Re: [PATCH v2 4/4] platform/x86: wmi: Add driver development guide

[Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 4/2/24 7:30 AM, Armin Wolf wrote:
> 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 article is outdated and causes people to still submit new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>
> ---

LGTM

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

> Changes since v1:
> - use footnote for lwn article link
> - wording fixes
> ---
>  .../wmi/driver-development-guide.rst          | 178 ++++++++++++++++++
>  Documentation/wmi/index.rst                   |   1 +
>  2 files changed, 179 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..429137b2f632
> --- /dev/null
> +++ b/Documentation/wmi/driver-development-guide.rst
> @@ -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/
> 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
>
>
-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer