1.. SPDX-License-Identifier: GPL-2.0-or-later 2 3============================ 4WMI driver development guide 5============================ 6 7The WMI subsystem provides a rich driver API for implementing WMI drivers, 8documented at Documentation/driver-api/wmi.rst. This document will serve 9as an introductory guide for WMI driver writers using this API. It is supposed 10to be a successor to the original LWN article [1]_ which deals with WMI drivers 11using the deprecated GUID-based WMI interface. 12 13Obtaining WMI device information 14-------------------------------- 15 16Before developing an WMI driver, information about the WMI device in question 17must be obtained. The `lswmi <https://pypi.org/project/lswmi>`_ utility can be 18used to extract detailed WMI device information using the following command: 19 20:: 21 22 lswmi -V 23 24The resulting output will contain information about all WMI devices available on 25a given machine, plus some extra information. 26 27In order to find out more about the interface used to communicate with a WMI device, 28the `bmfdec <https://github.com/pali/bmfdec>`_ utilities can be used to decode 29the Binary MOF (Managed Object Format) information used to describe WMI devices. 30The ``wmi-bmof`` driver exposes this information to userspace, see 31Documentation/wmi/devices/wmi-bmof.rst. 32 33In order to retrieve the decoded Binary MOF information, use the following command (requires root): 34 35:: 36 37 ./bmf2mof /sys/bus/wmi/devices/05901221-D566-11D1-B2F0-00A0C9062910[-X]/bmof 38 39Sometimes, looking at the disassembled ACPI tables used to describe the WMI device 40helps in understanding how the WMI device is supposed to work. The path of the ACPI 41method associated with a given WMI device can be retrieved using the ``lswmi`` utility 42as mentioned above. 43 44If you are attempting to port a driver to Linux and are working on a Windows 45system, `WMIExplorer <https://github.com/vinaypamnani/wmie2>`_ can be useful 46for inspecting available WMI methods and invoking them directly. 47 48Basic WMI driver structure 49-------------------------- 50 51The basic WMI driver is build around the struct wmi_driver, which is then bound 52to matching WMI devices using a struct wmi_device_id table: 53 54:: 55 56 static const struct wmi_device_id foo_id_table[] = { 57 { "936DA01F-9ABD-4D9D-80C7-02AF85C822A8", NULL }, 58 { } 59 }; 60 MODULE_DEVICE_TABLE(wmi, foo_id_table); 61 62 static struct wmi_driver foo_driver = { 63 .driver = { 64 .name = "foo", 65 .probe_type = PROBE_PREFER_ASYNCHRONOUS, /* recommended */ 66 .pm = pm_sleep_ptr(&foo_dev_pm_ops), /* optional */ 67 }, 68 .id_table = foo_id_table, 69 .probe = foo_probe, 70 .remove = foo_remove, /* optional, devres is preferred */ 71 .shutdown = foo_shutdown, /* optional, called during shutdown */ 72 .notify = foo_notify, /* optional, for event handling */ 73 .no_notify_data = true, /* optional, enables events containing no additional data */ 74 .no_singleton = true, /* required for new WMI drivers */ 75 }; 76 module_wmi_driver(foo_driver); 77 78The probe() callback is called when the WMI driver is bound to a matching WMI device. Allocating 79driver-specific data structures and initialising interfaces to other kernel subsystems should 80normally be done in this function. 81 82The remove() callback is then called when the WMI driver is unbound from a WMI device. In order 83to unregister interfaces to other kernel subsystems and release resources, devres should be used. 84This simplifies error handling during probe and often allows to omit this callback entirely, see 85Documentation/driver-api/driver-model/devres.rst for details. 86 87The shutdown() callback is called during shutdown, reboot or kexec. Its sole purpose is to disable 88the WMI device and put it in a well-known state for the WMI driver to pick up later after reboot 89or kexec. Most WMI drivers need no special shutdown handling and can thus omit this callback. 90 91Please note that new WMI drivers are required to be able to be instantiated multiple times, 92and are forbidden from using any deprecated GUID-based WMI functions. This means that the 93WMI driver should be prepared for the scenario that multiple matching WMI devices are present 94on a given machine. 