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Subject[PATCHv5 08/10] Thermal: Add Documentation to new APIs
This patch adds Documentation for the new APIs
introduced in this patch set. The documentation
also has a model sysfs structure for reference.

Signed-off-by: Durgadoss R <>
Documentation/thermal/sysfs-api2.txt | 240 ++++++++++++++++++++++++++++++++++
1 file changed, 240 insertions(+)
create mode 100644 Documentation/thermal/sysfs-api2.txt

diff --git a/Documentation/thermal/sysfs-api2.txt b/Documentation/thermal/sysfs-api2.txt
new file mode 100644
index 0000000..dd03297
--- /dev/null
+++ b/Documentation/thermal/sysfs-api2.txt
@@ -0,0 +1,240 @@
+Thermal Framework
+Written by Durgadoss R <>
+Copyright (c) 2014 Intel Corporation
+Created on: 15 January 2014
+0. Introduction
+The Linux thermal framework provides a set of interfaces for thermal
+sensors and thermal cooling devices (fan, processor...) to register
+with the thermal management solution and to be a part of it.
+This document focuses on how to enable new thermal sensors and cooling
+devices to participate in thermal management. This solution is intended
+to be 'light-weight' and platform/architecture independent. Any thermal
+sensor/cooling device should be able to use the infrastructure easily.
+The goal of thermal framework is to expose the thermal sensor/zone and
+cooling device attributes in a consistent way. This will help the
+thermal governors to make use of the information to manage platform
+thermals efficiently.
+The thermal sensor source file can be generic (can be any sensor driver,
+in any subsystem). This driver will use the sensor APIs and register with
+thermal framework to participate in platform Thermal management. This
+does not (and should not) know about which zone it belongs to, or any
+other information about platform thermals. A sensor driver is a standalone
+piece of code, which can optionally register with thermal framework.
+However, for any platform, there should be a platformX_thermal.c file,
+which will know about the platform thermal characteristics (e.g how many
+sensors, zones, cooling devices, etc.. And how they are related to each other
+i.e the mapping information). Only in this file, the zone level APIs should
+be used, in which case the file will have all information required to attach
+various sensors to a particular zone.
+This way, we can have one platform level thermal file, which can support
+multiple platforms (may be)using the same set of sensors (but)binded in
+a different way. This file can get the platform thermal information
+through Firmware, ACPI tables, device tree etc.
+Unfortunately, today we don't have many drivers that can be clearly
+differentiated as 'sensor_file.c' and 'platform_thermal_file.c'.
+But very soon we will need/have. We see a lot of chip drivers,
+starting to use thermal framework; we should keep it really
+light-weight for them to do so but at the same time provide all
+the necessary features to participate in platform thermal management.
+An Example: drivers/hwmon/emc1403.c - a generic thermal chip driver
+In one platform this sensor can belong to 'ZoneA' and in another the
+same can belong to 'ZoneB'. But, emc1403.c does not really care about
+where does it belong. It just reports temperature.
+1. Terminology
+This section describes the terminology used in the rest of this
+document as well as the thermal framework code.
+thermal_sensor: Hardware that can report temperature of a particular
+ spot in the platform, where it is placed. The temperature
+ reported by the sensor is the 'real' temperature reported
+ by the hardware.
+thermal_zone: A virtual area on the device, that gets heated up. It may
+ have one or more thermal sensors attached to it.
+cooling_device: Any component that can help in reducing the temperature of
+ a 'hot spot' either by reducing its performance (passive
+ cooling) or by other means(Active cooling E.g. Fan)
+trip_points: Various temperature levels for each sensor. As of now, we
+ have four levels namely active, passive, hot and critical.
+ Hot and critical trip point support only one value whereas
+ active and passive can have any number of values. These
+ temperature values can come from platform data, and are
+ exposed through sysfs in a consistent manner. Stand-alone
+ thermal sensor drivers are not expected to know these values.
+ These values are RO.
+thresholds: These are programmable temperature limits, on reaching which
+ the thermal sensor generates an interrupt. The framework is
+ notified about this interrupt to take appropriate action.
+ There can be as many number of thresholds as that of the
+ hardware supports. These values are RW.
+thermal_map: This provides the mapping (aka binding) information between
+ various sensors and cooling devices in a particular zone.
+ Typically, this also comes from platform data; Stand-alone
+ sensor drivers or cooling device drivers are not expected
+ to know these mapping information.
+2. Thermal framework APIs
+2.1: For Thermal Sensors
+2.1.1 thermal_sensor_register:
+ This function creates a new sensor directory under /sys/class/thermal/
+ as sensor[0-*]. This API is expected to be called by thermal sensor
+ drivers. These drivers may or may not be in thermal subsystem. This
+ function returns a thermal_sensor structure on success and appropriate
+ error on failure.
+ name: Name of the sensor
