thermal

Field	Value
Subsystem	thermal core, `drivers/thermal/thermal_sysfs.c` (zone, cooling-device, stats attrs); class registered in `drivers/thermal/thermal_core.c`
Config gate	`CONFIG_THERMAL` (`obj-$(CONFIG_THERMAL) += thermal_sys.o` in `drivers/thermal/Makefile`, torvalds/linux master)
ABI documentation	`Documentation/ABI/testing/sysfs-class-thermal`
Since	not stated: the ABI file carries no `Date:` or `KernelVersion:` lines
Live (5.10.134 host)	class directory present, 0 thermal zones and 0 cooling devices (container)
Ledger ID	SYSC-thermal

What it represents

One thermal_zoneX directory per temperature sensor managed by the thermal framework, and one cooling_deviceX per controllable consumer of thermal state (fans, cpufreq cooling, devfreq cooling). A governor per zone reads the temperature, compares it with trip points, and throttles bound cooling devices. There is no separate thermal_cooling class in mainline: v5.10 and master thermal_core.c register a single class named thermal, so cooling devices live in this directory too.

Relationship to the device model

thermal_zoneX and cooling_deviceX are class views over /sys/devices/... nodes created by zone drivers (ACPI, OF thermal, x86_pkg_temp, int340x, and so on). Each zone carries cdevY symlinks pointing at the cooling devices bound to it. No bus of its own and no /sys/dev char/block entry.

Attributes

27 attribute paths are documented in Documentation/ABI/testing/sysfs-class-thermal. The table below is representative.

Attribute	R/W	Unit/format	Conditional?	Source
`thermal_zoneX/type`	RO	short lowercase string (for example `acpitz`)	no (required)	ABI What `thermal_zoneX/type`
`thermal_zoneX/temp`	RO	millidegree Celsius	no (required)	ABI What `thermal_zoneX/temp`
`thermal_zoneX/mode`	RW	`enabled`/`disabled`	optional	ABI What `thermal_zoneX/mode`
`thermal_zoneX/policy`	RW	governor name	required	ABI What `thermal_zoneX/policy`
`thermal_zoneX/available_policies`	RO	governor list	required	ABI What `available_policies`
`thermal_zoneX/trip_point_Y_temp`	RO	millidegree Celsius	per trip point	ABI What `trip_point_Y_temp`
`thermal_zoneX/trip_point_Y_type`	RO	`critical`, `hot`, `passive`, `active[0-*]`	per trip point	ABI What `trip_point_Y_type`
`thermal_zoneX/trip_point_Y_hyst`	RW	degrees Celsius (integer)	per trip point	ABI What `trip_point_Y_hyst`
`thermal_zoneX/cdevY`	RO	symlink to cooling device	per bound cdev	ABI What `thermal_zoneX/cdevY`
`thermal_zoneX/cdevY_weight`	RW	relative influence value	per bound cdev	ABI What `cdevY_weight`
`thermal_zoneX/emul_temp`	WO	millidegree Celsius; 0 disables emulation	debugging hook	ABI What `emul_temp`
`thermal_zoneX/sustainable_power`	RW	milliwatts (power allocator governor)	power allocator zones	ABI What `sustainable_power`
`cooling_deviceX/type`	RO	string (for example `Processor`, `Fan`)	no (required)	ABI What `cooling_deviceX/type`
`cooling_deviceX/cur_state`	RW	integer 0..`max_state`	no (required)	ABI What `cur_state`
`cooling_deviceX/max_state`	RO	integer	no (required)	ABI What `max_state`
`cooling_deviceX/stats/*`	RO/WO	`time_in_state_ms`, `trans_table`, `total_trans`, `reset`	stats support	ABI What `stats/*` entries

The k_pu, k_i, k_d, integral_cutoff, slope, and offset attributes belong to power-allocator and hotspot-model zones and are optional.

Safety and writes

temp, type, and trip points are safe to read. cur_state writes manually throttle hardware (set 0 for no cooling); on a CPU cooling device this is effectively a cpufreq clamp. emul_temp carries an explicit ABI warning: flooding it with low values disables kernel thermal policy, which can overheat a production system. stats/reset wipes cooling statistics. All writes need root.

References

Documentation/ABI/testing/sysfs-class-thermal (cached copy: 259 lines)
drivers/thermal/thermal_sysfs.c, drivers/thermal/thermal_core.c, drivers/thermal/Makefile (v5.10 cross-check per worklog 2-b; master)
Documentation/driver-api/thermal/power_allocator.rst
Ledger row SYSC-thermal; worklog Task 2-b (no mainline thermal_cooling class)