Documentation/driver-api/thermal/sysfs-api.rst - linux - Git at Google

 ===================================
 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
 ---------------------------------

     ::

 	struct thermal_zone_device
 	*thermal_zone_device_register(char *type,
 				      int trips, int mask, void *devdata,
 				      struct thermal_zone_device_ops *ops,
 				      const struct thermal_zone_params *tzp,
 				      int passive_delay, int polling_delay))

     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.

     type:
 	the thermal zone type.
     trips:
 	the total number of trip points this thermal zone supports.
     mask:
 	Bit string: If 'n'th bit is set, then trip point 'n' is writable.
     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.
 	.set_trips:
 		    set the trip points window. Whenever the current temperature
 		    is updated, the trip points immediately below and above the
 		    current temperature are found.
 	.get_mode:
 		   get the current mode (enabled/disabled) of the thermal zone.

 			- "enabled" means the kernel thermal management is
 			  enabled.
 			- "disabled" will prevent kernel thermal driver action
 			  upon trip points so that user applications can take
 			  charge of thermal management.
 	.set_mode:
 		set the mode (enabled/disabled) 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.
 	.set_emul_temp:
 			set the emulation temperature which helps in debugging
 			different threshold temperature points.
     tzp:
 	thermal zone platform parameters.
     passive_delay:
 	number of milliseconds to wait between polls when
 	performing passive cooling.
     polling_delay:
 	number of milliseconds to wait between polls when checking
 	whether trip points have been crossed (0 for interrupt driven systems).

     ::

 	void thermal_zone_device_unregister(struct thermal_zone_device *tz)

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

 	::

 	   struct thermal_zone_device
 	   *thermal_zone_of_sensor_register(struct device *dev, int sensor_id,
 				void *data,
 				const struct thermal_zone_of_device_ops *ops)

 	This interface adds a new sensor to a DT thermal zone.
 	This function will search the list of thermal zones described in
 	device tree and look for the zone that refer to the sensor device
 	pointed by dev->of_node as temperature providers. For the zone
 	pointing to the sensor node, the sensor will be added to the DT
 	thermal zone device.

 	The parameters for this interface are:

 	dev:
 			Device node of sensor containing valid node pointer in
 			dev->of_node.
 	sensor_id:
 			a sensor identifier, in case the sensor IP has more
 			than one sensors
 	data:
 			a private pointer (owned by the caller) that will be
 			passed back, when a temperature reading is needed.
 	ops:
 			`struct thermal_zone_of_device_ops *`.

 			==============  =======================================
 			get_temp	a pointer to a function that reads the
 					sensor temperature. This is mandatory
 					callback provided by sensor driver.
 			set_trips	a pointer to a function that sets a
 					temperature window. When this window is
 					left the driver must inform the thermal
 					core via thermal_zone_device_update.
 			get_trend 	a pointer to a function that reads the
 					sensor temperature trend.
 			set_emul_temp	a pointer to a function that sets
 					sensor emulated temperature.
 			==============  =======================================

 	The thermal zone temperature is provided by the get_temp() function
 	pointer of thermal_zone_of_device_ops. When called, it will
 	have the private pointer @data back.

 	It returns error pointer if fails otherwise valid thermal zone device
 	handle. Caller should check the return handle with IS_ERR() for finding
 	whether success or not.

 	::

 	    void thermal_zone_of_sensor_unregister(struct device *dev,
 						   struct thermal_zone_device *tzd)

 	This interface unregisters a sensor from a DT thermal zone which was
 	successfully added by interface thermal_zone_of_sensor_register().
 	This function removes the sensor callbacks and private data from the
 	thermal zone device registered with thermal_zone_of_sensor_register()
 	interface. It will also silent the zone by remove the .get_temp() and
 	get_trend() thermal zone device callbacks.

 	::

 	  struct thermal_zone_device
 	  *devm_thermal_zone_of_sensor_register(struct device *dev,
 				int sensor_id,
 				void *data,
 				const struct thermal_zone_of_device_ops *ops)

 	This interface is resource managed version of
 	thermal_zone_of_sensor_register().

