Documentation/power/power_supply_class.rst - linux - Git at Google

 ========================
 Linux power supply class
 ========================

 Synopsis
 ~~~~~~~~
 Power supply class used to represent battery, UPS, AC or DC power supply
 properties to user-space.

 It defines core set of attributes, which should be applicable to (almost)
 every power supply out there. Attributes are available via sysfs and uevent
 interfaces.

 Each attribute has well defined meaning, up to unit of measure used. While
 the attributes provided are believed to be universally applicable to any
 power supply, specific monitoring hardware may not be able to provide them
 all, so any of them may be skipped.

 Power supply class is extensible, and allows to define drivers own attributes.
 The core attribute set is subject to the standard Linux evolution (i.e.
 if it will be found that some attribute is applicable to many power supply
 types or their drivers, it can be added to the core set).

 It also integrates with LED framework, for the purpose of providing
 typically expected feedback of battery charging/fully charged status and
 AC/USB power supply online status. (Note that specific details of the
 indication (including whether to use it at all) are fully controllable by
 user and/or specific machine defaults, per design principles of LED
 framework).


 Attributes/properties
 ~~~~~~~~~~~~~~~~~~~~~
 Power supply class has predefined set of attributes, this eliminates code
 duplication across drivers. Power supply class insist on reusing its
 predefined attributes *and* their units.

 So, userspace gets predictable set of attributes and their units for any
 kind of power supply, and can process/present them to a user in consistent
 manner. Results for different power supplies and machines are also directly
 comparable.

 See drivers/power/supply/ds2760_battery.c and drivers/power/supply/pda_power.c
 for the example how to declare and handle attributes.


