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

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Since 2010, an LWN article covering WMI drivers exists:


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article is outdated and causes people to still submit new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>
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
+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
+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>
+.. [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

+   driver-development-guide

 .. only::  subproject and html

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