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 44Basic WMI driver structure 45-------------------------- 46 47The basic WMI driver is build around the struct wmi_driver, which is then bound 48to matching WMI devices using a struct wmi_device_id table: 49 50:: 51 52 static const struct wmi_device_id foo_id_table[] = { 53 { "936DA01F-9ABD-4D9D-80C7-02AF85C822A8", NULL }, 54 { } 55 }; 56 MODULE_DEVICE_TABLE(wmi, foo_id_table); 57 58 static struct wmi_driver foo_driver = { 59 .driver = { 60 .name = "foo", 61 .probe_type = PROBE_PREFER_ASYNCHRONOUS, /* recommended */ 62 .pm = pm_sleep_ptr(&foo_dev_pm_ops), /* optional */ 63 }, 64 .id_table = foo_id_table, 65 .probe = foo_probe, 66 .remove = foo_remove, /* optional, devres is preferred */ 67 .notify = foo_notify, /* optional, for event handling */ 68 .no_notify_data = true, /* optional, enables events containing no additional data */ 69 .no_singleton = true, /* required for new WMI drivers */ 70 }; 71 module_wmi_driver(foo_driver); 72 73The probe() callback is called when the WMI driver is bound to a matching WMI device. Allocating 74driver-specific data structures and initialising interfaces to other kernel subsystems should 75normally be done in this function. 76 77The remove() callback is then called when the WMI driver is unbound from a WMI device. In order 78to unregister interfaces to other kernel subsystems and release resources, devres should be used. 79This simplifies error handling during probe and often allows to omit this callback entirely, see 80Documentation/driver-api/driver-model/devres.rst for details. 81 82Please note that new WMI drivers are required to be able to be instantiated multiple times, 83and are forbidden from using any deprecated GUID-based WMI functions. This means that the 84WMI driver should be prepared for the scenario that multiple matching WMI devices are present 85on a given machine. 86 87Because of this, WMI drivers should use the state container design pattern as described in 88Documentation/driver-api/driver-model/design-patterns.rst. 89 90WMI method drivers 91------------------ 92 93WMI drivers can call WMI device methods using wmidev_evaluate_method(), the 94structure of the ACPI buffer passed to this function is device-specific and usually 95needs some tinkering to get right. Looking at the ACPI tables containing the WMI 96device usually helps here. The method id and instance number passed to this function 97are also device-specific, looking at the decoded Binary MOF is usually enough to 98find the right values. 99 100The maximum instance number can be retrieved during runtime using wmidev_instance_count(). 101 102Take a look at drivers/platform/x86/inspur_platform_profile.c for an example WMI method driver. 103 104WMI data block drivers 105---------------------- 106 107WMI drivers can query WMI device data blocks using wmidev_block_query(), the 108structure of the returned ACPI object is again device-specific. Some WMI devices 109also allow for setting data blocks using wmidev_block_set(). 110 111The maximum instance number can also be retrieved using wmidev_instance_count(). 112 113Take a look at drivers/platform/x86/intel/wmi/sbl-fw-update.c for an example 114WMI data block driver. 115 116WMI event drivers 117----------------- 118 119WMI drivers can receive WMI events via the notify() callback inside the struct wmi_driver. 120The WMI subsystem will then take care of setting up the WMI event accordingly. Please note that 121the structure of the ACPI object passed to this callback is device-specific, and freeing the 122ACPI object is being done by the WMI subsystem, not the driver. 123 124The WMI driver core will take care that the notify() callback will only be called after 125the probe() callback has been called, and that no events are being received by the driver 126right before and after calling its remove() callback. 127 128However WMI driver developers should be aware that multiple WMI events can be received concurrently, 129so any locking (if necessary) needs to be provided by the WMI driver itself. 130 131In order to be able to receive WMI events containing no additional event data, 132the ``no_notify_data`` flag inside struct wmi_driver should be set to ``true``. 133 134Take a look at drivers/platform/x86/xiaomi-wmi.c for an example WMI event driver. 135 136Handling multiple WMI devices at once 137------------------------------------- 138 139There are many cases of firmware vendors using multiple WMI devices to control different aspects 140of a single physical device. This can make developing WMI drivers complicated, as those drivers 141might need to communicate with each other to present a unified interface to userspace. 142 143On such case involves a WMI event device which needs to talk to a WMI data block device or WMI 144method device upon receiving an WMI event. In such a case, two WMI drivers should be developed, 145one for the WMI event device and one for the other WMI device. 146 147The WMI event device driver has only one purpose: to receive WMI events, validate any additional 148event data and invoke a notifier chain. The other WMI driver adds itself to this notifier chain 149during probing and thus gets notified every time a WMI event is received. This WMI driver might 150then process the event further for example by using an input device. 151 152For other WMI device constellations, similar mechanisms can be used. 153 154Things to avoid 155--------------- 156 157When developing WMI drivers, there are a couple of things which should be avoided: 158 159- usage of the deprecated GUID-based WMI interface which uses GUIDs instead of WMI device structs 160- bypassing of the WMI subsystem when talking to WMI devices 161- WMI drivers which cannot be instantiated multiple times. 162 163Many older WMI drivers violate one or more points from this list. The reason for 164this is that the WMI subsystem evolved significantly over the last two decades, 165so there is a lot of legacy cruft inside older WMI drivers. 166 167New WMI drivers are also required to conform to the linux kernel coding style as specified in 168Documentation/process/coding-style.rst. The checkpatch utility can catch many common coding style 169violations, you can invoke it with the following command: 170 171:: 172 173 ./scripts/checkpatch.pl --strict <path to driver file> 174 175References 176========== 177 178.. [1] https://lwn.net/Articles/391230/ 179