hwmon: add HP WMI Sensors driver

Hewlett-Packard (and some HP Compaq) business-class computers report
hardware monitoring information via WMI. This driver exposes that
information to hwmon.

Initial support is provided for temperature, fan speed, and intrusion
sensor types. Provisional support is provided for voltage and current
sensor types.

HP's WMI implementation permits many other types of numeric sensors.
Therefore, a debugfs interface is also provided to enumerate and
inspect all numeric sensors visible on the WMI side. This should
facilitate adding support for other sensor types in the future.

Tested on a HP Z420, a HP EliteOne 800 G1, and a HP Compaq Elite 8300
SFF.

Note that provisionally supported sensor types are untested and seem
to be rare-to-nonexistent in the wild, having been encountered
neither on test systems nor in ACPI dumps from the Linux Hardware
Database. They are included because their popularity in general makes
their presence on past or future HP systems plausible and because no
doubt exists as to how the sensors themselves would be represented in
WMI (alarm attributes will need to wait for hardware to be located).
A 2005 HP whitepaper gives the relevant sensor object MOF definition
and sensor value scaling calculation, and both this driver and the
official HP Performance Advisor utility comply with them (confirmed
in the latter case by reverse engineering).

Link: https://h20331.www2.hp.com/hpsub/downloads/cmi_whitepaper.pdf
Signed-off-by: James Seo <james@equiv.tech>
Link: https://lore.kernel.org/r/20230522115645.509701-1-james@equiv.tech
[groeck: Set error return value for intrusion writes to -EINVAL.
	 Always accept writes of 0 even if there was no intrusion. ]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

