===================================
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_bind_params

    This structure defines the following parameters that are used to bind
    a zone with a cooling device for a particular trip point.

    .cdev:
	     The cooling device pointer
    .weight:
	     The 'influence' of a particular cooling device on this
	     zone. This is relative to the rest of the cooling
	     devices. For example, if all cooling devices have a
	     weight of 1, then they all contribute the same. You can
	     use percentages if you want, but it's not mandatory. A
	     weight of 0 means that this cooling device doesn't
	     contribute to the cooling of this zone unless all cooling
	     devices have a weight of 0. If all weights are 0, then
	     they all contribute the same.
    .trip_mask:
	       This is a bit mask that gives the binding relation between
	       this thermal zone and cdev, for a particular trip point.
	       If nth bit is set, then the cdev and thermal zone are bound
	       for trip point n.
    .binding_limits:
		     This is an array of cooling state limits. Must have
		     exactly 2 * thermal_zone.number_of_trip_points. It is an
		     array consisting of tuples <lower-state upper-state> of
		     state limits. Each trip will be associated with one state
		     limit tuple when binding. A NULL pointer means
		     <THERMAL_NO_LIMITS THERMAL_NO_LIMITS> on all trips.
		     These limits are used when binding a cdev to a trip point.
    .match:
	    This call back returns success(0) if the 'tz and cdev' need to
	    be bound, as per platform data.

    ::

	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).
    .num_tbps:
	       Number of thermal_bind_params entries for this zone
    .tbp:
	       thermal_bind_params entries

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.

::

  /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.

Thermal zone attributes
-----------------------

type
	Strings which represent the thermal zone type.
	This is given by thermal zone driver as part of registration.
	E.g: "acpitz" indicates it's an ACPI thermal device.
	In order to keep it consistent with hwmon sys attribute; this should
	be a short, lowercase string, not containing spaces nor dashes.
	RO, Required

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

mode
	One of the predefined values in [enabled, disabled].
	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.

	enabled
			  enable Kernel Thermal management.
	disabled
			  Preventing kernel thermal zone driver actions upon
			  trip points so that user application can take full
			  charge of the thermal management.

	RW, Optional

policy
	One of the various thermal governors used for a particular zone.

	RW, Required

available_policies
	Available thermal governors which can be used for a particular zone.

	RO, Required

`trip_point_[0-*]_temp`
	The temperature above which trip point will be fired.

	Unit: millidegree 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

`trip_point_[0-*]_hyst`
	The hysteresis value for a trip point, represented as an integer
	Unit: Celsius
	RW, 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 in this thermal zone which `cdev[0-*]` is associated
	with; -1 means the cooling device is not associated with any trip
	point.

	RO, Optional

`cdev[0-*]_weight`
	The influence of `cdev[0-*]` in this thermal zone. This value
	is relative to the rest of cooling devices in the thermal
	zone. For example, if a cooling device has a weight double
	than that of other, it's twice as effective in cooling the
	thermal zone.

	RW, Optional

emul_temp
	Interface to set the emulated temperature method in thermal zone
	(sensor). After setting this temperature, the thermal zone may pass
	this temperature to platform emulation function if registered or
	cache it locally. This is useful in debugging different temperature
	threshold and its associated cooling action. This is write only node
	and writing 0 on this node should disable emulation.
	Unit: millidegree Celsius

	WO, Optional

	  WARNING:
	    Be careful while enabling this option on production systems,
	    because userland can easily disable the thermal policy by simply
	    flooding this sysfs node with low temperature values.

sustainable_power
	An estimate of the sustained power that can be dissipated by
	the thermal zone. Used by the power allocator governor. For
	more information see Documentation/driver-api/thermal/power_allocator.rst

	Unit: milliwatts

	RW, Optional

k_po
	The proportional term of the power allocator governor's PID
	controller during temperature overshoot. Temperature overshoot
	is when the current temperature is above the "desired
	temperature" trip point. For more information see
	Documentation/driver-api/thermal/power_allocator.rst

	RW, Optional

k_pu
	The proportional term of the power allocator governor's PID
	controller during temperature undershoot. Temperature undershoot
	is when the current temperature is below the "desired
	temperature" trip point. For more information see
	Documentation/driver-api/thermal/power_allocator.rst

	RW, Optional

k_i
	The integral term of the power allocator governor's PID
	controller. This term allows the PID controller to compensate
	for long term drift. For more information see
	Documentation/driver-api/thermal/power_allocator.rst

	RW, Optional

k_d
	The derivative term of the power allocator governor's PID
	controller. For more information see
	Documentation/driver-api/thermal/power_allocator.rst

	RW, Optional

integral_cutoff
	Temperature offset from the desired temperature trip point
	above which the integral term of the power allocator
	governor's PID controller starts accumulating errors. For
	example, if integral_cutoff is 0, then the integral term only
	accumulates error when temperature is above the desired
	temperature trip point. For more information see
	Documentation/driver-api/thermal/power_allocator.rst

	Unit: millidegree Celsius

	RW, Optional

slope
	The slope constant used in a linear extrapolation model
	to determine a hotspot temperature based off the sensor's
	raw readings. It is up to the device driver to determine
	the usage of these values.

	RW, Optional

offset
	The offset constant used in a linear extrapolation model
	to determine a hotspot temperature based off the sensor's
	raw readings. It is up to the device driver to determine
	the usage of these values.

	RW, Optional

Cooling device attributes
-------------------------

type
	String which represents the type of device, e.g:

	- for generic ACPI: should be "Fan", "Processor" or "LCD"
	- for memory controller device on intel_menlow platform:
	  should be "Memory controller".

	RO, Required

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

stats/reset
	Writing any value resets the cooling device's statistics.
	WO, Required

stats/time_in_state_ms:
	The amount of time spent by the cooling device in various cooling
	states. The output will have "<state> <time>" pair in each line, which
	will mean this cooling device spent <time> msec of time at <state>.
	Output will have one line for each of the supported states.
	RO, Required


stats/total_trans:
	A single positive value showing the total number of times the state of a
	cooling device is changed.

	RO, Required

stats/trans_table:
	This gives fine grained information about all the cooling state
	transitions. The cat output here is a two dimensional matrix, where an
	entry <i,j> (row i, column j) represents the number of transitions from
	State_i to State_j. If the transition table is bigger than PAGE_SIZE,
	reading this will return an -EFBIG error.
	RO, 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. 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. Thermal framework
allows the system to shutdown gracefully by calling orderly_poweroff().
In the event of a failure of orderly_poweroff() to shut down the system
we are in danger of keeping the system alive at undesirably high
temperatures. To mitigate this high risk scenario we program a work
queue to fire after a pre-determined number of seconds to start
an emergency shutdown of the device using the kernel_power_off()
function. In case kernel_power_off() fails then finally
emergency_restart() is called in the worst case.

The delay should be carefully profiled so as to give adequate time for
orderly_poweroff(). In case of failure of an orderly_poweroff() the
emergency poweroff kicks in after the delay has elapsed and shuts down
the system.

If 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.
