linux/Documentation/thermal/sysfs-api.txt
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   1Generic Thermal Sysfs driver How To
   2===================================
   3
   4Written by Sujith Thomas <sujith.thomas@intel.com>, Zhang Rui <rui.zhang@intel.com>
   5
   6Updated: 2 January 2008
   7
   8Copyright (c)  2008 Intel Corporation
   9
  10
  110. Introduction
  12
  13The generic thermal sysfs provides a set of interfaces for thermal zone
  14devices (sensors) and thermal cooling devices (fan, processor...) to register
  15with the thermal management solution and to be a part of it.
  16
  17This how-to focuses on enabling new thermal zone and cooling devices to
  18participate in thermal management.
  19This solution is platform independent and any type of thermal zone devices
  20and cooling devices should be able to make use of the infrastructure.
  21
  22The main task of the thermal sysfs driver is to expose thermal zone attributes
  23as well as cooling device attributes to the user space.
  24An intelligent thermal management application can make decisions based on
  25inputs from thermal zone attributes (the current temperature and trip point
  26temperature) and throttle appropriate devices.
  27
  28[0-*]   denotes any positive number starting from 0
  29[1-*]   denotes any positive number starting from 1
  30
  311. thermal sysfs driver interface functions
  32
  331.1 thermal zone device interface
  341.1.1 struct thermal_zone_device *thermal_zone_device_register(char *type,
  35                int trips, int mask, void *devdata,
  36                struct thermal_zone_device_ops *ops,
  37                const struct thermal_zone_params *tzp,
  38                int passive_delay, int polling_delay))
  39
  40    This interface function adds a new thermal zone device (sensor) to
  41    /sys/class/thermal folder as thermal_zone[0-*]. It tries to bind all the
  42    thermal cooling devices registered at the same time.
  43
  44    type: the thermal zone type.
  45    trips: the total number of trip points this thermal zone supports.
  46    mask: Bit string: If 'n'th bit is set, then trip point 'n' is writeable.
  47    devdata: device private data
  48    ops: thermal zone device call-backs.
  49        .bind: bind the thermal zone device with a thermal cooling device.
  50        .unbind: unbind the thermal zone device with a thermal cooling device.
  51        .get_temp: get the current temperature of the thermal zone.
  52        .set_trips: set the trip points window. Whenever the current temperature
  53                    is updated, the trip points immediately below and above the
  54                    current temperature are found.
  55        .get_mode: get the current mode (enabled/disabled) of the thermal zone.
  56            - "enabled" means the kernel thermal management is enabled.
  57            - "disabled" will prevent kernel thermal driver action upon trip points
  58              so that user applications can take charge of thermal management.
  59        .set_mode: set the mode (enabled/disabled) of the thermal zone.
  60        .get_trip_type: get the type of certain trip point.
  61        .get_trip_temp: get the temperature above which the certain trip point
  62                        will be fired.
  63        .set_emul_temp: set the emulation temperature which helps in debugging
  64                        different threshold temperature points.
  65    tzp: thermal zone platform parameters.
  66    passive_delay: number of milliseconds to wait between polls when
  67        performing passive cooling.
  68    polling_delay: number of milliseconds to wait between polls when checking
  69        whether trip points have been crossed (0 for interrupt driven systems).
  70
  71
  721.1.2 void thermal_zone_device_unregister(struct thermal_zone_device *tz)
  73
  74    This interface function removes the thermal zone device.
  75    It deletes the corresponding entry from /sys/class/thermal folder and
  76    unbinds all the thermal cooling devices it uses.
  77
  781.1.3 struct thermal_zone_device *thermal_zone_of_sensor_register(
  79                struct device *dev, int sensor_id, void *data,
  80                const struct thermal_zone_of_device_ops *ops)
  81
  82        This interface adds a new sensor to a DT thermal zone.
  83        This function will search the list of thermal zones described in
  84        device tree and look for the zone that refer to the sensor device
  85        pointed by dev->of_node as temperature providers. For the zone
  86        pointing to the sensor node, the sensor will be added to the DT
  87        thermal zone device.
  88
  89        The parameters for this interface are:
  90        dev:            Device node of sensor containing valid node pointer in
  91                        dev->of_node.
  92        sensor_id:      a sensor identifier, in case the sensor IP has more
  93                        than one sensors
  94        data:           a private pointer (owned by the caller) that will be
  95                        passed back, when a temperature reading is needed.
  96        ops:            struct thermal_zone_of_device_ops *.