 	All details of thermal_zone_of_sensor_register() described in
 	section 1.1.3 is applicable here.

 	The benefit of using this interface to register sensor is that it
 	is not require to explicitly call thermal_zone_of_sensor_unregister()
 	in error path or during driver unbinding as this is done by driver
 	resource manager.

 	::

 		void devm_thermal_zone_of_sensor_unregister(struct device *dev,
 						struct thermal_zone_device *tzd)

 	This interface is resource managed version of
 	thermal_zone_of_sensor_unregister().
 	All details of thermal_zone_of_sensor_unregister() described in
 	section 1.1.4 is applicable here.
 	Normally this function will not need to be called and the resource
 	management code will ensure that the resource is freed.

 	::

 		int thermal_zone_get_slope(struct thermal_zone_device *tz)

 	This interface is used to read the slope attribute value
 	for the thermal zone device, which might be useful for platform
 	drivers for temperature calculations.

 	::

 		int thermal_zone_get_offset(struct thermal_zone_device *tz)

 	This interface is used to read the offset attribute value
 	for the thermal zone device, which might be useful for platform
 	drivers for temperature calculations.

 1.2 thermal cooling device interface
 ------------------------------------


     ::

 	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 registered 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 Currently requested throttle state of the
 		cooling device.
 	.set_cur_state:
 		set the Current throttle state of the cooling device.

     ::

 	void thermal_cooling_device_unregister(struct thermal_cooling_device *cdev)

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

 1.3 interface for binding a thermal zone device with a thermal cooling device
 -----------------------------------------------------------------------------

     ::

 	int thermal_zone_bind_cooling_device(struct thermal_zone_device *tz,
 		int trip, struct thermal_cooling_device *cdev,
 		unsigned long upper, unsigned long lower, unsigned int weight);

     This interface function binds a thermal cooling device to a particular 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 in this thermal zone the cooling device
 	  is associated with.
     upper:
 	  the Maximum cooling state for this trip point.
 	  THERMAL_NO_LIMIT means no upper limit,
 	  and the cooling device can be in max_state.
     lower:
 	  the Minimum cooling state can be used for this trip point.
 	  THERMAL_NO_LIMIT means no lower limit,
 	  and the cooling device can be in cooling state 0.
     weight:
 	  the influence of this cooling device in this thermal
 	  zone.  See 1.4.1 below for more information.

     ::

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

     This interface function unbinds a thermal cooling device from a particular
     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 in this thermal zone the cooling device
 	is associated with.

 1.4 Thermal Zone Parameters
 ---------------------------

     ::

 	struct thermal_zone_params

     This structure defines the platform level parameters for a thermal zone.
     This data, for each thermal zone should come from the platform layer.
     This is an optional feature where some platforms can choose not to
     provide this data.

     .governor_name:
 	       Name of the thermal governor used for this zone
     .no_hwmon:
 	       a boolean to indicate if the thermal to hwmon sysfs interface
 	       is required. when no_hwmon == false, a hwmon sysfs interface
 	       will be created. when no_hwmon == true, nothing will be done.
 	       In case the thermal_zone_params is NULL, the hwmon interface
 	       will be created (for backward compatibility).

 2. sysfs attributes structure
 =============================

 ==	================
 RO	read only value
 WO	write only value
 RW	read/write value
 ==	================

 Thermal sysfs attributes will be represented under /sys/class/thermal.
 Hwmon sysfs I/F extension is also available under /sys/class/hwmon
 if hwmon is compiled in or built as a module.

 Thermal zone device sys I/F, created once it's registered::

   /sys/class/thermal/thermal_zone[0-*]:
     |---type:			Type of the thermal zone
     |---temp:			Current temperature
     |---mode:			Working mode of the thermal zone
     |---policy:			Thermal governor used for this zone
     |---available_policies:	Available thermal governors for this zone
     |---trip_point_[0-*]_temp:	Trip point temperature
     |---trip_point_[0-*]_type:	Trip point type
     |---trip_point_[0-*]_hyst:	Hysteresis value for this trip point
     |---emul_temp:		Emulated temperature set node
     |---sustainable_power:      Sustainable dissipatable power
     |---k_po:                   Proportional term during temperature overshoot
     |---k_pu:                   Proportional term during temperature undershoot
     |---k_i:                    PID's integral term in the power allocator gov
     |---k_d:                    PID's derivative term in the power allocator
     |---integral_cutoff:        Offset above which errors are accumulated
     |---slope:                  Slope constant applied as linear extrapolation
     |---offset:                 Offset constant applied as linear extrapolation

