WMI driver development guide

The WMI subsystem provides a rich driver API for implementing WMI drivers, documented at WMI Driver API. This document will serve as an introductory guide for WMI driver writers using this API. It is supposed to be a successor to the original LWN article 1 which deals with WMI drivers using the deprecated GUID-based WMI interface.

Obtaining WMI device information

Before developing an WMI driver, information about the WMI device in question must be obtained. The lswmi utility can be used to extract detailed WMI device information using the following command:

lswmi -V

The resulting output will contain information about all WMI devices available on a given machine, plus some extra information.

In order to find out more about the interface used to communicate with a WMI device, the bmfdec utilities can be used to decode the Binary MOF (Managed Object Format) information used to describe WMI devices. The wmi-bmof driver exposes this information to userspace, see WMI embedded Binary MOF driver.

In order to retrieve the decoded Binary MOF information, use the following command (requires root):

./bmf2mof /sys/bus/wmi/devices/05901221-D566-11D1-B2F0-00A0C9062910[-X]/bmof

Sometimes, looking at the disassembled ACPI tables used to describe the WMI device helps in understanding how the WMI device is supposed to work. The path of the ACPI method associated with a given WMI device can be retrieved using the lswmi utility as mentioned above.

Basic WMI driver structure

The basic WMI driver is build around the struct wmi_driver, which is then bound to matching WMI devices using a struct wmi_device_id table:

static const struct wmi_device_id foo_id_table[] = {
       { "936DA01F-9ABD-4D9D-80C7-02AF85C822A8", NULL },
       { }
MODULE_DEVICE_TABLE(wmi, foo_id_table);

static struct wmi_driver foo_driver = {
      .driver = {
              .name = "foo",
              .probe_type = PROBE_PREFER_ASYNCHRONOUS,        /* recommended */
              .pm = pm_sleep_ptr(&foo_dev_pm_ops),            /* optional */
      .id_table = foo_id_table,
      .probe = foo_probe,
      .remove = foo_remove,         /* optional, devres is preferred */
      .notify = 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 */

The probe() callback is called when the WMI driver is bound to a matching WMI device. Allocating driver-specific data structures and initialising interfaces to other kernel subsystems should normally be done in this function.

The remove() callback is then called when the WMI driver is unbound from a WMI device. In order to unregister interfaces to other kernel subsystems and release resources, devres should be used. This simplifies error handling during probe and often allows to omit this callback entirely, see Devres - Managed Device Resource for details.

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.

Because of this, WMI drivers should use the state container design pattern as described in Device Driver Design Patterns.

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.

The maximum instance number can be retrieved during runtime using wmidev_instance_count().

Take a look at drivers/platform/x86/inspur_platform_profile.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().

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.

WMI event drivers

WMI drivers can receive WMI events via the notify() 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 WMI driver core will take care that the notify() 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() callback.

However WMI driver developers should be aware that multiple WMI events can be received concurrently, so any locking (if necessary) needs to be provided by the WMI driver itself.

In order to be able to receive WMI events containing no additional event data, 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.

Handling multiple WMI devices at once

There are many cases of firmware vendors using multiple WMI devices to control different aspects of a single physical device. This can make developing WMI drivers complicated, as those drivers might need to communicate with each other to present a unified interface to userspace.

On such case involves a WMI event device which needs to talk to a WMI data block device or WMI method device upon receiving an WMI event. In such a case, two WMI drivers should be developed, one for the WMI event device and one for the other WMI device.

The WMI event device driver has only one purpose: to receive WMI events, validate any additional event data and invoke a notifier chain. The other WMI driver adds itself to this notifier chain during probing and thus gets notified every time a WMI event is received. This WMI driver might then process the event further for example by using an input device.

For other WMI device constellations, similar mechanisms can be used.

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

  • bypassing of the WMI subsystem when talking to WMI devices

  • WMI drivers which cannot be instantiated multiple times.

Many older WMI drivers violate one or more points from this list. The reason for this is that the WMI subsystem evolved significantly over the last two decades, so there is a lot of legacy cruft inside older WMI drivers.

New WMI drivers are also required to conform to the linux kernel coding style as specified in Linux kernel coding style. The checkpatch utility can catch many common coding style violations, you can invoke it with the following command:

./scripts/checkpatch.pl --strict <path to driver file>