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the introduction of the modern bus-based interface and other userspace tooling (bmfdec, lswmi, ...), this article is outdated and causes people to still submit new WMI drivers using the deprecated GUID-based interface.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@xxxxxx> --- .../wmi/driver-development-guide.rst | 173 ++++++++++++++++++ Documentation/wmi/index.rst | 1 + 2 files changed, 174 insertions(+) create mode 100644 Documentation/wmi/driver-development-guide.rst diff --git a/Documentation/wmi/driver-development-guide.rst b/Documentation/wmi/driver-development-guide.rst new file mode 100644 index 000000000000..9a1b0a8490bb --- /dev/null +++ b/Documentation/wmi/driver-development-guide.rst @@ -0,0 +1,173 @@ +.. SPDX-License-Identifier: GPL-2.0-or-later + +============================ +WMI driver development guide +============================ + +The WMI subsystem provides a rich driver API for implementing WMI drivers, +documented at Documentation/driver-api/wmi.rst. This document will serve +as an introductory guide for WMI driver writers using this API. It is supposed +to be an successor to the original `LWN article <https://lwn.net/Articles/391230/>`_ +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 <https://pypi.org/project/lswmi>`_ utility can be +used to display 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 <https://github.com/pali/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 +Documentation/wmi/devices/wmi-bmof.rst. + +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 */ + }; + module_wmi_driver(foo_driver); + +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 +Documentation/driver-api/driver-model/devres.rst 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 +Documentation/driver-api/driver-model/design-patterns.rst. + +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 by providing 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 containg 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 +Documentation/process/coding-style.rst. 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> diff --git a/Documentation/wmi/index.rst b/Documentation/wmi/index.rst index 537cff188e14..fec4b6ae97b3 100644 --- a/Documentation/wmi/index.rst +++ b/Documentation/wmi/index.rst @@ -8,6 +8,7 @@ WMI Subsystem :maxdepth: 1 acpi-interface + driver-development-guide devices/index .. only:: subproject and html -- 2.39.2