  97
  98                        get_temp:       a pointer to a function that reads the
  99                                        sensor temperature. This is mandatory
 100                                        callback provided by sensor driver.
 101                        set_trips:      a pointer to a function that sets a
 102                                        temperature window. When this window is
 103                                        left the driver must inform the thermal
 104                                        core via thermal_zone_device_update.
 105                        get_trend:      a pointer to a function that reads the
 106                                        sensor temperature trend.
 107                        set_emul_temp:  a pointer to a function that sets
 108                                        sensor emulated temperature.
 109        The thermal zone temperature is provided by the get_temp() function
 110        pointer of thermal_zone_of_device_ops. When called, it will
 111        have the private pointer @data back.
 112
 113        It returns error pointer if fails otherwise valid thermal zone device
 114        handle. Caller should check the return handle with IS_ERR() for finding
 115        whether success or not.
 116
 1171.1.4 void thermal_zone_of_sensor_unregister(struct device *dev,
 118                struct thermal_zone_device *tzd)
 119
 120        This interface unregisters a sensor from a DT thermal zone which was
 121        successfully added by interface thermal_zone_of_sensor_register().
 122        This function removes the sensor callbacks and private data from the
 123        thermal zone device registered with thermal_zone_of_sensor_register()
 124        interface. It will also silent the zone by remove the .get_temp() and
 125        get_trend() thermal zone device callbacks.
 126
 1271.1.5 struct thermal_zone_device *devm_thermal_zone_of_sensor_register(
 128                struct device *dev, int sensor_id,
 129                void *data, const struct thermal_zone_of_device_ops *ops)
 130
 131        This interface is resource managed version of
 132        thermal_zone_of_sensor_register().
 133        All details of thermal_zone_of_sensor_register() described in
 134        section 1.1.3 is applicable here.
 135        The benefit of using this interface to register sensor is that it
 136        is not require to explicitly call thermal_zone_of_sensor_unregister()
 137        in error path or during driver unbinding as this is done by driver
 138        resource manager.
 139
 1401.1.6 void devm_thermal_zone_of_sensor_unregister(struct device *dev,
 141                struct thermal_zone_device *tzd)
 142
 143        This interface is resource managed version of
 144        thermal_zone_of_sensor_unregister().
 145        All details of thermal_zone_of_sensor_unregister() described in
 146        section 1.1.4 is applicable here.
 147        Normally this function will not need to be called and the resource
 148        management code will ensure that the resource is freed.
 149
 1501.1.7 int thermal_zone_get_slope(struct thermal_zone_device *tz)
 151
 152        This interface is used to read the slope attribute value
 153        for the thermal zone device, which might be useful for platform
 154        drivers for temperature calculations.
 155
 1561.1.8 int thermal_zone_get_offset(struct thermal_zone_device *tz)
 157
 158        This interface is used to read the offset attribute value
 159        for the thermal zone device, which might be useful for platform
 160        drivers for temperature calculations.
 161
 1621.2 thermal cooling device interface
 1631.2.1 struct thermal_cooling_device *thermal_cooling_device_register(char *name,
 164                void *devdata, struct thermal_cooling_device_ops *)
 165
 166    This interface function adds a new thermal cooling device (fan/processor/...)
 167    to /sys/class/thermal/ folder as cooling_device[0-*]. It tries to bind itself
 168    to all the thermal zone devices registered at the same time.
 169    name: the cooling device name.
 170    devdata: device private data.
 171    ops: thermal cooling devices call-backs.
 172        .get_max_state: get the Maximum throttle state of the cooling device.
 173        .get_cur_state: get the Currently requested throttle state of the cooling device.
 174        .set_cur_state: set the Current throttle state of the cooling device.
 175
 1761.2.2 void thermal_cooling_device_unregister(struct thermal_cooling_device *cdev)
 177
 178    This interface function removes the thermal cooling device.
 179    It deletes the corresponding entry from /sys/class/thermal folder and
 180    unbinds itself from all the thermal zone devices using it.
 181
 1821.3 interface for binding a thermal zone device with a thermal cooling device
 1831.3.1 int thermal_zone_bind_cooling_device(struct thermal_zone_device *tz,
 184        int trip, struct thermal_cooling_device *cdev,
 185        unsigned long upper, unsigned long lower, unsigned int weight);
 186
 187    This interface function binds a thermal cooling device to a particular trip
 188    point of a thermal zone device.
