devres.rst for details.
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Please note that new WMI drivers are required to be able to be instantiated multiple times,
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and are forbidden from using any deprecated GUID-based WMI functions. This means that the
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WMI driver should be prepared for the scenario that multiple matching WMI devices are present
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on a given machine.
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Because of this, WMI drivers should use the state container design pattern as described in
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Documentation/driver-api/driver-model/design-patterns.rst.
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WMI method drivers
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------------------
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WMI drivers can call WMI device methods using wmidev_evaluate_method(), the
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structure of the ACPI buffer passed to this function is device-specific and usually
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needs some tinkering to get right. Looking at the ACPI tables containing the WMI
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device usually helps here. The method id and instance number passed to this function
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are also device-specific, looking at the decoded Binary MOF is usually enough to
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find the right values.
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The maximum instance number can be retrieved during runtime using wmidev_instance_count().
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Take a look at drivers/platform/x86/inspur_platform_profile.c for an example WMI method driver.
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WMI data block drivers
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----------------------
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WMI drivers can query WMI device data blocks using wmidev_block_query(), the
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structure of the returned ACPI object is again device-specific. Some WMI devices
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also allow for setting data blocks using wmidev_block_set().
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The maximum instance number can also be retrieved using wmidev_instance_count().
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Take a look at drivers/platform/x86/intel/wmi/sbl-fw-update.c for an example
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WMI data block driver.
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WMI event drivers
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-----------------
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WMI drivers can receive WMI events via the notify() callback inside the struct wmi_driver.
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The WMI subsystem will then take care of setting up the WMI event accordingly. Please note that
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the structure of the ACPI object passed to this callback is device-specific, and freeing the
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ACPI object is being done by the WMI subsystem, not the driver.
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The WMI driver core will take care that the notify() callback will only be called after
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the probe() callback has been called, and that no events are being received by the driver
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right before and after calling its remove() callback.
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However WMI driver developers should be aware that multiple WMI events can be received concurrently,
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so any locking (if necessary) needs to be provided by the WMI driver itself.
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In order to be able to receive WMI events containing no additional event data,
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the ``no_notify_data`` flag inside struct wmi_driver should be set to ``true``.
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Take a look at drivers/platform/x86/xiaomi-wmi.c for an example WMI event driver.
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Handling multiple WMI devices at once
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-------------------------------------
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There are many cases of firmware vendors using multiple WMI devices to control different aspects
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of a single physical device. This can make developing WMI drivers complicated, as those drivers
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might need to communicate with each other to present a unified interface to userspace.
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On such case involves a WMI event device which needs to talk to a WMI data block device or WMI
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method device upon receiving an WMI event. In such a case, two WMI drivers should be developed,
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one for the WMI event device and one for the other WMI device.
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The WMI event device driver has only one purpose: to receive WMI events, validate any additional
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event data and invoke a notifier chain. The other WMI driver adds itself to this notifier chain
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during probing and thus gets notified every time a WMI event is received. This WMI driver might
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then process the event further for example by using an input device.
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For other WMI device constellations, similar mechanisms can be used.
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Things to avoid
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---------------
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When developing WMI drivers, there are a couple of things which should be avoided:
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- usage of the deprecated GUID-based WMI interface which uses GUIDs instead of WMI device structs
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- bypassing of the WMI subsystem when talking to WMI devices
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- WMI drivers which cannot be instantiated multiple times.
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Many older WMI drivers violate one or more points from this list. The reason for
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this is that the WMI subsystem evolved significantly over the last two decades,
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so there is a lot of legacy cruft inside older WMI drivers.
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New WMI drivers are also required to conform to the linux kernel coding style as specified in
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Documentation/process/coding-style.rst. The checkpatch utility can catch many common coding style
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violations, you can invoke it with the following command:
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::
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./scripts/checkpatch.pl --strict <path to driver file>
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References
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==========
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.. [1] https://lwn.net/Articles/391230/
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@ -8,6 +8,7 @@ WMI Subsystem
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:maxdepth: 1
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acpi-interface
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driver-development-guide
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devices/index
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.. only:: subproject and html
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