2 files changed, 141 insertions, 0 deletions

diff --git a/Documentation/hwmon/hp-wmi-sensors.rst b/Documentation/hwmon/hp-wmi-sensors.rst
new file mode 100644
index 000000000000..a6bca9aecdde
--- /dev/null
+++ b/Documentation/hwmon/hp-wmi-sensors.rst
@@ -0,0 +1,140 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+.. include:: <isonum.txt>
+
+===========================
+Linux HP WMI Sensors Driver
+===========================
+
+:Copyright: |copy| 2023 James Seo <james@equiv.tech>
+
+Description
+===========
+
+Hewlett-Packard (and some HP Compaq) business-class computers report hardware
+monitoring information via Windows Management Instrumentation (WMI).
+This driver exposes that information to the Linux hwmon subsystem, allowing
+userspace utilities like ``sensors`` to gather numeric sensor readings.
+
+sysfs interface
+===============
+
+When the driver is loaded, it discovers the sensors available on the
+system and creates the following sysfs attributes as necessary within
+``/sys/class/hwmon/hwmon[X]``:
+
+(``[X]`` is some number that depends on other system components.)
+
+======================= ======= ===================================
+Name                    Perm    Description
+======================= ======= ===================================
+``curr[X]_input``       RO      Current in milliamperes (mA).
+``curr[X]_label``       RO      Current sensor label.
+``fan[X]_input``        RO      Fan speed in RPM.
+``fan[X]_label``        RO      Fan sensor label.
+``fan[X]_fault``        RO      Fan sensor fault indicator.
+``fan[X]_alarm``        RO      Fan sensor alarm indicator.
+``in[X]_input``         RO      Voltage in millivolts (mV).
+``in[X]_label``         RO      Voltage sensor label.
+``temp[X]_input``       RO      Temperature in millidegrees Celsius
+                                (m\ |deg|\ C).
+``temp[X]_label``       RO      Temperature sensor label.
+``temp[X]_fault``       RO      Temperature sensor fault indicator.
+``temp[X]_alarm``       RO      Temperature sensor alarm indicator.
+``intrusion[X]_alarm``  RW      Chassis intrusion alarm indicator.
+======================= ======= ===================================
+
+``fault`` attributes
+  Reading ``1`` instead of ``0`` as the ``fault`` attribute for a sensor
+  indicates that it has encountered some issue during operation such that
+  measurements from it should not be trusted. If a sensor with the fault
+  condition recovers later, reading this attribute will return ``0`` again.
+
+``alarm`` attributes
+  Reading ``1`` instead of ``0`` as the ``alarm`` attribute for a sensor
+  indicates that one of the following has occurred, depending on its type:
+
+  - ``fan``: The fan has stalled or has been disconnected while running.
+  - ``temp``: The sensor reading has reached a critical threshold.
+    The exact threshold is system-dependent.
+  - ``intrusion``: The system's chassis has been opened.
+
+  After ``1`` is read from an ``alarm`` attribute, the attribute resets itself
+  and returns ``0`` on subsequent reads. As an exception, an
+  ``intrusion[X]_alarm`` can only be manually reset by writing ``0`` to it.
+
+debugfs interface
+=================
+
+.. warning:: The debugfs interface is subject to change without notice
+             and is only available when the kernel is compiled with
+             ``CONFIG_DEBUG_FS`` defined.
+
+The standard hwmon interface in sysfs exposes sensors of several common types
+that are connected as of driver initialization. However, there are usually
+other sensors in WMI that do not meet these criteria. In addition, a number of
+system-dependent "platform events objects" used for ``alarm`` attributes may
+be present. A debugfs interface is therefore provided for read-only access to
+all available HP WMI sensors and platform events objects.
+
+``/sys/kernel/debug/hp-wmi-sensors-[X]/sensor``
+contains one numbered entry per sensor with the following attributes:
+
+=============================== =======================================
+Name                            Example
+=============================== =======================================
+``name``                        ``CPU0 Fan``
+``description``                 ``Reports CPU0 fan speed``
+``sensor_type``                 ``12``
+``other_sensor_type``           (an empty string)
+``operational_status``          ``2``
+``possible_states``             ``Normal,Caution,Critical,Not Present``
+``current_state``               ``Normal``
+``base_units``                  ``19``
+``unit_modifier``               ``0``
+``current_reading``             ``1008``
+``rate_units``                  ``0`` (only exists on some systems)
+=============================== =======================================
+
+If platform events objects are available,
+``/sys/kernel/debug/hp-wmi-sensors-[X]/platform_events``
+contains one numbered entry per object with the following attributes:
+
+=============================== ====================
+Name                            Example
+=============================== ====================
+``name``                        ``CPU0 Fan Stall``
+``description``                 ``CPU0 Fan Speed``
+``source_namespace``            ``root\wmi``
+``source_class``                ``HPBIOS_BIOSEvent``
+``category``                    ``3``
+``possible_severity``           ``25``
+``possible_status``             ``5``
+=============================== ====================
+
+These represent the properties of the underlying ``HPBIOS_BIOSNumericSensor``
+and ``HPBIOS_PlatformEvents`` WMI objects, which vary between systems.
+See [#]_ for more details and Managed Object Format (MOF) definitions.
+
+Known issues and limitations
+============================
+
+- If the existing hp-wmi driver for non-business-class HP systems is already
+  loaded, ``alarm`` attributes will be unavailable even on systems that
+  support them. This is because the same WMI event GUID used by this driver
+  for ``alarm`` attributes is used on those systems for e.g. laptop hotkeys.
+- Dubious sensor hardware and inconsistent BIOS WMI implementations have been
+  observed to cause inaccurate readings and peculiar behavior, such as alarms
+  failing to occur or occurring only once per boot.
+- Only temperature, fan speed, and intrusion sensor types have been seen in
+  the wild so far. Support for voltage and current sensors is therefore
+  provisional.
+- Although HP WMI sensors may claim to be of any type, any oddball sensor
+  types unknown to hwmon will not be supported.
+
+References
+==========
+
+.. [#] Hewlett-Packard Development Company, L.P.,
+       "HP Client Management Interface Technical White Paper", 2005. [Online].
+       Available: https://h20331.www2.hp.com/hpsub/downloads/cmi_whitepaper.pdf
diff --git a/Documentation/hwmon/index.rst b/Documentation/hwmon/index.rst
index b3af03214b3e..042e1cf9501b 100644
--- a/Documentation/hwmon/index.rst
+++ b/Documentation/hwmon/index.rst
@@ -77,6 +77,7 @@ Hardware Monitoring Kernel Drivers
    gl518sm
    gxp-fan-ctrl
    hih6130
+   hp-wmi-sensors
    ibmaem
    ibm-cffps
    ibmpowernv