FreeBSD/Linux Kernel Cross Reference
sys/contrib/device-tree/Bindings/thermal/thermal.txt

     * Thermal Framework Device Tree descriptor
     
     This file describes a generic binding to provide a way of
     defining hardware thermal structure using device tree.
     A thermal structure includes thermal zones and their components,
     such as trip points, polling intervals, sensors and cooling devices
     binding descriptors.
     
     The target of device tree thermal descriptors is to describe only
    the hardware thermal aspects. The thermal device tree bindings are
    not about how the system must control or which algorithm or policy
    must be taken in place.
    
    There are five types of nodes involved to describe thermal bindings:
    - thermal sensors: devices which may be used to take temperature
      measurements.
    - cooling devices: devices which may be used to dissipate heat.
    - trip points: describe key temperatures at which cooling is recommended. The
      set of points should be chosen based on hardware limits.
    - cooling maps: used to describe links between trip points and cooling devices;
    - thermal zones: used to describe thermal data within the hardware;
    
    The following is a description of each of these node types.
    
    * Thermal sensor devices
    
    Thermal sensor devices are nodes providing temperature sensing capabilities on
    thermal zones. Typical devices are I2C ADC converters and bandgaps. These are
    nodes providing temperature data to thermal zones. Thermal sensor devices may
    control one or more internal sensors.
    
    Required property:
    - #thermal-sensor-cells: Used to provide sensor device specific information
      Type: unsigned         while referring to it. Typically 0 on thermal sensor
      Size: one cell         nodes with only one sensor, and at least 1 on nodes
                             with several internal sensors, in order
                             to identify uniquely the sensor instances within
                             the IC. See thermal zone binding for more details
                             on how consumers refer to sensor devices.
    
    * Cooling device nodes
    
    Cooling devices are nodes providing control on power dissipation. There
    are essentially two ways to provide control on power dissipation. First
    is by means of regulating device performance, which is known as passive
    cooling. A typical passive cooling is a CPU that has dynamic voltage and
    frequency scaling (DVFS), and uses lower frequencies as cooling states.
    Second is by means of activating devices in order to remove
    the dissipated heat, which is known as active cooling, e.g. regulating
    fan speeds. In both cases, cooling devices shall have a way to determine
    the state of cooling in which the device is.
    
    Any cooling device has a range of cooling states (i.e. different levels
    of heat dissipation). For example a fan's cooling states correspond to
    the different fan speeds possible. Cooling states are referred to by
    single unsigned integers, where larger numbers mean greater heat
    dissipation. The precise set of cooling states associated with a device
    should be defined in a particular device's binding.
    For more examples of cooling devices, refer to the example sections below.
    
    Required properties:
    - #cooling-cells:       Used to provide cooling device specific information
      Type: unsigned        while referring to it. Must be at least 2, in order
      Size: one cell        to specify minimum and maximum cooling state used
                            in the reference. The first cell is the minimum
                            cooling state requested and the second cell is
                            the maximum cooling state requested in the reference.
                            See Cooling device maps section below for more details
                            on how consumers refer to cooling devices.
    
    * Trip points
    
    The trip node is a node to describe a point in the temperature domain
    in which the system takes an action. This node describes just the point,
    not the action.
    
    Required properties:
    - temperature:          An integer indicating the trip temperature level,
      Type: signed          in millicelsius.
      Size: one cell
    
    - hysteresis:           A low hysteresis value on temperature property (above).
      Type: unsigned        This is a relative value, in millicelsius.
      Size: one cell
    
    - type:                 a string containing the trip type. Expected values are:
            "active":       A trip point to enable active cooling
            "passive":      A trip point to enable passive cooling
            "hot":          A trip point to notify emergency
            "critical":     Hardware not reliable.
      Type: string
    
    * Cooling device maps
    
    The cooling device maps node is a node to describe how cooling devices
    get assigned to trip points of the zone. The cooling devices are expected
    to be loaded in the target system.
    
    Required properties:
   - cooling-device:       A list of phandles of cooling devices with their specifiers,
     Type: phandle +       referring to which cooling devices are used in this
       cooling specifier   binding. In the cooling specifier, the first cell
                           is the minimum cooling state and the second cell
                           is the maximum cooling state used in this map.
   - trip:                 A phandle of a trip point node within the same thermal
     Type: phandle of      zone.
      trip point node
   
   Optional property:
   - contribution:         The cooling contribution to the thermal zone of the
     Type: unsigned        referred cooling device at the referred trip point.
     Size: one cell        The contribution is a ratio of the sum
                           of all cooling contributions within a thermal zone.
   
   Note: Using the THERMAL_NO_LIMIT (-1UL) constant in the cooling-device phandle
   limit specifier means:
   (i)   - minimum state allowed for minimum cooling state used in the reference.
   (ii)  - maximum state allowed for maximum cooling state used in the reference.
   Refer to include/dt-bindings/thermal/thermal.h for definition of this constant.
   
