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 *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 writeable.
    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 (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).


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,
	unsigned long upper, unsigned long lower);

    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.
    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.

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.

1.4 Thermal Zone Parameters
1.4.1 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 on a percentage scale. The sum of all these weights
             (for a particular zone) cannot exceed 100.
    .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.
    .match: This call back returns success(0) if the 'tz and cdev' need to
	    be bound, as per platform data.
1.4.2 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
    .num_tbps: Number of thermal_bind_params entries for this zone
    .tbp: thermal_bind_params entries

2. sysfs attributes structure

RO	read 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
    |---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

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


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

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 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

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

passive
	Attribute is only present for zones in which the passive cooling
	policy is not supported by native thermal driver. Default is zero
	and can be set to a temperature (in millidegrees) to enable a
	passive trip point for the zone. Activation is done by polling with
	an interval of 1 second.
	Unit: millidegrees Celsius
	Valid values: 0 (disabled) or greater than 1000
	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.

*****************************
* 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

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:			acpitz
    |---temp:			37000
    |---mode:			enabled
    |---policy:			step_wise
    |---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 */
    |---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

/sys/class/hwmon:

|hwmon0:
    |---name:			acpitz
    |---temp1_input:		37000
    |---temp1_crit:		100000

4. Event Notification

The framework includes a simple notification mechanism, in the form of a
netlink event. Netlink socket initialization is done during the _init_
of the framework. Drivers which intend to use the notification mechanism
just need to call thermal_generate_netlink_event() with two arguments viz
(originator, event). The originator is a pointer to struct thermal_zone_device
from where the event has been originated. An integer which represents the
thermal zone device will be used in the message to identify the zone. The
event will be one of:{THERMAL_AUX0, THERMAL_AUX1, THERMAL_CRITICAL,
THERMAL_DEV_FAULT}. Notification can be sent when the current temperature
crosses any of the configured thresholds.

5. Export Symbol APIs:

5.1: 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.

5.2: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.

5.3:thermal_notify_framework:
This function handles the trip events from sensor drivers. It starts
throttling the cooling devices according to the policy configured.
For CRITICAL and HOT trip points, this notifies the respective drivers,
and does actual throttling for other trip points i.e ACTIVE and PASSIVE.
The throttling policy is based on the configured platform data; if no
platform data is provided, this uses the step_wise throttling policy.

5.4: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.