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

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

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_with_trips(const char *type,
					const struct thermal_trip *trips,
					int num_trips, void *devdata,
					const struct thermal_zone_device_ops *ops,
					const struct thermal_zone_params *tzp,
					unsigned int passive_delay,
					unsigned int polling_delay)

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

    type:
	the thermal zone type.
    trips:
	the table of trip points for this thermal zone.
    devdata:
	device private data
    ops:
	thermal zone device call-backs.

	.should_bind:
		check whether or not a given cooling device should be bound to
		a given trip point in this thermal zone.
	.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.
	.change_mode:
		change the mode (enabled/disabled) of the thermal zone.
	.set_trip_temp:
		set the temperature of a given trip point.
	.get_crit_temp:
		get the critical temperature for this thermal zone.
	.set_emul_temp:
		set the emulation temperature which helps in debugging
		different threshold temperature points.
	.get_trend:
		get the trend of most recent zone temperature changes.
	.hot:
		hot trip point crossing handler.
	.critical:
		critical trip point crossing handler.
    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.

1.4 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.

::

  /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. 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. Critical Events
==================

On an event of critical trip temperature crossing, the thermal framework
will trigger a hardware protection power-off (shutdown) or reboot,
depending on configuration.

At first, the kernel will attempt an orderly power-off or reboot, but
accepts a delay after which it proceeds to do a forced power-off or
reboot, respectively. If this fails, ``emergency_restart()`` is invoked
as last resort.

The delay should be carefully profiled so as to give adequate time for
orderly power-off or reboot.

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