 189    This function is usually called in the thermal zone device .bind callback.
 190    tz: the thermal zone device
 191    cdev: thermal cooling device
 192    trip: indicates which trip point in this thermal zone the cooling device
 193          is associated with.
 194    upper:the Maximum cooling state for this trip point.
 195          THERMAL_NO_LIMIT means no upper limit,
 196          and the cooling device can be in max_state.
 197    lower:the Minimum cooling state can be used for this trip point.
 198          THERMAL_NO_LIMIT means no lower limit,
 199          and the cooling device can be in cooling state 0.
 200    weight: the influence of this cooling device in this thermal
 201            zone.  See 1.4.1 below for more information.
 202
 2031.3.2 int thermal_zone_unbind_cooling_device(struct thermal_zone_device *tz,
 204                int trip, struct thermal_cooling_device *cdev);
 205
 206    This interface function unbinds a thermal cooling device from a particular
 207    trip point of a thermal zone device. This function is usually called in
 208    the thermal zone device .unbind callback.
 209    tz: the thermal zone device
 210    cdev: thermal cooling device
 211    trip: indicates which trip point in this thermal zone the cooling device
 212          is associated with.
 213
 2141.4 Thermal Zone Parameters
 2151.4.1 struct thermal_bind_params
 216    This structure defines the following parameters that are used to bind
 217    a zone with a cooling device for a particular trip point.
 218    .cdev: The cooling device pointer
 219    .weight: The 'influence' of a particular cooling device on this
 220             zone. This is relative to the rest of the cooling
 221             devices. For example, if all cooling devices have a
 222             weight of 1, then they all contribute the same. You can
 223             use percentages if you want, but it's not mandatory. A
 224             weight of 0 means that this cooling device doesn't
 225             contribute to the cooling of this zone unless all cooling
 226             devices have a weight of 0. If all weights are 0, then
 227             they all contribute the same.
 228    .trip_mask:This is a bit mask that gives the binding relation between
 229               this thermal zone and cdev, for a particular trip point.
 230               If nth bit is set, then the cdev and thermal zone are bound
 231               for trip point n.
 232    .binding_limits: This is an array of cooling state limits. Must have
 233                     exactly 2 * thermal_zone.number_of_trip_points. It is an
 234                     array consisting of tuples <lower-state upper-state> of
 235                     state limits. Each trip will be associated with one state
 236                     limit tuple when binding. A NULL pointer means
 237                     <THERMAL_NO_LIMITS THERMAL_NO_LIMITS> on all trips.
 238                     These limits are used when binding a cdev to a trip point.
 239    .match: This call back returns success(0) if the 'tz and cdev' need to
 240            be bound, as per platform data.
 2411.4.2 struct thermal_zone_params
 242    This structure defines the platform level parameters for a thermal zone.
 243    This data, for each thermal zone should come from the platform layer.
 244    This is an optional feature where some platforms can choose not to
 245    provide this data.
 246    .governor_name: Name of the thermal governor used for this zone
 247    .no_hwmon: a boolean to indicate if the thermal to hwmon sysfs interface
 248               is required. when no_hwmon == false, a hwmon sysfs interface
 249               will be created. when no_hwmon == true, nothing will be done.
 250               In case the thermal_zone_params is NULL, the hwmon interface
 251               will be created (for backward compatibility).
 252    .num_tbps: Number of thermal_bind_params entries for this zone
 253    .tbp: thermal_bind_params entries
 254
 2552. sysfs attributes structure
 256
 257RO      read only value
 258RW      read/write value
 259
 260Thermal sysfs attributes will be represented under /sys/class/thermal.
 261Hwmon sysfs I/F extension is also available under /sys/class/hwmon
 262if hwmon is compiled in or built as a module.
 263
 264Thermal zone device sys I/F, created once it's registered:
 265/sys/class/thermal/thermal_zone[0-*]:
 266    |---type:                   Type of the thermal zone
 267    |---temp:                   Current temperature
 268    |---mode:                   Working mode of the thermal zone
 269    |---policy:                 Thermal governor used for this zone
 270    |---available_policies:     Available thermal governors for this zone
 271    |---trip_point_[0-*]_temp:  Trip point temperature
 272    |---trip_point_[0-*]_type:  Trip point type
 273    |---trip_point_[0-*]_hyst:  Hysteresis value for this trip point
 274    |---emul_temp:              Emulated temperature set node
 275    |---sustainable_power:      Sustainable dissipatable power
 276    |---k_po:                   Proportional term during temperature overshoot
 277    |---k_pu:                   Proportional term during temperature undershoot
 278    |---k_i:                    PID's integral term in the power allocator gov
 279    |---k_d:                    PID's derivative term in the power allocator
 280    |---integral_cutoff:        Offset above which errors are accumulated
 281    |---slope:                  Slope constant applied as linear extrapolation
 282    |---offset:                 Offset constant applied as linear extrapolation
 283
 284Thermal cooling device sys I/F, created once it's registered:
 285/sys/class/thermal/cooling_device[0-*]:
 286    |---type:                   Type of the cooling device(processor/fan/...)