 Thermal cooling device sys I/F, created once it's registered::

   /sys/class/thermal/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
     |---stats:			Directory containing cooling device's statistics
     |---stats/reset:		Writing any value resets the statistics
     |---stats/time_in_state_ms:	Time (msec) spent in various cooling states
     |---stats/total_trans:	Total number of times cooling state is changed
     |---stats/trans_table:	Cooling state transition table


 Then next 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 successful execution of
 thermal_zone_bind_cooling_device/thermal_zone_unbind_cooling_device.

 ::

   /sys/class/thermal/thermal_zone[0-*]:
     |---cdev[0-*]:		[0-*]th cooling device in current thermal zone
     |---cdev[0-*]_trip_point:	Trip point that cdev[0-*] is associated with
     |---cdev[0-*]_weight:       Influence of the cooling device in
 				this thermal zone

 Besides the thermal zone device sysfs I/F and cooling device sysfs I/F,
 the generic thermal driver also creates a hwmon sysfs I/F for each _type_
 of thermal zone device. E.g. the generic thermal driver registers one hwmon
 class device and build the associated hwmon sysfs I/F for all the registered
 ACPI thermal zones.

 Please read Documentation/ABI/testing/sysfs-class-thermal for thermal
 zone and cooling device attribute details.

 ::

   /sys/class/hwmon/hwmon[0-*]:
     |---name:			The type of the thermal zone devices
     |---temp[1-*]_input:	The current temperature of thermal zone [1-*]
     |---temp[1-*]_critical:	The critical trip point of thermal zone [1-*]

 Please read Documentation/hwmon/sysfs-interface.rst for additional information.

 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. Both are considered to have the same
 effectiveness in cooling the thermal zone.

 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:			acpitz
     |---temp:			37000
     |---mode:			enabled
     |---policy:			step_wise
     |---available_policies:	step_wise fair_share
     |---trip_point_0_temp:	100000
     |---trip_point_0_type:	critical
     |---trip_point_1_temp:	80000
     |---trip_point_1_type:	passive
     |---trip_point_2_temp:	70000
     |---trip_point_2_type:	active0
     |---trip_point_3_temp:	60000
     |---trip_point_3_type:	active1
     |---cdev0:			--->/sys/class/thermal/cooling_device0
     |---cdev0_trip_point:	1	/* cdev0 can be used for passive */
     |---cdev0_weight:           1024
     |---cdev1:			--->/sys/class/thermal/cooling_device3
     |---cdev1_trip_point:	2	/* cdev1 can be used for active[0]*/
     |---cdev1_weight:           1024

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

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

  /sys/class/hwmon:
   |hwmon0:
     |---name:			acpitz
     |---temp1_input:		37000
     |---temp1_crit:		100000

 4. Export Symbol APIs
 =====================

 4.1. get_tz_trend
 -----------------

 This function returns the trend of a thermal zone, i.e the rate of change
 of temperature of the thermal zone. Ideally, the thermal sensor drivers
 are supposed to implement the callback. If they don't, the thermal
 framework calculated the trend by comparing the previous and the current
 temperature values.

 4.2. get_thermal_instance
 -------------------------

 This function returns the thermal_instance corresponding to a given
 {thermal_zone, cooling_device, trip_point} combination. Returns NULL
 if such an instance does not exist.

 4.3. thermal_cdev_update
 ------------------------

 This function serves as an arbitrator to set the state of a cooling
 device. It sets the cooling device to the deepest cooling state if
 possible.

 5. thermal_emergency_poweroff
 =============================

 On an event of critical trip temperature crossing the thermal framework
 shuts down the system by calling hw_protection_shutdown(). The
 hw_protection_shutdown() first attempts to perform an orderly shutdown
 but accepts a delay after which it proceeds doing a forced power-off
 or as last resort an emergency_restart.