 Units
 ~~~~~
 Quoting include/linux/power_supply.h:

   All voltages, currents, charges, energies, time and temperatures in µV,
   µA, µAh, µWh, seconds and tenths of degree Celsius unless otherwise
   stated. It's driver's job to convert its raw values to units in which
   this class operates.


 Attributes/properties detailed
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 +--------------------------------------------------------------------------+
 |               **Charge/Energy/Capacity - how to not confuse**            |
 +--------------------------------------------------------------------------+
 | **Because both "charge" (µAh) and "energy" (µWh) represents "capacity"   |
 | of battery, this class distinguish these terms. Don't mix them!**        |
 |                                                                          |
 | - `CHARGE_*`                                                             |
 |	attributes represents capacity in µAh only.                        |
 | - `ENERGY_*`                                                             |
 |	attributes represents capacity in µWh only.                        |
 | - `CAPACITY`                                                             |
 |	attribute represents capacity in *percents*, from 0 to 100.        |
 +--------------------------------------------------------------------------+

 Postfixes:

 _AVG
   *hardware* averaged value, use it if your hardware is really able to
   report averaged values.
 _NOW
   momentary/instantaneous values.

 STATUS
   this attribute represents operating status (charging, full,
   discharging (i.e. powering a load), etc.). This corresponds to
   `BATTERY_STATUS_*` values, as defined in battery.h.

 CHARGE_TYPE
   batteries can typically charge at different rates.
   This defines trickle and fast charges.  For batteries that
   are already charged or discharging, 'n/a' can be displayed (or
   'unknown', if the status is not known).

 AUTHENTIC
   indicates the power supply (battery or charger) connected
   to the platform is authentic(1) or non authentic(0).

 HEALTH
   represents health of the battery, values corresponds to
   POWER_SUPPLY_HEALTH_*, defined in battery.h.

 VOLTAGE_OCV
   open circuit voltage of the battery.

 VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN
   design values for maximal and minimal power supply voltages.
   Maximal/minimal means values of voltages when battery considered
   "full"/"empty" at normal conditions. Yes, there is no direct relation
   between voltage and battery capacity, but some dumb
   batteries use voltage for very approximated calculation of capacity.
   Battery driver also can use this attribute just to inform userspace
   about maximal and minimal voltage thresholds of a given battery.

 VOLTAGE_MAX, VOLTAGE_MIN
   same as _DESIGN voltage values except that these ones should be used
   if hardware could only guess (measure and retain) the thresholds of a
   given power supply.

 VOLTAGE_BOOT
   Reports the voltage measured during boot

 CURRENT_BOOT
   Reports the current measured during boot

 CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN
   design charge values, when battery considered full/empty.

 ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN
   same as above but for energy.

 CHARGE_FULL, CHARGE_EMPTY
   These attributes means "last remembered value of charge when battery
   became full/empty". It also could mean "value of charge when battery
   considered full/empty at given conditions (temperature, age)".
   I.e. these attributes represents real thresholds, not design values.

 ENERGY_FULL, ENERGY_EMPTY
   same as above but for energy.

 CHARGE_COUNTER
   the current charge counter (in µAh).  This could easily
   be negative; there is no empty or full value.  It is only useful for
   relative, time-based measurements.

 PRECHARGE_CURRENT
   the maximum charge current during precharge phase of charge cycle
   (typically 20% of battery capacity).

 CHARGE_TERM_CURRENT
   Charge termination current. The charge cycle terminates when battery
   voltage is above recharge threshold, and charge current is below
   this setting (typically 10% of battery capacity).

 CONSTANT_CHARGE_CURRENT
   constant charge current programmed by charger.


 CONSTANT_CHARGE_CURRENT_MAX
   maximum charge current supported by the power supply object.

 CONSTANT_CHARGE_VOLTAGE
   constant charge voltage programmed by charger.
 CONSTANT_CHARGE_VOLTAGE_MAX
   maximum charge voltage supported by the power supply object.

 INPUT_CURRENT_LIMIT
   input current limit programmed by charger. Indicates
   the current drawn from a charging source.
 INPUT_VOLTAGE_LIMIT
   input voltage limit programmed by charger. Indicates
   the voltage limit from a charging source.
 INPUT_POWER_LIMIT
   input power limit programmed by charger. Indicates
   the power limit from a charging source.

 CHARGE_CONTROL_LIMIT
   current charge control limit setting
 CHARGE_CONTROL_LIMIT_MAX
   maximum charge control limit setting

 CALIBRATE
   battery or coulomb counter calibration status

 CAPACITY
   capacity in percents.
 CAPACITY_ALERT_MIN
   minimum capacity alert value in percents.
 CAPACITY_ALERT_MAX
   maximum capacity alert value in percents.
 CAPACITY_LEVEL
   capacity level. This corresponds to POWER_SUPPLY_CAPACITY_LEVEL_*.

 TEMP
   temperature of the power supply.
 TEMP_ALERT_MIN
   minimum battery temperature alert.
 TEMP_ALERT_MAX
   maximum battery temperature alert.
 TEMP_AMBIENT
   ambient temperature.
 TEMP_AMBIENT_ALERT_MIN
   minimum ambient temperature alert.
 TEMP_AMBIENT_ALERT_MAX
   maximum ambient temperature alert.
 TEMP_MIN
   minimum operatable temperature
 TEMP_MAX
   maximum operatable temperature

 TIME_TO_EMPTY
   seconds left for battery to be considered empty
   (i.e. while battery powers a load)
 TIME_TO_FULL
   seconds left for battery to be considered full
   (i.e. while battery is charging)


 Battery <-> external power supply interaction
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Often power supplies are acting as supplies and supplicants at the same
 time. Batteries are good example. So, batteries usually care if they're
 externally powered or not.

 For that case, power supply class implements notification mechanism for
 batteries.

 External power supply (AC) lists supplicants (batteries) names in
 "supplied_to" struct member, and each power_supply_changed() call
 issued by external power supply will notify supplicants via
 external_power_changed callback.


 Devicetree battery characteristics
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Drivers should call power_supply_get_battery_info() to obtain battery
 characteristics from a devicetree battery node, defined in
 Documentation/devicetree/bindings/power/supply/battery.txt. This is
 implemented in drivers/power/supply/bq27xxx_battery.c.

 Properties in struct power_supply_battery_info and their counterparts in the
 battery node have names corresponding to elements in enum power_supply_property,
 for naming consistency between sysfs attributes and battery node properties.


 QA
 ~~

 Q:
    Where is POWER_SUPPLY_PROP_XYZ attribute?
 A:
    If you cannot find attribute suitable for your driver needs, feel free
    to add it and send patch along with your driver.

    The attributes available currently are the ones currently provided by the
    drivers written.

    Good candidates to add in future: model/part#, cycle_time, manufacturer,
    etc.


 Q:
    I have some very specific attribute (e.g. battery color), should I add
    this attribute to standard ones?
 A:
    Most likely, no. Such attribute can be placed in the driver itself, if
    it is useful. Of course, if the attribute in question applicable to
    large set of batteries, provided by many drivers, and/or comes from
    some general battery specification/standard, it may be a candidate to
    be added to the core attribute set.


 Q:
    Suppose, my battery monitoring chip/firmware does not provides capacity
    in percents, but provides charge_{now,full,empty}. Should I calculate
    percentage capacity manually, inside the driver, and register CAPACITY
    attribute? The same question about time_to_empty/time_to_full.
 A:
    Most likely, no. This class is designed to export properties which are
    directly measurable by the specific hardware available.

    Inferring not available properties using some heuristics or mathematical
    model is not subject of work for a battery driver. Such functionality
    should be factored out, and in fact, apm_power, the driver to serve
    legacy APM API on top of power supply class, uses a simple heuristic of
    approximating remaining battery capacity based on its charge, current,
    voltage and so on. But full-fledged battery model is likely not subject
    for kernel at all, as it would require floating point calculation to deal
    with things like differential equations and Kalman filters. This is
    better be handled by batteryd/libbattery, yet to be written.
	========================
	Linux power supply class
	========================

	Synopsis
	~~~~~~~~
	Power supply class used to represent battery, UPS, AC or DC power supply
	properties to user-space.

