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 .shutdown = foo_shutdown, /* optional, called during shutdown */ 68 .notify = foo_notify, /* optional, for event handling */ 69 .no_notify_data = true, /* optional, enables events containing no additional data */ 70 .no_singleton = true, /* required for new WMI drivers */ 71 }; 72 module_wmi_driver(foo_driver); 73 74The probe() callback is called when the WMI driver is bound to a matching WMI device. Allocating 75driver-specific data structures and initialising interfaces to other kernel subsystems should 76normally be done in this function. 77 78The remove() callback is then called when the WMI driver is unbound from a WMI device. In order 79to unregister interfaces to other kernel subsystems and release resources, devres should be used. 80This simplifies error handling during probe and often allows to omit this callback entirely, see 81Documentation/driver-api/driver-model/devres.rst for details. 82 83The shutdown() callback is called during shutdown, reboot or kexec. Its sole purpose is to disable 84the WMI device and put it in a well-known state for the WMI driver to pick up later after reboot 85or kexec. Most WMI drivers need no special shutdown handling and can thus omit this callback. 86 87Please note that new WMI drivers are required to be able to be instantiated multiple times, 88and are forbidden from using any deprecated GUID-based WMI functions. This means that the 89WMI driver should be prepared for the scenario that multiple matching WMI devices are present 90on a given machine. 91 92Because of this, WMI drivers should use the state container design pattern as described in 93Documentation/driver-api/driver-model/design-patterns.rst. 94 95WMI method drivers 96------------------ 97 98WMI drivers can call WMI device methods using wmidev_evaluate_method(), the 99structure of the ACPI buffer passed to this function is device-specific and usually 100needs some tinkering to get right. Looking at the ACPI tables containing the WMI 101device usually helps here. The method id and instance number passed to this function 102are also device-specific, looking at the decoded Binary MOF is usually enough to 103find the right values. 104 105The maximum instance number can be retrieved during runtime using wmidev_instance_count(). 106 107Take a look at drivers/platform/x86/inspur_platform_profile.c for an example WMI method driver. 108 109WMI data block drivers 110---------------------- 111 112WMI drivers can query WMI device data blocks using wmidev_block_query(), the 113structure of the returned ACPI object is again device-specific. Some WMI devices 114also allow for setting data blocks using wmidev_block_set(). 115 116The maximum instance number can also be retrieved using wmidev_instance_count(). 117 118Take a look at drivers/platform/x86/intel/wmi/sbl-fw-update.c for an example 119WMI data block driver. 120 121WMI event drivers 122----------------- 123 124WMI drivers can receive WMI events via the notify() callback inside the struct wmi_driver. 125The WMI subsystem will then take care of setting up the WMI event accordingly. Please note that 126the structure of the ACPI object passed to this callback is device-specific, and freeing the 127ACPI object is being done by the WMI subsystem, not the driver. 128 129The WMI driver core will take care that the notify() callback will only be called after 130the probe() callback has been called, and that no events are being received by the driver 131right before and after calling its remove() or shutdown() callback. 132 133However WMI driver developers should be aware that multiple WMI events can be received concurrently, 134so any locking (if necessary) needs to be provided by the WMI driver itself. 135 136In order to be able to receive WMI events containing no additional event data, 137the ``no_notify_data`` flag inside struct wmi_driver should be set to ``true``. 138 139Take a look at drivers/platform/x86/xiaomi-wmi.c for an example WMI event driver. 140 141Handling multiple WMI devices at once 142------------------------------------- 143 144There are many cases of firmware vendors using multiple WMI devices to control different aspects 145of a single physical device. This can make developing WMI drivers complicated, as those drivers 146might need to communicate with each other to present a unified interface to userspace. 147 148On such case involves a WMI event device which needs to talk to a WMI data block device or WMI 149method device upon receiving an WMI event. In such a case, two WMI drivers should be developed, 150one for the WMI event device and one for the other WMI device. 151 152The WMI event device driver has only one purpose: to receive WMI events, validate any additional 153event data and invoke a notifier chain. The other WMI driver adds itself to this notifier chain 154during probing and thus gets notified every time a WMI event is received. This WMI driver might 155then process the event further for example by using an input device. 156 157For other WMI device constellations, similar mechanisms can be used. 158 159Things to avoid 160--------------- 161 162When developing WMI drivers, there are a couple of things which should be avoided: 163 164- usage of the deprecated GUID-based WMI interface which uses GUIDs instead of WMI device structs 165- bypassing of the WMI subsystem when talking to WMI devices 166- WMI drivers which cannot be instantiated multiple times. 167 168Many older WMI drivers violate one or more points from this list. The reason for 169this is that the WMI subsystem evolved significantly over the last two decades, 170so there is a lot of legacy cruft inside older WMI drivers. 171 172New WMI drivers are also required to conform to the linux kernel coding style as specified in 173Documentation/process/coding-style.rst. The checkpatch utility can catch many common coding style 174violations, you can invoke it with the following command: 175 176:: 177 178 ./scripts/checkpatch.pl --strict <path to driver file> 179 180References 181========== 182 183.. [1] https://lwn.net/Articles/391230/ 184