95 96Because of this, WMI drivers should use the state container design pattern as described in 97Documentation/driver-api/driver-model/design-patterns.rst. 98 99WMI method drivers 100------------------ 101 102WMI drivers can call WMI device methods using wmidev_evaluate_method(), the 103structure of the ACPI buffer passed to this function is device-specific and usually 104needs some tinkering to get right. Looking at the ACPI tables containing the WMI 105device usually helps here. The method id and instance number passed to this function 106are also device-specific, looking at the decoded Binary MOF is usually enough to 107find the right values. 108 109The maximum instance number can be retrieved during runtime using wmidev_instance_count(). 110 111Take a look at drivers/platform/x86/inspur_platform_profile.c for an example WMI method driver. 112 113WMI data block drivers 114---------------------- 115 116WMI drivers can query WMI device data blocks using wmidev_block_query(), the 117structure of the returned ACPI object is again device-specific. Some WMI devices 118also allow for setting data blocks using wmidev_block_set(). 119 120The maximum instance number can also be retrieved using wmidev_instance_count(). 121 122Take a look at drivers/platform/x86/intel/wmi/sbl-fw-update.c for an example 123WMI data block driver. 124 125WMI event drivers 126----------------- 127 128WMI drivers can receive WMI events via the notify() callback inside the struct wmi_driver. 129The WMI subsystem will then take care of setting up the WMI event accordingly. Please note that 130the structure of the ACPI object passed to this callback is device-specific, and freeing the 131ACPI object is being done by the WMI subsystem, not the driver. 132 133The WMI driver core will take care that the notify() callback will only be called after 134the probe() callback has been called, and that no events are being received by the driver 135right before and after calling its remove() or shutdown() callback. 136 137However WMI driver developers should be aware that multiple WMI events can be received concurrently, 138so any locking (if necessary) needs to be provided by the WMI driver itself. 139 140In order to be able to receive WMI events containing no additional event data, 141the ``no_notify_data`` flag inside struct wmi_driver should be set to ``true``. 142 143Take a look at drivers/platform/x86/xiaomi-wmi.c for an example WMI event driver. 144 145Handling multiple WMI devices at once 146------------------------------------- 147 148There are many cases of firmware vendors using multiple WMI devices to control different aspects 149of a single physical device. This can make developing WMI drivers complicated, as those drivers 150might need to communicate with each other to present a unified interface to userspace. 151 152On such case involves a WMI event device which needs to talk to a WMI data block device or WMI 153method device upon receiving an WMI event. In such a case, two WMI drivers should be developed, 154one for the WMI event device and one for the other WMI device. 155 156The WMI event device driver has only one purpose: to receive WMI events, validate any additional 157event data and invoke a notifier chain. The other WMI driver adds itself to this notifier chain 158during probing and thus gets notified every time a WMI event is received. This WMI driver might 159then process the event further for example by using an input device. 160 161For other WMI device constellations, similar mechanisms can be used. 162 163Things to avoid 164--------------- 165 166When developing WMI drivers, there are a couple of things which should be avoided: 167 168- usage of the deprecated GUID-based WMI interface which uses GUIDs instead of WMI device structs 169- bypassing of the WMI subsystem when talking to WMI devices 170- WMI drivers which cannot be instantiated multiple times. 171 172Many older WMI drivers violate one or more points from this list. The reason for 173this is that the WMI subsystem evolved significantly over the last two decades, 174so there is a lot of legacy cruft inside older WMI drivers. 175 176New WMI drivers are also required to conform to the linux kernel coding style as specified in 177Documentation/process/coding-style.rst. The checkpatch utility can catch many common coding style 178violations, you can invoke it with the following command: 179 180:: 181 182 ./scripts/checkpatch.pl --strict <path to driver file> 183 184References 185========== 186 187.. [1] https://lwn.net/Articles/391230/ 188