On Thu, 7 Dec 2023, 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 (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 article is outdated and causes people to still submit new 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 Too used to typing guid(?), here you want "guide" instead. :-D (I know that feeling when my fingers type something else than I think). > 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, API > +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/>`_ t -> to > +which deals with WMI drivers using the deprecated GUID-based WMI interface. > + > +Optaining WMI device information Obtaining > +-------------------------------- > + > +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 obtained > +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 (Managed Object Format) > +exposes this information to userspace, see Documentation/ABI/stable/sysfs-platform-wmi-bmof. This should use a true link to the file. > +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 an struct -> a struct > +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. Just voicing wouldn't it be more useful to not burden new stuff with this at all and construct the opposite list instead with the GUIDs that have a driver that don't support duplicates? It's the existing set of GUIDs we have in-tree minus those currently on the list, correct? > +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 Plase -> Please > +is optional and its structure device-specific. It also does not need to be freed, structure is device-specific. > +the WMI subsystem takes care of that. I'd state the freeing part more strongly: Releasing the ACPI object is handled by the WMI subsystem, not the driver. > + > +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 It would be nice to be more specific because it's far from obvious at this point how to differentiate. So perhaps adding something like this would help: (avoid functions with wmi_ prefix that input GUID converting it into a wmi_device using wmi_find_device_by_guid()). > +- 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. -- i.
