Linux HP WMI Sensors Driver

Copyright:

© 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°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 [1] 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

[1]

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