 287    |---max_state:              Maximum cooling state of the cooling device
 288    |---cur_state:              Current cooling state of the cooling device
 289
 290
 291Then next two dynamic attributes are created/removed in pairs. They represent
 292the relationship between a thermal zone and its associated cooling device.
 293They are created/removed for each successful execution of
 294thermal_zone_bind_cooling_device/thermal_zone_unbind_cooling_device.
 295
 296/sys/class/thermal/thermal_zone[0-*]:
 297    |---cdev[0-*]:              [0-*]th cooling device in current thermal zone
 298    |---cdev[0-*]_trip_point:   Trip point that cdev[0-*] is associated with
 299    |---cdev[0-*]_weight:       Influence of the cooling device in
 300                                this thermal zone
 301
 302Besides the thermal zone device sysfs I/F and cooling device sysfs I/F,
 303the generic thermal driver also creates a hwmon sysfs I/F for each _type_
 304of thermal zone device. E.g. the generic thermal driver registers one hwmon
 305class device and build the associated hwmon sysfs I/F for all the registered
 306ACPI thermal zones.
 307
 308/sys/class/hwmon/hwmon[0-*]:
 309    |---name:                   The type of the thermal zone devices
 310    |---temp[1-*]_input:        The current temperature of thermal zone [1-*]
 311    |---temp[1-*]_critical:     The critical trip point of thermal zone [1-*]
 312
 313Please read Documentation/hwmon/sysfs-interface for additional information.
 314
 315***************************
 316* Thermal zone attributes *
 317***************************
 318
 319type
 320        Strings which represent the thermal zone type.
 321        This is given by thermal zone driver as part of registration.
 322        E.g: "acpitz" indicates it's an ACPI thermal device.
 323        In order to keep it consistent with hwmon sys attribute; this should
 324        be a short, lowercase string, not containing spaces nor dashes.
 325        RO, Required
 326
 327temp
 328        Current temperature as reported by thermal zone (sensor).
 329        Unit: millidegree Celsius
 330        RO, Required
 331
 332mode
 333        One of the predefined values in [enabled, disabled].
 334        This file gives information about the algorithm that is currently
 335        managing the thermal zone. It can be either default kernel based
 336        algorithm or user space application.
 337        enabled         = enable Kernel Thermal management.
 338        disabled        = Preventing kernel thermal zone driver actions upon
 339                          trip points so that user application can take full
 340                          charge of the thermal management.
 341        RW, Optional
 342
 343policy
 344        One of the various thermal governors used for a particular zone.
 345        RW, Required
 346
 347available_policies
 348        Available thermal governors which can be used for a particular zone.
 349        RO, Required
 350
 351trip_point_[0-*]_temp
 352        The temperature above which trip point will be fired.
 353        Unit: millidegree Celsius
 354        RO, Optional
 355
 356trip_point_[0-*]_type
 357        Strings which indicate the type of the trip point.
 358        E.g. it can be one of critical, hot, passive, active[0-*] for ACPI
 359        thermal zone.
 360        RO, Optional
 361
 362trip_point_[0-*]_hyst
 363        The hysteresis value for a trip point, represented as an integer
 364        Unit: Celsius
 365        RW, Optional
 366
 367cdev[0-*]
 368        Sysfs link to the thermal cooling device node where the sys I/F
 369        for cooling device throttling control represents.
 370        RO, Optional
 371
 372cdev[0-*]_trip_point
 373        The trip point in this thermal zone which cdev[0-*] is associated
 374        with; -1 means the cooling device is not associated with any trip
 375        point.
 376        RO, Optional
 377
 378cdev[0-*]_weight
 379        The influence of cdev[0-*] in this thermal zone. This value
 380        is relative to the rest of cooling devices in the thermal
 381        zone. For example, if a cooling device has a weight double
 382        than that of other, it's twice as effective in cooling the
 383        thermal zone.