   * Thermal zone nodes
   
   The thermal zone node is the node containing all the required info
   for describing a thermal zone, including its cooling device bindings. The
   thermal zone node must contain, apart from its own properties, one sub-node
   containing trip nodes and one sub-node containing all the zone cooling maps.
   
   Required properties:
   - polling-delay:        The maximum number of milliseconds to wait between polls
     Type: unsigned        when checking this thermal zone.
     Size: one cell
   
   - polling-delay-passive: The maximum number of milliseconds to wait
     Type: unsigned        between polls when performing passive cooling.
     Size: one cell
   
   - thermal-sensors:      A list of thermal sensor phandles and sensor specifier
     Type: list of         used while monitoring the thermal zone.
     phandles + sensor
     specifier
   
   - trips:                A sub-node which is a container of only trip point nodes
     Type: sub-node        required to describe the thermal zone.
   
   Optional property:
   - cooling-maps:         A sub-node which is a container of only cooling device
     Type: sub-node        map nodes, used to describe the relation between trips
                           and cooling devices.
   
   - coefficients:         An array of integers (one signed cell) containing
     Type: array           coefficients to compose a linear relation between
     Elem size: one cell   the sensors listed in the thermal-sensors property.
     Elem type: signed     Coefficients defaults to 1, in case this property
                           is not specified. A simple linear polynomial is used:
                           Z = c0 * x0 + c1 * x1 + ... + c(n-1) * x(n-1) + cn.
   
                           The coefficients are ordered and they match with sensors
                           by means of sensor ID. Additional coefficients are
                           interpreted as constant offset.
   
   - sustainable-power:    An estimate of the sustainable power (in mW) that the
     Type: unsigned        thermal zone can dissipate at the desired
     Size: one cell        control temperature.  For reference, the
                           sustainable power of a 4'' phone is typically
                           2000mW, while on a 10'' tablet is around
                           4500mW.
   
   Note: The delay properties are bound to the maximum dT/dt (temperature
   derivative over time) in two situations for a thermal zone:
   (i)  - when passive cooling is activated (polling-delay-passive); and
   (ii) - when the zone just needs to be monitored (polling-delay) or
   when active cooling is activated.
   
   The maximum dT/dt is highly bound to hardware power consumption and dissipation
   capability. The delays should be chosen to account for said max dT/dt,
   such that a device does not cross several trip boundaries unexpectedly
   between polls. Choosing the right polling delays shall avoid having the
   device in temperature ranges that may damage the silicon structures and
   reduce silicon lifetime.
   
   * The thermal-zones node
   
   The "thermal-zones" node is a container for all thermal zone nodes. It shall
   contain only sub-nodes describing thermal zones as in the section
   "Thermal zone nodes". The "thermal-zones" node appears under "/".
   
   * Examples
   
   Below are several examples on how to use thermal data descriptors
   using device tree bindings:
   
   (a) - CPU thermal zone
   
   The CPU thermal zone example below describes how to setup one thermal zone
   using one single sensor as temperature source and many cooling devices and
   power dissipation control sources.
   
   #include <dt-bindings/thermal/thermal.h>
   
   cpus {
           /*
            * Here is an example of describing a cooling device for a DVFS
            * capable CPU. The CPU node describes its four OPPs.
            * The cooling states possible are 0..3, and they are
            * used as OPP indexes. The minimum cooling state is 0, which means
            * all four OPPs can be available to the system. The maximum
            * cooling state is 3, which means only the lowest OPPs (198MHz@0.85V)
            * can be available in the system.
            */
           cpu0: cpu@0 {
                   ...
                   operating-points = <
                           /* kHz    uV */
                           970000  1200000
                           792000  1100000
                           396000  950000
                           198000  850000
                   >;
                   #cooling-cells = <2>; /* min followed by max */
           };
           ...
   };
   
   &i2c1 {
           ...
           /*
            * A simple fan controller which supports 10 speeds of operation
            * (represented as 0-9).
            */
           fan0: fan@48 {
                   ...
                   #cooling-cells = <2>; /* min followed by max */
           };
   };
   
   ocp {
           ...
           /*
            * A simple IC with a single bandgap temperature sensor.
            */
           bandgap0: bandgap@0000ed00 {
                   ...
                   #thermal-sensor-cells = <0>;
           };
   };
   
   thermal-zones {
           cpu_thermal: cpu-thermal {
                   polling-delay-passive = <250>; /* milliseconds */
                   polling-delay = <1000>; /* milliseconds */
   
                   thermal-sensors = <&bandgap0>;
   
                   trips {
                           cpu_alert0: cpu-alert0 {
                                   temperature = <90000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "active";
                           };
                           cpu_alert1: cpu-alert1 {
                                   temperature = <100000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "passive";
                           };
                           cpu_crit: cpu-crit {
                                   temperature = <125000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "critical";
                           };
                   };
   
                   cooling-maps {
                           map0 {
                                   trip = <&cpu_alert0>;
                                   cooling-device = <&fan0 THERMAL_NO_LIMIT 4>;
                           };
                           map1 {
                                   trip = <&cpu_alert1>;
                                   cooling-device = <&fan0 5 THERMAL_NO_LIMIT>, <&cpu0 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>;
                           };
                   };
           };
   };
   