+ count: Number of programmable thresholds associated with this sensor
+ devdata: Device private data
+ ops: Thermal sensor callbacks
+ .get_temp: obtain the current temperature of the sensor
+ .get_trend: obtain the trend of the sensor
+ .get_threshold: get a particular threshold temperature
+ .set_threshold: set a particular threshold temperature
+ .get_hyst: get hysteresis value associated with a threshold
+ .set_hyst: set hysteresis value associated with a threshold
+2.1.2 thermal_sensor_unregister:
+ This function deletes the sensor directory under /sys/class/thermal/
+ for the given sensor. Thermal sensor drivers may call this API
+ during the driver's 'exit' routine.
+ ts: Thermal sensor that has to be unregistered
+2.2: For Cooling Devices
+2.2.1 thermal_cdev_register:
+ This function adds a new thermal cooling device (fan/processor/...)
+ to /sys/class/thermal/ folder as cooling_device[0-*]. This function
+ is expected to be called by cooling device drivers that may be
+ present in other subsystems also.
+ name: the cooling device name
+ devdata: device private data
+ ops: thermal cooling devices callbacks
+ .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
+2.2.2 thermal_cdev_unregister:
+ This function deletes the given cdev entry form /sys/class/thermal;
+ and also cleans all the symlinks referred from various zones.
+ cdev: Cooling device to be unregistered
+2.3: For Thermal Zones
+2.3.1 thermal_create_thermal_zone:
+ This function adds a new 'zone' under /sys/class/thermal/
+ directory as zone[0-*]. This zone has at least one thermal
+ sensor and at most MAX_SENSORS_PER_ZONE number of sensors
+ attached to it. Similarly, this zone has at least one cdev
+ and at most MAX_CDEVS_PER_ZONE number of cdevs attached to it.
+ As of now, MAX_*_PER_ZONE values are hard-coded to 5. We can
+ make them configurable, through Kconfig option(during 'menuconfig').
+ name: Name of the thermal zone
+ devdata: Device private data
+2.3.2 thermal_add_sensor_to_zone
+ This function adds a 'sensorX' entry under /sys/class/thermal/
+ zoneY/ directory. This 'sensorX' is a symlink to the actual
+ sensor entry under /sys/class/thermal/. Correspondingly, the
+ method remove_sensor_from_zone deletes the symlink.
+ tz: thermal zone structure
+ ts: thermal sensor structure
+2.3.3 thermal_add_cdev_to_zone
+ This function adds a 'cdevX' entry under /sys/class/thermal/
+ zoneY/ directory. This 'cdevX' is a symlink to the actual
+ cdev entry under /sys/class/thermal/. Correspondingly, the
+ method remove_cdev_from_zone deletes the symlink.
+ tz: thermal zone structure
+ cdev: thermal cooling device structure
+2.4 For Thermal Trip
+2.4.1 thermal_add_sensor_trip_info
+ This function adds trip point information for the given sensor,
+ (under a given zone) under /sys/class/thermal/zoneX/.
+ This API creates sysfs attributes namely:
+ sensorX_trip_activeY, sensorX_trip_passiveY, sensorX_trip_hot,
+ sensorX_trip_critical. Each of these hold one trip point temperature
+ (in mC) values, as provided from platform data. As of now, we
+ support many Active and Passive trip points but only one hot
+ one critical trip point.
+ tz: thermal zone structure
+ ts: thermal sensor to which the trip points are attached
+ trip: trip point structure. Usually obtained from platform data
+2.5 For Thermal Map
+2.5.1 thermal_add_map_entry
+ This function adds a 'map[0-*]' sysfs attribute under
+ /sys/class/thermal/zoneX/mapY_*. Each map attribute helps
+ to describe the binding relationship between a sensor and
+ a cdev in the given zone. The map structure is typically
+ obtained as platform data. For example, through ACPI tables,
+ SFI tables, Device tree etc. The trip mask is a hex value;
+ if 'n' th bit (from LSB) is set, then for trip point 'n' this
+ cdev is throttled with the given weight[n].
+ tz: thermal zone to which a 'map' is being added
+ map: thermal_map structure
+3. Sysfs attributes structure
+Thermal sysfs attributes will be represented under /sys/class/thermal.
+3.1: For Thermal Sensors
+ /sys/class/thermal/sensor[0-*]:
+ |---type: Name of the thermal sensor
+ |---temp_input: Current temperature in mC
+ |---threshold[0-*]: Threshold temperature in mC
+ |---threshold[0-*]_hyst:Optional hysteresis value in mC
+3.2: For Thermal Cooling Devices
+ /sys/class/thermal/cooling_device[0-*]:
+ |---type: Type of the cooling device
+ |---max_state: Maximum throttle state of the cdev
+ |---cur_state: Current throttle state of the cdev
+3.3: For Thermal Zones
+ /sys/class/thermal/zone[0-*]:
+ |---zone_name: Name of the thermal zone
+ |---sensorX: Symlink to ../sensorX
+ |---cdevY: Symlink to ../cdevY
+ |---sensorX_trip_activeM: Active trip point for sensorX
+ |---sensorX_trip_passiveN: Passive trip point for sensorX
+ |---sensorX_trip_hot: Hot trip point for sensorX
+ |---sensorX_trip_critical: Critical trip point for sensorX
+ |---mapX_sensor_name: Sensor Name
+ |---mapX_cdev_name: Cooling device Name
+ |---mapX_trip_type: Trip point type
+ |---mapX_trip_mask: Trip point mask
+ |---mapX_weightY: Weights (used by governors)
+3.5: For Thermal Map
+ Each attribute represents the mapping/binding information between
+ a sensor and a cdev, together with a trip type.
+ /sys/class/thermal/zoneX/:
+ |---mapY_trip_type: active/passive
+ |---mapY_sensor_name: cpu
+ |---mapY_cdev_name: proc
+ |---mapY_trip_mask: 0x03
+ |---mapY_weight0: 50
+ |---mapY_weight1: 30

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