 384        RW, Optional
 385
 386passive
 387        Attribute is only present for zones in which the passive cooling
 388        policy is not supported by native thermal driver. Default is zero
 389        and can be set to a temperature (in millidegrees) to enable a
 390        passive trip point for the zone. Activation is done by polling with
 391        an interval of 1 second.
 392        Unit: millidegrees Celsius
 393        Valid values: 0 (disabled) or greater than 1000
 394        RW, Optional
 395
 396emul_temp
 397        Interface to set the emulated temperature method in thermal zone
 398        (sensor). After setting this temperature, the thermal zone may pass
 399        this temperature to platform emulation function if registered or
 400        cache it locally. This is useful in debugging different temperature
 401        threshold and its associated cooling action. This is write only node
 402        and writing 0 on this node should disable emulation.
 403        Unit: millidegree Celsius
 404        WO, Optional
 405
 406          WARNING: Be careful while enabling this option on production systems,
 407          because userland can easily disable the thermal policy by simply
 408          flooding this sysfs node with low temperature values.
 409
 410sustainable_power
 411        An estimate of the sustained power that can be dissipated by
 412        the thermal zone. Used by the power allocator governor. For
 413        more information see Documentation/thermal/power_allocator.txt
 414        Unit: milliwatts
 415        RW, Optional
 416
 417k_po
 418        The proportional term of the power allocator governor's PID
 419        controller during temperature overshoot. Temperature overshoot
 420        is when the current temperature is above the "desired
 421        temperature" trip point. For more information see
 422        Documentation/thermal/power_allocator.txt
 423        RW, Optional
 424
 425k_pu
 426        The proportional term of the power allocator governor's PID
 427        controller during temperature undershoot. Temperature undershoot
 428        is when the current temperature is below the "desired
 429        temperature" trip point. For more information see
 430        Documentation/thermal/power_allocator.txt
 431        RW, Optional
 432
 433k_i
 434        The integral term of the power allocator governor's PID
 435        controller. This term allows the PID controller to compensate
 436        for long term drift. For more information see
 437        Documentation/thermal/power_allocator.txt
 438        RW, Optional
 439
 440k_d
 441        The derivative term of the power allocator governor's PID
 442        controller. For more information see
 443        Documentation/thermal/power_allocator.txt
 444        RW, Optional
 445
 446integral_cutoff
 447        Temperature offset from the desired temperature trip point
 448        above which the integral term of the power allocator
 449        governor's PID controller starts accumulating errors. For
 450        example, if integral_cutoff is 0, then the integral term only
 451        accumulates error when temperature is above the desired
 452        temperature trip point. For more information see
 453        Documentation/thermal/power_allocator.txt
 454        Unit: millidegree Celsius
 455        RW, Optional
 456
 457slope
 458        The slope constant used in a linear extrapolation model
 459        to determine a hotspot temperature based off the sensor's
 460        raw readings. It is up to the device driver to determine
 461        the usage of these values.
 462        RW, Optional
 463
 464offset
 465        The offset constant used in a linear extrapolation model
 466        to determine a hotspot temperature based off the sensor's
 467        raw readings. It is up to the device driver to determine
 468        the usage of these values.
 469        RW, Optional
 470
 471*****************************
 472* Cooling device attributes *
 473*****************************
 474
 475type
 476        String which represents the type of device, e.g:
 477        - for generic ACPI: should be "Fan", "Processor" or "LCD"
 478        - for memory controller device on intel_menlow platform:
 479          should be "Memory controller".
 480        RO, Required
 481
 482max_state
 483        The maximum permissible cooling state of this cooling device.
 484        RO, Required
 485
 486cur_state
 487        The current cooling state of this cooling device.
 488        The value can any integer numbers between 0 and max_state:
 489        - cur_state == 0 means no cooling
 490        - cur_state == max_state means the maximum cooling.
 491        RW, Required
 492
 4933. A simple implementation
 494
 495ACPI thermal zone may support multiple trip points like critical, hot,
 496passive, active. If an ACPI thermal zone supports critical, passive,
 497active[0] and active[1] at the same time, it may register itself as a
 498thermal_zone_device (thermal_zone1) with 4 trip points in all.
 499It has one processor and one fan, which are both registered as
 500thermal_cooling_device. Both are considered to have the same
 501effectiveness in cooling the thermal zone.