 The delay should be carefully profiled so as to give adequate time for
 orderly poweroff.

 If the delay is set to 0 emergency poweroff will not be supported. So a
 carefully profiled non-zero positive value is a must for emergency
 poweroff to be triggered.
	===================================
	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
	---------------------------------

	::

	struct thermal_zone_device
	thermal_zone_device_register(char type,
	int trips, int mask, void *devdata,
	struct thermal_zone_device_ops *ops,
	const struct thermal_zone_params *tzp,
	int passive_delay, int polling_delay))

	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.

	type:
	the thermal zone type.
	trips:
	the total number of trip points this thermal zone supports.
	mask:
	Bit string: If 'n'th bit is set, then trip point 'n' is writable.
	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.
	.set_trips:
	set the trip points window. Whenever the current temperature
	is updated, the trip points immediately below and above the
	current temperature are found.
	.get_mode:
	get the current mode (enabled/disabled) of the thermal zone.

	- "enabled" means the kernel thermal management is
	enabled.
	- "disabled" will prevent kernel thermal driver action
	upon trip points so that user applications can take
	charge of thermal management.
	.set_mode:
	set the mode (enabled/disabled) 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.
	.set_emul_temp:
	set the emulation temperature which helps in debugging
	different threshold temperature points.
	tzp:
	thermal zone platform parameters.
	passive_delay:
	number of milliseconds to wait between polls when
	performing passive cooling.
	polling_delay:
	number of milliseconds to wait between polls when checking
	whether trip points have been crossed (0 for interrupt driven systems).

	::

	void thermal_zone_device_unregister(struct thermal_zone_device *tz)

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

	::

	struct thermal_zone_device
	thermal_zone_of_sensor_register(struct device dev, int sensor_id,
	void *data,
	const struct thermal_zone_of_device_ops *ops)

	This interface adds a new sensor to a DT thermal zone.
	This function will search the list of thermal zones described in
	device tree and look for the zone that refer to the sensor device
	pointed by dev->of_node as temperature providers. For the zone
	pointing to the sensor node, the sensor will be added to the DT
	thermal zone device.

	The parameters for this interface are:

	dev:
	Device node of sensor containing valid node pointer in
	dev->of_node.
	sensor_id:
	a sensor identifier, in case the sensor IP has more
	than one sensors
	data:
	a private pointer (owned by the caller) that will be
	passed back, when a temperature reading is needed.
	ops:
	`struct thermal_zone_of_device_ops *`.

	============== =======================================
	get_temp a pointer to a function that reads the
	sensor temperature. This is mandatory
	callback provided by sensor driver.
	set_trips a pointer to a function that sets a
	temperature window. When this window is
	left the driver must inform the thermal
	core via thermal_zone_device_update.
	get_trend a pointer to a function that reads the
	sensor temperature trend.
	set_emul_temp a pointer to a function that sets
	sensor emulated temperature.
	============== =======================================

	The thermal zone temperature is provided by the get_temp() function
	pointer of thermal_zone_of_device_ops. When called, it will
	have the private pointer @data back.

	It returns error pointer if fails otherwise valid thermal zone device
	handle. Caller should check the return handle with IS_ERR() for finding
	whether success or not.

	::

	void thermal_zone_of_sensor_unregister(struct device *dev,
	struct thermal_zone_device *tzd)

	This interface unregisters a sensor from a DT thermal zone which was
	successfully added by interface thermal_zone_of_sensor_register().
	This function removes the sensor callbacks and private data from the
	thermal zone device registered with thermal_zone_of_sensor_register()
	interface. It will also silent the zone by remove the .get_temp() and
	get_trend() thermal zone device callbacks.

	::

	struct thermal_zone_device
	devm_thermal_zone_of_sensor_register(struct device dev,
	int sensor_id,
	void *data,
	const struct thermal_zone_of_device_ops *ops)

	This interface is resource managed version of
	thermal_zone_of_sensor_register().

	All details of thermal_zone_of_sensor_register() described in
	section 1.1.3 is applicable here.

