Generic Thermal Sysfs driver How To
=========================

Written by Sujith Thomas <sujith.thomas@intel.com>, Zhang Rui <rui.zhang@intel.com>

Updated: 2 January 2008

Copyright (c)  2008 Intel Corporation


0. Introduction

The generic thermal sysfs provides a set of interfaces for thermal zone devices (sensors)
and thermal cooling devices (fan, processor...) to register with the thermal management
solution and to be a part of it.

This how-to focuses on enabling new thermal zone and cooling devices to participate
in thermal management.
This solution is platform independent and any type of thermal zone devices and
cooling devices should be able to make use of the infrastructure.

The main task of the thermal sysfs driver is to expose thermal zone attributes as well
as cooling device attributes to the user space.
An intelligent thermal management application can make decisions based on inputs
from thermal zone attributes (the current temperature and trip point temperature)
and throttle appropriate devices.

[0-*]	denotes any positive number starting from 0
[1-*]	denotes any positive number starting from 1

1. thermal sysfs driver interface functions

1.1 thermal zone device interface
1.1.1 struct thermal_zone_device *thermal_zone_device_register(char *name, int trips,
				void *devdata, struct thermal_zone_device_ops *ops)

	This interface function adds a new thermal zone device (sensor) to
	/sys/class/thermal folder as thermal_zone[0-*].
	It tries to bind all the thermal cooling devices registered at the same time.

	name: the thermal zone name.
	trips: the total number of trip points this thermal zone supports.
	devdata: device private data
	ops: thermal zone device call-backs.
		.bind: bind the thermal zone device with a thermal cooling device.
		.unbind: unbind the thermal zone device with a thermal cooling device.
		.get_temp: get the current temperature of the thermal zone.
		.get_mode: get the current mode (user/kernel) of the thermal zone.
			   "kernel" means thermal management is done in kernel.
			   "user" will prevent kernel thermal driver actions upon trip points
			   so that user applications can take charge of thermal management.
		.set_mode: set the mode (user/kernel) of the thermal zone.
		.get_trip_type: get the type of certain trip point.
		.get_trip_temp: get the temperature above which the certain trip point
				will be fired.

1.1.2 void thermal_zone_device_unregister(struct thermal_zone_device *tz)

	This interface function removes the thermal zone device.
	It deletes the corresponding entry form /sys/class/thermal folder and unbind all
	the thermal cooling devices it uses.

1.2 thermal cooling device interface
1.2.1 struct thermal_cooling_device *thermal_cooling_device_register(char *name,
					void *devdata, struct thermal_cooling_device_ops *)

	This interface function adds a new thermal cooling device (fan/processor/...) to
	/sys/class/thermal/ folder as cooling_device[0-*].
	It tries to bind itself to all the thermal zone devices register at the same time.
	name: the cooling device name.
	devdata: device private data.
	ops: thermal cooling devices call-backs.
		.get_max_state: get the Maximum throttle state of the cooling device.
		.get_cur_state: get the Current throttle state of the cooling device.
		.set_cur_state: set the Current throttle state of the cooling device.

1.2.2 void thermal_cooling_device_unregister(struct thermal_cooling_device *cdev)

	This interface function remove the thermal cooling device.
	It deletes the corresponding entry form /sys/class/thermal folder and unbind
	itself from all	the thermal zone devices using it.

1.3 interface for binding a thermal zone device with a thermal cooling device
1.3.1 int thermal_zone_bind_cooling_device(struct thermal_zone_device *tz,
			int trip, struct thermal_cooling_device *cdev);

	This interface function bind a thermal cooling device to the certain trip point
	of a thermal zone device.
	This function is usually called in the thermal zone device .bind callback.
	tz: the thermal zone device
	cdev: thermal cooling device
	trip: indicates which trip point the cooling devices is associated with
		 in this thermal zone.

1.3.2 int thermal_zone_unbind_cooling_device(struct thermal_zone_device *tz,
				int trip, struct thermal_cooling_device *cdev);

	This interface function unbind a thermal cooling device from the certain trip point
	of a thermal zone device.
	This function is usually called in the thermal zone device .unbind callback.
	tz: the thermal zone device
	cdev: thermal cooling device
	trip: indicates which trip point the cooling devices is associated with
		in this thermal zone.