 502
 503If the processor is listed in _PSL method, and the fan is listed in _AL0
 504method, the sys I/F structure will be built like this:
 505
 506/sys/class/thermal:
 507
 508|thermal_zone1:
 509    |---type:                   acpitz
 510    |---temp:                   37000
 511    |---mode:                   enabled
 512    |---policy:                 step_wise
 513    |---available_policies:     step_wise fair_share
 514    |---trip_point_0_temp:      100000
 515    |---trip_point_0_type:      critical
 516    |---trip_point_1_temp:      80000
 517    |---trip_point_1_type:      passive
 518    |---trip_point_2_temp:      70000
 519    |---trip_point_2_type:      active0
 520    |---trip_point_3_temp:      60000
 521    |---trip_point_3_type:      active1
 522    |---cdev0:                  --->/sys/class/thermal/cooling_device0
 523    |---cdev0_trip_point:       1       /* cdev0 can be used for passive */
 524    |---cdev0_weight:           1024
 525    |---cdev1:                  --->/sys/class/thermal/cooling_device3
 526    |---cdev1_trip_point:       2       /* cdev1 can be used for active[0]*/
 527    |---cdev1_weight:           1024
 528
 529|cooling_device0:
 530    |---type:                   Processor
 531    |---max_state:              8
 532    |---cur_state:              0
 533
 534|cooling_device3:
 535    |---type:                   Fan
 536    |---max_state:              2
 537    |---cur_state:              0
 538
 539/sys/class/hwmon:
 540
 541|hwmon0:
 542    |---name:                   acpitz
 543    |---temp1_input:            37000
 544    |---temp1_crit:             100000
 545
 5464. Event Notification
 547
 548The framework includes a simple notification mechanism, in the form of a
 549netlink event. Netlink socket initialization is done during the _init_
 550of the framework. Drivers which intend to use the notification mechanism
 551just need to call thermal_generate_netlink_event() with two arguments viz
 552(originator, event). The originator is a pointer to struct thermal_zone_device
 553from where the event has been originated. An integer which represents the
 554thermal zone device will be used in the message to identify the zone. The
 555event will be one of:{THERMAL_AUX0, THERMAL_AUX1, THERMAL_CRITICAL,
 556THERMAL_DEV_FAULT}. Notification can be sent when the current temperature
 557crosses any of the configured thresholds.
 558
 5595. Export Symbol APIs:
 560
 5615.1: get_tz_trend:
 562This function returns the trend of a thermal zone, i.e the rate of change
 563of temperature of the thermal zone. Ideally, the thermal sensor drivers
 564are supposed to implement the callback. If they don't, the thermal
 565framework calculated the trend by comparing the previous and the current
 566temperature values.
 567
 5685.2:get_thermal_instance:
 569This function returns the thermal_instance corresponding to a given
 570{thermal_zone, cooling_device, trip_point} combination. Returns NULL
 571if such an instance does not exist.
 572
 5735.3:thermal_notify_framework:
 574This function handles the trip events from sensor drivers. It starts
 575throttling the cooling devices according to the policy configured.
 576For CRITICAL and HOT trip points, this notifies the respective drivers,
 577and does actual throttling for other trip points i.e ACTIVE and PASSIVE.
 578The throttling policy is based on the configured platform data; if no
 579platform data is provided, this uses the step_wise throttling policy.
 580
 5815.4:thermal_cdev_update:
 582This function serves as an arbitrator to set the state of a cooling
 583device. It sets the cooling device to the deepest cooling state if
 584possible.
 585
 5866. thermal_emergency_poweroff:
 587
 588On an event of critical trip temperature crossing. Thermal framework
 589allows the system to shutdown gracefully by calling orderly_poweroff().
 590In the event of a failure of orderly_poweroff() to shut down the system
 591we are in danger of keeping the system alive at undesirably high
 592temperatures. To mitigate this high risk scenario we program a work
 593queue to fire after a pre-determined number of seconds to start
 594an emergency shutdown of the device using the kernel_power_off()
 595function. In case kernel_power_off() fails then finally
 596emergency_restart() is called in the worst case.
 597
 598The delay should be carefully profiled so as to give adequate time for
 599orderly_poweroff(). In case of failure of an orderly_poweroff() the
 600emergency poweroff kicks in after the delay has elapsed and shuts down
 601the system.
 602
 603If set to 0 emergency poweroff will not be supported. So a carefully
 604profiled non-zero positive value is a must for emergerncy poweroff to be
 605triggered.
 606
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