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

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

Updated: 2 January 2008

Copyright (c)  2008 Intel Corporation


0. Introduction

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

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

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

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

1. thermal sysfs driver interface functions

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

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

    name: the thermal zone name.
    trips: the total number of trip points this thermal zone supports.
    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.

1.1.2 void thermal_zone_device_unregister(struct thermal_zone_device *tz)

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

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

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

1.2.2 void thermal_cooling_device_unregister(struct thermal_cooling_device *cdev)

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

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

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

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

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

2. sysfs attributes structure

RO      read only value
RW      read/write value

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
    |---trip_point_[0-*]_temp:  Trip point temperature
    |---trip_point_[0-*]_type:  Trip point type
    |---trip_point_[0-*]_hyst:  Hysteresis value for this trip point

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

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

*****************************
* 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
    |---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). Typically the originator will be an integer assigned
to a thermal_zone_device when it registers itself with the framework. 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.