hwmon

Field	Value
Subsystem	hardware monitoring (hwmon) core, `drivers/hwmon/hwmon.c`
Config gate	`CONFIG_HWMON` (`obj-$(CONFIG_HWMON) += hwmon.o` in `drivers/hwmon/Makefile`, torvalds/linux master)
ABI documentation	`Documentation/ABI/testing/sysfs-class-hwmon`
Since	not stated: the ABI file carries no `KernelVersion:` lines (What: entries only)
Live (5.10.134 host)	class directory present, 0 devices (no sensor chip driver bound in the container)
Ledger ID	SYSC-hwmon

What it represents

One directory per hardware monitoring chip registered with the hwmon core: voltage, temperature, fan, current, power, energy, and humidity sensors plus fan-speed (PWM) controls. Chip drivers register a hwmon device; the core creates hwmonX entries and forwards attribute names to the driver. This is the interface sensors (lm-sensors) and fancontrol read.

Relationship to the device model

/sys/class/hwmon/hwmonX is a class view of a device that also lives under /sys/devices/... (the device link inside hwmonX points at the parent, typically an i2c or platform device). There is no bus of its own and no /sys/dev entry: hwmon devices carry no major:minor. The legacy per-driver sysfs layout is described in Documentation/hwmon/sysfs-interface.rst, which the class ABI mirrors.

Attributes

120 attribute paths spell /sys/class/hwmon/... in What: lines across the ABI tree (119 of them in sysfs-class-hwmon itself, 1 in a driver ABI file); the sysfs-attrs.json cross-check and the ledger both record 120. The table below is representative.

Attribute	R/W	Unit/format	Conditional?	Source
`hwmonX/name`	RO	short lowercase chip name; only mandatory attribute	no	ABI What `hwmonX/name`
`hwmonX/label`	RO	free-form text	driver-dependent	ABI What `hwmonX/label`
`hwmonX/update_interval`	RW	milliseconds	driver-dependent	ABI What `hwmonX/update_interval`
`hwmonX/tempY_input`	RO	millidegree Celsius	per channel Y	ABI What `tempY_input`
`hwmonX/tempY_max`	RW	millidegree Celsius	per channel	ABI What `tempY_max`
`hwmonX/tempY_max_alarm`	RO	0/1 flag	per channel	ABI What `tempY_max_alarm`
`hwmonX/fanY_input`	RO	revolution/min (RPM)	per channel	ABI What `fanY_input`
`hwmonX/fanY_fault`	RO	0/1 flag	per channel	ABI What `fanY_fault`
`hwmonX/pwmY`	RW	0-255 (255 = 100%)	fan-control chips	ABI What `pwmY`
`hwmonX/pwmY_enable`	RW	0 none, 1 manual, 2+ automatic	fan-control chips	ABI What `pwmY_enable`
`hwmonX/inY_input`	RO	millivolt (board scaling not applied)	per channel	ABI What `inY_input`
`hwmonX/currY_input`	RO	milliampere	per channel	ABI What `currY_input`
`hwmonX/powerY_input`	RO	microWatt; `energyY_input` is microJoule	per channel	ABI What `powerY_input`, `energyY_input`
`hwmonX/humidityY_input`	RO	milli-percent (per cent mille)	per channel	ABI What `humidityY_input`
`hwmonX/intrusionY_alarm`	RW	0/1; sticks until cleared by writing 0	chassis intrusion support	ABI What `intrusionY_alarm`

Channel numbering (Y) and the subset present are driver-specific; limits, alarms, _enable, _label, and rated/history variants follow the same per-family naming. Live values were not sampled: the host has no hwmon device.

Safety and writes

All readings are safe to read as any user. Writes (limits, pwmY, *_enable, history resets) affect sensor policy or fan control and need root; writing a wrong pwmY_enable mode can force fans to full speed or hand control to the chip. Alarm flags other than intrusionY_alarm clear on read; the intrusion flag persists until explicitly cleared. powerY_cap and friends state that the system reacts when limits are crossed, so treat them as safety-critical configuration, not telemetry.

References

Documentation/ABI/testing/sysfs-class-hwmon (cached copy: 1089 lines, What: inventory)
drivers/hwmon/hwmon.c, drivers/hwmon/Makefile (torvalds/linux master)
Documentation/hwmon/sysfs-interface.rst
Ledger row SYSC-hwmon (src/linux/sysfs/ledger.md); prior research: worklog Task 2-b