	The benefit of using this interface to register sensor is that it
	is not require to explicitly call thermal_zone_of_sensor_unregister()
	in error path or during driver unbinding as this is done by driver
	resource manager.

	::

	void devm_thermal_zone_of_sensor_unregister(struct device *dev,
	struct thermal_zone_device *tzd)

	This interface is resource managed version of
	thermal_zone_of_sensor_unregister().
	All details of thermal_zone_of_sensor_unregister() described in
	section 1.1.4 is applicable here.
	Normally this function will not need to be called and the resource
	management code will ensure that the resource is freed.

	::

	int thermal_zone_get_slope(struct thermal_zone_device *tz)

	This interface is used to read the slope attribute value
	for the thermal zone device, which might be useful for platform
	drivers for temperature calculations.

	::

	int thermal_zone_get_offset(struct thermal_zone_device *tz)

	This interface is used to read the offset attribute value
	for the thermal zone device, which might be useful for platform
	drivers for temperature calculations.

	1.2 thermal cooling device interface
	------------------------------------


	::

	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 registered 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 Currently requested throttle state of the
	cooling device.
	.set_cur_state:
	set the Current throttle state of the cooling device.

	::

	void thermal_cooling_device_unregister(struct thermal_cooling_device *cdev)

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

	1.3 interface for binding a thermal zone device with a thermal cooling device
	-----------------------------------------------------------------------------

	::

	int thermal_zone_bind_cooling_device(struct thermal_zone_device *tz,
	int trip, struct thermal_cooling_device *cdev,
	unsigned long upper, unsigned long lower, unsigned int weight);

	This interface function binds a thermal cooling device to a particular 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 in this thermal zone the cooling device
	is associated with.
	upper:
	the Maximum cooling state for this trip point.
	THERMAL_NO_LIMIT means no upper limit,
	and the cooling device can be in max_state.
	lower:
	the Minimum cooling state can be used for this trip point.
	THERMAL_NO_LIMIT means no lower limit,
	and the cooling device can be in cooling state 0.
	weight:
	the influence of this cooling device in this thermal
	zone. See 1.4.1 below for more information.

	::

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

	This interface function unbinds a thermal cooling device from a particular
	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 in this thermal zone the cooling device
	is associated with.

	1.4 Thermal Zone Parameters
	---------------------------

	::

	struct thermal_zone_params

	This structure defines the platform level parameters for a thermal zone.
	This data, for each thermal zone should come from the platform layer.
	This is an optional feature where some platforms can choose not to
	provide this data.

	.governor_name:
	Name of the thermal governor used for this zone
	.no_hwmon:
	a boolean to indicate if the thermal to hwmon sysfs interface
	is required. when no_hwmon == false, a hwmon sysfs interface
	will be created. when no_hwmon == true, nothing will be done.
	In case the thermal_zone_params is NULL, the hwmon interface
	will be created (for backward compatibility).

	2. sysfs attributes structure
	=============================

	== ================
	RO read only value
	WO write only value
	RW read/write value
	== ================

	Thermal sysfs attributes will be represented under /sys/class/thermal.
	Hwmon sysfs I/F extension is also available under /sys/class/hwmon
	if hwmon is compiled in or built as a module.

	Thermal zone device sys I/F, created once it's registered::

	/sys/class/thermal/thermal_zone[0-*]:
	\|---type: Type of the thermal zone
	\|---temp: Current temperature
	\|---mode: Working mode of the thermal zone
	\|---policy: Thermal governor used for this zone
	\|---available_policies: Available thermal governors for this zone
	\|---trip_point_[0-*]_temp: Trip point temperature
	\|---trip_point_[0-*]_type: Trip point type
	\|---trip_point_[0-*]_hyst: Hysteresis value for this trip point
	\|---emul_temp: Emulated temperature set node
	\|---sustainable_power: Sustainable dissipatable power
	\|---k_po: Proportional term during temperature overshoot
	\|---k_pu: Proportional term during temperature undershoot
	\|---k_i: PID's integral term in the power allocator gov
	\|---k_d: PID's derivative term in the power allocator
	\|---integral_cutoff: Offset above which errors are accumulated
	\|---slope: Slope constant applied as linear extrapolation
	\|---offset: Offset constant applied as linear extrapolation

	Thermal cooling device sys I/F, created once it's registered::

	/sys/class/thermal/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
	\|---stats: Directory containing cooling device's statistics
	\|---stats/reset: Writing any value resets the statistics
	\|---stats/time_in_state_ms: Time (msec) spent in various cooling states
	\|---stats/total_trans: Total number of times cooling state is changed
	\|---stats/trans_table: Cooling state transition table


	Then next 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 successful execution of
	thermal_zone_bind_cooling_device/thermal_zone_unbind_cooling_device.