   In the example above, the ADC sensor (bandgap0) at address 0x0000ED00 is
   used to monitor the zone 'cpu-thermal' using its sole sensor. A fan
   device (fan0) is controlled via I2C bus 1, at address 0x48, and has ten
   different cooling states 0-9. It is used to remove the heat out of
   the thermal zone 'cpu-thermal' using its cooling states
   from its minimum to 4, when it reaches trip point 'cpu_alert0'
   at 90C, as an example of active cooling. The same cooling device is used at
   'cpu_alert1', but from 5 to its maximum state. The cpu@0 device is also
   linked to the same thermal zone, 'cpu-thermal', as a passive cooling device,
   using all its cooling states at trip point 'cpu_alert1',
   which is a trip point at 100C. On the thermal zone 'cpu-thermal', at the
   temperature of 125C, represented by the trip point 'cpu_crit', the silicon
   is not reliable anymore.
   
   (b) - IC with several internal sensors
   
   The example below describes how to deploy several thermal zones based off a
   single sensor IC, assuming it has several internal sensors. This is a common
   case on SoC designs with several internal IPs that may need different thermal
   requirements, and thus may have their own sensor to monitor or detect internal
   hotspots in their silicon.
   
   #include <dt-bindings/thermal/thermal.h>
   
   ocp {
           ...
           /*
            * A simple IC with several bandgap temperature sensors.
            */
           bandgap0: bandgap@0000ed00 {
                   ...
                   #thermal-sensor-cells = <1>;
           };
   };
   
   thermal-zones {
           cpu_thermal: cpu-thermal {
                   polling-delay-passive = <250>; /* milliseconds */
                   polling-delay = <1000>; /* milliseconds */
   
                                   /* sensor       ID */
                   thermal-sensors = <&bandgap0     0>;
   
                   trips {
                           /* each zone within the SoC may have its own trips */
                           cpu_alert: cpu-alert {
                                   temperature = <100000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "passive";
                           };
                           cpu_crit: cpu-crit {
                                   temperature = <125000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "critical";
                           };
                   };
   
                   cooling-maps {
                           /* each zone within the SoC may have its own cooling */
                           ...
                   };
           };
   
           gpu_thermal: gpu-thermal {
                   polling-delay-passive = <120>; /* milliseconds */
                   polling-delay = <1000>; /* milliseconds */
   
                                   /* sensor       ID */
                   thermal-sensors = <&bandgap0     1>;
   
                   trips {
                           /* each zone within the SoC may have its own trips */
                           gpu_alert: gpu-alert {
                                   temperature = <90000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "passive";
                           };
                           gpu_crit: gpu-crit {
                                   temperature = <105000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "critical";
                           };
                   };
   
                   cooling-maps {
                           /* each zone within the SoC may have its own cooling */
                           ...
                   };
           };
   
           dsp_thermal: dsp-thermal {
                   polling-delay-passive = <50>; /* milliseconds */
                   polling-delay = <1000>; /* milliseconds */
   
                                   /* sensor       ID */
                   thermal-sensors = <&bandgap0     2>;
   
                   trips {
                           /* each zone within the SoC may have its own trips */
                           dsp_alert: dsp-alert {
                                   temperature = <90000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "passive";
                           };
                           dsp_crit: gpu-crit {
                                   temperature = <135000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "critical";
                           };
                   };
   
                   cooling-maps {
                           /* each zone within the SoC may have its own cooling */
                           ...
                   };
           };
   };
   
   In the example above, there is one bandgap IC which has the capability to
   monitor three sensors. The hardware has been designed so that sensors are
   placed on different places in the DIE to monitor different temperature
   hotspots: one for CPU thermal zone, one for GPU thermal zone and the
   other to monitor a DSP thermal zone.
   
   Thus, there is a need to assign each sensor provided by the bandgap IC
   to different thermal zones. This is achieved by means of using the
   #thermal-sensor-cells property and using the first cell of the sensor
   specifier as sensor ID. In the example, then, <bandgap 0> is used to
   monitor CPU thermal zone, <bandgap 1> is used to monitor GPU thermal
   zone and <bandgap 2> is used to monitor DSP thermal zone. Each zone
   may be uncorrelated, having its own dT/dt requirements, trips
   and cooling maps.
   
