[PATCH 5/5] platform/x86: wmi: Add driver development guide

[Armin Wolf]

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 (fwts wmi, bmfdec, ...), 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article is outdated and causes people to still submit new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 guid 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          | 126 ++++++++++++++++++
 Documentation/wmi/index.rst                   |   1 +
 2 files changed, 127 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..a831e2728d25
--- /dev/null
+++ b/Documentation/wmi/driver-development-guide.rst
@@ -0,0 +1,126 @@
+.. 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
+t 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.
+
+Optaining WMI device information
+--------------------------------
+
+Before developing an WMI driver, information about the WMI device in question
+must be optained. 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 inside 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 information used to describe WMI devices. The ``wmi-bmof`` driver
+exposes this information to userspace, see Documentation/ABI/stable/sysfs-platform-wmi-bmof.
+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. To find out which
+ACPI method handles which WMI device, the `fwts <https://github.com/fwts/fwts>`_
+program can be used with the following command (requires root):
+
+::
+
+  fwts wmi -
+
+Basic WMI driver structure
+--------------------------
+
+The basic WMI driver is build around the struct wmi_driver, which is then bound
+to matching WMI devices using an struct wmi_device_id table. Please note that each
+WMI driver should be able to be instantiated multiple times.
+
+::
+
+  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,        /* optional */
+                .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 */
+  };
+  module_wmi_driver(foo_driver);
+
+If your WMI driver is not using any deprecated GUID-based WMI functions and is
+able to be instantiated multiple times, please add its GUID to ``allow_duplicates``
+inside drivers/platform/x86/wmi.c, so that the WMI subsystem does not block duplicate
+GUIDs for it.
+
+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 event notifications by providing the notify() callback
+inside the struct wmi_driver. The WMI subsystem will then take care of setting
+up the WMI event accordingly. Plase note that the ACPI object passed to this callback
+is optional and its structure device-specific. It also does not need to be freed,
+the WMI subsystem takes care of that.
+
+Take a look at drivers/platform/x86/xiaomi-wmi.c for an example WMI event driver.
+
+Things to avoid
+---------------
+
+When developing WMI drivers, there are a couple of things which should be avoid
+if feasible:
+
+- usage of the deprecated GUID-based WMI interface
+- 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.
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