mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
synced 2024-12-29 17:22:07 +00:00
platform/x86: wmi: Add driver development guide
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the introduction of the modern bus-based interface and other userspace tooling (bmfdec, lswmi, ...), this article is outdated and causes people to still submit new WMI drivers using the deprecated GUID-based interface.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@gmx.de> Link: https://lore.kernel.org/r/20240402143059.8456-4-W_Armin@gmx.de Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com> Reviewed-by: Hans de Goede <hdegoede@redhat.com> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
This commit is contained in:
Armin Wolf
Hans de Goede
parent
c5e160ff34
commit
a582a43e0d
178
Documentation/wmi/driver-development-guide.rst
Normal file
178
Documentation/wmi/driver-development-guide.rst
Normal file
@ -0,0 +1,178 @@
|
||||
.. 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 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 <https://pypi.org/project/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 <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 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
|
||||
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>
|
||||
|
||||
References
|
||||
==========
|
||||
|
||||
.. [1] https://lwn.net/Articles/391230/
|
||||
@ -8,6 +8,7 @@ WMI Subsystem
|
||||
:maxdepth: 1
|
||||
|
||||
acpi-interface
|
||||
driver-development-guide
|
||||
devices/index
|
||||
|
||||
.. only:: subproject and html
|
||||
|
||||
Loading…
Reference in New Issue
Block a user