2. sysfs attributes structure

RO	read only value
RW	read/write value

All thermal sysfs attributes will be represented under /sys/class/thermal

Thermal zone device sys I/F, created once it's registered:
|thermal_zone[0-*]:
	|-----type:			Type of the thermal zone
	|-----temp:			Current temperature
	|-----mode:			Working mode of the thermal zone
	|-----trip_point_[0-*]_temp:	Trip point temperature
	|-----trip_point_[0-*]_type:	Trip point type

Thermal cooling device sys I/F, created once it's registered:
|cooling_device[0-*]:
	|-----type :			Type of the cooling device(processor/fan/...)
	|-----max_state:		Maximum cooling state of the cooling device
	|-----cur_state:		Current cooling state of the cooling device


These two dynamic attributes are created/removed in pairs.
They represent the relationship between a thermal zone and its associated cooling device.
They are created/removed for each
thermal_zone_bind_cooling_device/thermal_zone_unbind_cooling_device successful execution.

|thermal_zone[0-*]
	|-----cdev[0-*]:		The [0-*]th cooling device in the current thermal zone
	|-----cdev[0-*]_trip_point:	Trip point that cdev[0-*] is associated with


***************************
* Thermal zone attributes *
***************************

type				Strings which represent the thermal zone type.
				This is given by thermal zone driver as part of registration.
				Eg: "ACPI thermal zone" indicates it's a ACPI thermal device
				RO
				Optional

temp				Current temperature as reported by thermal zone (sensor)
				Unit: degree Celsius
				RO
				Required

mode				One of the predefined values in [kernel, user]
				This file gives information about the algorithm
				that is currently managing the thermal zone.
				It can be either default kernel based algorithm
				or user space application.
				RW
				Optional
				kernel	= Thermal management in kernel thermal zone driver.
				user	= Preventing kernel thermal zone driver actions upon
					  trip points so that user application can take full
					  charge of the thermal management.

trip_point_[0-*]_temp		The temperature above which trip point will be fired
				Unit: degree Celsius
				RO
				Optional

trip_point_[0-*]_type 		Strings which indicate the type of the trip point
				E.g. it can be one of critical, hot, passive,
				    active[0-*] for ACPI thermal zone.
				RO
				Optional

cdev[0-*]			Sysfs link to the thermal cooling device node where the sys I/F
				for cooling device throttling control represents.
				RO
				Optional

cdev[0-*]_trip_point		The trip point with which cdev[0-*] is associated in this thermal zone
				-1 means the cooling device is not associated with any trip point.
				RO
				Optional

******************************
* Cooling device  attributes *
******************************

type				String which represents the type of device
				eg: For generic ACPI: this should be "Fan",
				"Processor" or "LCD"
				eg. For memory controller device on intel_menlow platform:
				this should be "Memory controller"
				RO
				Optional

max_state			The maximum permissible cooling state of this cooling device.
				RO
				Required

cur_state			The current cooling state of this cooling device.
				the value can any integer numbers between 0 and max_state,
				cur_state == 0 means no cooling
				cur_state == max_state means the maximum cooling.
				RW
				Required

3. A simple implementation

ACPI thermal zone may support multiple trip points like critical/hot/passive/active.
If an ACPI thermal zone supports critical, passive, active[0] and active[1] at the same time,
it may register itself as a thermal_zone_device (thermal_zone1) with 4 trip points in all.
It has one processor and one fan, which are both registered as thermal_cooling_device.
If the processor is listed in _PSL method, and the fan is listed in _AL0 method,
the sys I/F structure will be built like this:

/sys/class/thermal:

|thermal_zone1:
	|-----type:			ACPI thermal zone
	|-----temp:			37
	|-----mode:			kernel
	|-----trip_point_0_temp:	100
	|-----trip_point_0_type:	critical
	|-----trip_point_1_temp:	80
	|-----trip_point_1_type:	passive
	|-----trip_point_2_temp:	70
	|-----trip_point_2_type:	active[0]
	|-----trip_point_3_temp:	60
	|-----trip_point_3_type:	active[1]
	|-----cdev0:			--->/sys/class/thermal/cooling_device0
	|-----cdev0_trip_point:		1	/* cdev0 can be used for passive */
	|-----cdev1:			--->/sys/class/thermal/cooling_device3
	|-----cdev1_trip_point:		2	/* cdev1 can be used for active[0]*/

|cooling_device0:
	|-----type:			Processor
	|-----max_state:		8
	|-----cur_state:		0

|cooling_device3:
	|-----type:			Fan
	|-----max_state:		2
	|-----cur_state:		0