	::

	/sys/class/thermal/thermal_zone[0-*]:
	\|---cdev[0-]: [0-]th cooling device in current thermal zone
	\|---cdev[0-]_trip_point: Trip point that cdev[0-] is associated with
	\|---cdev[0-*]_weight: Influence of the cooling device in
	this thermal zone

	Besides the thermal zone device sysfs I/F and cooling device sysfs I/F,
	the generic thermal driver also creates a hwmon sysfs I/F for each _type_
	of thermal zone device. E.g. the generic thermal driver registers one hwmon
	class device and build the associated hwmon sysfs I/F for all the registered
	ACPI thermal zones.

	Please read Documentation/ABI/testing/sysfs-class-thermal for thermal
	zone and cooling device attribute details.

	::

	/sys/class/hwmon/hwmon[0-*]:
	\|---name: The type of the thermal zone devices
	\|---temp[1-]_input: The current temperature of thermal zone [1-]
	\|---temp[1-]_critical: The critical trip point of thermal zone [1-]

	Please read Documentation/hwmon/sysfs-interface.rst for additional information.

	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. Both are considered to have the same
	effectiveness in cooling the thermal zone.

	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: acpitz
	\|---temp: 37000
	\|---mode: enabled
	\|---policy: step_wise
	\|---available_policies: step_wise fair_share
	\|---trip_point_0_temp: 100000
	\|---trip_point_0_type: critical
	\|---trip_point_1_temp: 80000
	\|---trip_point_1_type: passive
	\|---trip_point_2_temp: 70000
	\|---trip_point_2_type: active0
	\|---trip_point_3_temp: 60000
	\|---trip_point_3_type: active1
	\|---cdev0: --->/sys/class/thermal/cooling_device0
	\|---cdev0_trip_point: 1 /* cdev0 can be used for passive */
	\|---cdev0_weight: 1024
	\|---cdev1: --->/sys/class/thermal/cooling_device3
	\|---cdev1_trip_point: 2 /* cdev1 can be used for active[0]*/
	\|---cdev1_weight: 1024

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

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

	/sys/class/hwmon:
	\|hwmon0:
	\|---name: acpitz
	\|---temp1_input: 37000
	\|---temp1_crit: 100000

	4. Export Symbol APIs
	=====================

	4.1. get_tz_trend
	-----------------

	This function returns the trend of a thermal zone, i.e the rate of change
	of temperature of the thermal zone. Ideally, the thermal sensor drivers
	are supposed to implement the callback. If they don't, the thermal
	framework calculated the trend by comparing the previous and the current
	temperature values.

	4.2. get_thermal_instance
	-------------------------

	This function returns the thermal_instance corresponding to a given
	{thermal_zone, cooling_device, trip_point} combination. Returns NULL
	if such an instance does not exist.

	4.3. thermal_cdev_update
	------------------------

	This function serves as an arbitrator to set the state of a cooling
	device. It sets the cooling device to the deepest cooling state if
	possible.

	5. thermal_emergency_poweroff
	=============================

	On an event of critical trip temperature crossing the thermal framework
	shuts down the system by calling hw_protection_shutdown(). The
	hw_protection_shutdown() first attempts to perform an orderly shutdown
	but accepts a delay after which it proceeds doing a forced power-off
	or as last resort an emergency_restart.

	The delay should be carefully profiled so as to give adequate time for
	orderly poweroff.

	If the delay is set to 0 emergency poweroff will not be supported. So a
	carefully profiled non-zero positive value is a must for emergency
	poweroff to be triggered.