   
   (c) - Several sensors within one single thermal zone
   
   The example below illustrates how to use more than one sensor within
   one thermal zone.
   
   #include <dt-bindings/thermal/thermal.h>
   
   &i2c1 {
           ...
           /*
            * A simple IC with a single temperature sensor.
            */
           adc: sensor@49 {
                   ...
                   #thermal-sensor-cells = <0>;
           };
   };
   
   ocp {
           ...
           /*
            * A simple IC with a single bandgap temperature sensor.
            */
           bandgap0: bandgap@0000ed00 {
                   ...
                   #thermal-sensor-cells = <0>;
           };
   };
   
   thermal-zones {
           cpu_thermal: cpu-thermal {
                   polling-delay-passive = <250>; /* milliseconds */
                   polling-delay = <1000>; /* milliseconds */
   
                   thermal-sensors = <&bandgap0>,  /* cpu */
                                     <&adc>;       /* pcb north */
   
                   /* hotspot = 100 * bandgap - 120 * adc + 484 */
                   coefficients =          <100    -120    484>;
   
                   trips {
                           ...
                   };
   
                   cooling-maps {
                           ...
                   };
           };
   };
   
   In some cases, there is a need to use more than one sensor to extrapolate
   a thermal hotspot in the silicon. The above example illustrates this situation.
   For instance, it may be the case that a sensor external to CPU IP may be placed
   close to CPU hotspot and together with internal CPU sensor, it is used
   to determine the hotspot. Assuming this is the case for the above example,
   the hypothetical extrapolation rule would be:
                   hotspot = 100 * bandgap - 120 * adc + 484
   
   In other context, the same idea can be used to add fixed offset. For instance,
   consider the hotspot extrapolation rule below:
                   hotspot = 1 * adc + 6000
   
   In the above equation, the hotspot is always 6C higher than what is read
   from the ADC sensor. The binding would be then:
           thermal-sensors =  <&adc>;
   
                   /* hotspot = 1 * adc + 6000 */
           coefficients =          <1      6000>;
   
   (d) - Board thermal
   
   The board thermal example below illustrates how to setup one thermal zone
   with many sensors and many cooling devices.
   
   #include <dt-bindings/thermal/thermal.h>
   
   &i2c1 {
           ...
           /*
            * An IC with several temperature sensor.
            */
           adc_dummy: sensor@50 {
                   ...
                   #thermal-sensor-cells = <1>; /* sensor internal ID */
           };
   };
   
   thermal-zones {
           batt-thermal {
                   polling-delay-passive = <500>; /* milliseconds */
                   polling-delay = <2500>; /* milliseconds */
   
                                   /* sensor       ID */
                   thermal-sensors = <&adc_dummy     4>;
   
                   trips {
                           ...
                   };
   
                   cooling-maps {
                           ...
                   };
           };
   
           board_thermal: board-thermal {
                   polling-delay-passive = <1000>; /* milliseconds */
                   polling-delay = <2500>; /* milliseconds */
   
                                   /* sensor       ID */
                   thermal-sensors = <&adc_dummy     0>, /* pcb top edge */
                                     <&adc_dummy     1>, /* lcd */
                                     <&adc_dummy     2>; /* back cover */
                   /*
                    * An array of coefficients describing the sensor
                    * linear relation. E.g.:
                    * z = c1*x1 + c2*x2 + c3*x3
                    */
                   coefficients =          <1200   -345    890>;
   
                   sustainable-power = <2500>;
   
                   trips {
                           /* Trips are based on resulting linear equation */
                           cpu_trip: cpu-trip {
                                   temperature = <60000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "passive";
                           };
                           gpu_trip: gpu-trip {
                                   temperature = <55000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "passive";
                           }
                           lcd_trip: lcp-trip {
                                   temperature = <53000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "passive";
                           };
                           crit_trip: crit-trip {
                                   temperature = <68000>; /* millicelsius */
                                   hysteresis = <2000>; /* millicelsius */
                                   type = "critical";
                           };
                   };
   
                   cooling-maps {
                           map0 {
                                   trip = <&cpu_trip>;
                                   cooling-device = <&cpu0 0 2>;
                                   contribution = <55>;
                           };
                           map1 {
                                   trip = <&gpu_trip>;
                                   cooling-device = <&gpu0 0 2>;
                                   contribution = <20>;
                           };
                           map2 {
                                   trip = <&lcd_trip>;
                                   cooling-device = <&lcd0 5 10>;
                                   contribution = <15>;
                           };
                   };
           };
   };
   
   The above example is a mix of previous examples, a sensor IP with several internal
   sensors used to monitor different zones, one of them is composed by several sensors and
   with different cooling devices.

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