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+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.
+ .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.
+ .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 from /sys/class/thermal folder and
+ unbinds all the thermal cooling devices it uses.
+
+1.1.3 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.
+
+1.1.4 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.
+
+1.1.5 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.
+
+1.1.6 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.
+
+1.1.7 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.
+
+1.1.8 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
+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 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.
+
+1.2.2 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.
+
+1.3 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, unsigned int weight);
+
+ This interface function binds a thermal cooling device to a particular 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 in this thermal zone the cooling device
+ is associated with.
+ 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.
+ weight: the influence of this cooling device in this thermal
+ zone. See 1.4.1 below for more information.
+
+1.3.2 int thermal_zone_unbind_cooling_device(struct thermal_zone_device *tz,
+ int trip, struct thermal_cooling_device *cdev);
+
+ This interface function unbinds a thermal cooling device from a particular
+ 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 in this thermal zone the cooling device
+ is associated with.
+
+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 relative to the rest of the cooling
+ devices. For example, if all cooling devices have a
+ weight of 1, then they all contribute the same. You can
+ use percentages if you want, but it's not mandatory. A
+ weight of 0 means that this cooling device doesn't
+ contribute to the cooling of this zone unless all cooling
+ devices have a weight of 0. If all weights are 0, then
+ they all contribute the same.
+ .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.
+ .binding_limits: This is an array of cooling state limits. Must have
+ exactly 2 * thermal_zone.number_of_trip_points. It is an
+ array consisting of tuples <lower-state upper-state> of
+ state limits. Each trip will be associated with one state
+ limit tuple when binding. A NULL pointer means
+ <THERMAL_NO_LIMITS THERMAL_NO_LIMITS> on all trips.
+ These limits are used when binding a cdev to a trip point.
+ .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
+ .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).
+ .num_tbps: Number of thermal_bind_params entries for this zone
+ .tbp: thermal_bind_params entries
+
+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: Cooing 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.
+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
+ |---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.
+
+/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
+
+available_policies
+ Available thermal governors which can be used for a particular zone.
+ RO, 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 in this thermal zone which cdev[0-*] is associated
+ with; -1 means the cooling device is not associated with any trip
+ point.
+ RO, Optional
+
+cdev[0-*]_weight
+ The influence of cdev[0-*] in this thermal zone. This value
+ is relative to the rest of cooling devices in the thermal
+ zone. For example, if a cooling device has a weight double
+ than that of other, it's twice as effective in cooling the
+ thermal zone.
+ RW, 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.
+
+sustainable_power
+ An estimate of the sustained power that can be dissipated by
+ the thermal zone. Used by the power allocator governor. For
+ more information see Documentation/thermal/power_allocator.txt
+ Unit: milliwatts
+ RW, Optional
+
+k_po
+ The proportional term of the power allocator governor's PID
+ controller during temperature overshoot. Temperature overshoot
+ is when the current temperature is above the "desired
+ temperature" trip point. For more information see
+ Documentation/thermal/power_allocator.txt
+ RW, Optional
+
+k_pu
+ The proportional term of the power allocator governor's PID
+ controller during temperature undershoot. Temperature undershoot
+ is when the current temperature is below the "desired
+ temperature" trip point. For more information see
+ Documentation/thermal/power_allocator.txt
+ RW, Optional
+
+k_i
+ The integral term of the power allocator governor's PID
+ controller. This term allows the PID controller to compensate
+ for long term drift. For more information see
+ Documentation/thermal/power_allocator.txt
+ RW, Optional
+
+k_d
+ The derivative term of the power allocator governor's PID
+ controller. For more information see
+ Documentation/thermal/power_allocator.txt
+ RW, Optional
+
+integral_cutoff
+ Temperature offset from the desired temperature trip point
+ above which the integral term of the power allocator
+ governor's PID controller starts accumulating errors. For
+ example, if integral_cutoff is 0, then the integral term only
+ accumulates error when temperature is above the desired
+ temperature trip point. For more information see
+ Documentation/thermal/power_allocator.txt
+ Unit: millidegree Celsius
+ RW, Optional
+
+slope
+ The slope constant used in a linear extrapolation model
+ to determine a hotspot temperature based off the sensor's
+ raw readings. It is up to the device driver to determine
+ the usage of these values.
+ RW, Optional
+
+offset
+ The offset constant used in a linear extrapolation model
+ to determine a hotspot temperature based off the sensor's
+ raw readings. It is up to the device driver to determine
+ the usage of these values.
+ 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
+
+stats/reset
+ Writing any value resets the cooling device's statistics.
+ WO, Required
+
+stats/time_in_state_ms:
+ The amount of time spent by the cooling device in various cooling
+ states. The output will have "<state> <time>" pair in each line, which
+ will mean this cooling device spent <time> msec of time at <state>.
+ Output will have one line for each of the supported states. usertime
+ units here is 10mS (similar to other time exported in /proc).
+ RO, Required
+
+stats/total_trans:
+ A single positive value showing the total number of times the state of a
+ cooling device is changed.
+ RO, Required
+
+stats/trans_table:
+ This gives fine grained information about all the cooling state
+ transitions. The cat output here is a two dimensional matrix, where an
+ entry <i,j> (row i, column j) represents the number of transitions from
+ State_i to State_j. If the transition table is bigger than PAGE_SIZE,
+ reading this will return an -EFBIG error.
+ RO, 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. 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. 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.
+
+6. thermal_emergency_poweroff:
+
+On an event of critical trip temperature crossing. Thermal framework
+allows the system to shutdown gracefully by calling orderly_poweroff().
+In the event of a failure of orderly_poweroff() to shut down the system
+we are in danger of keeping the system alive at undesirably high
+temperatures. To mitigate this high risk scenario we program a work
+queue to fire after a pre-determined number of seconds to start
+an emergency shutdown of the device using the kernel_power_off()
+function. In case kernel_power_off() fails then finally
+emergency_restart() is called in the worst case.
+
+The delay should be carefully profiled so as to give adequate time for
+orderly_poweroff(). In case of failure of an orderly_poweroff() the
+emergency poweroff kicks in after the delay has elapsed and shuts down
+the system.
+
+If set to 0 emergency poweroff will not be supported. So a carefully
+profiled non-zero positive value is a must for emergerncy poweroff to be
+triggered.