	It defines core set of attributes, which should be applicable to (almost)
	every power supply out there. Attributes are available via sysfs and uevent
	interfaces.

	Each attribute has well defined meaning, up to unit of measure used. While
	the attributes provided are believed to be universally applicable to any
	power supply, specific monitoring hardware may not be able to provide them
	all, so any of them may be skipped.

	Power supply class is extensible, and allows to define drivers own attributes.
	The core attribute set is subject to the standard Linux evolution (i.e.
	if it will be found that some attribute is applicable to many power supply
	types or their drivers, it can be added to the core set).

	It also integrates with LED framework, for the purpose of providing
	typically expected feedback of battery charging/fully charged status and
	AC/USB power supply online status. (Note that specific details of the
	indication (including whether to use it at all) are fully controllable by
	user and/or specific machine defaults, per design principles of LED
	framework).


	Attributes/properties
	~~~~~~~~~~~~~~~~~~~~~
	Power supply class has predefined set of attributes, this eliminates code
	duplication across drivers. Power supply class insist on reusing its
	predefined attributes and their units.

	So, userspace gets predictable set of attributes and their units for any
	kind of power supply, and can process/present them to a user in consistent
	manner. Results for different power supplies and machines are also directly
	comparable.

	See drivers/power/supply/ds2760_battery.c and drivers/power/supply/pda_power.c
	for the example how to declare and handle attributes.


	Units
	~~~~~
	Quoting include/linux/power_supply.h:

	All voltages, currents, charges, energies, time and temperatures in µV,
	µA, µAh, µWh, seconds and tenths of degree Celsius unless otherwise
	stated. It's driver's job to convert its raw values to units in which
	this class operates.


	Attributes/properties detailed
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

	+--------------------------------------------------------------------------+
	\| Charge/Energy/Capacity - how to not confuse \|
	+--------------------------------------------------------------------------+
	\| **Because both "charge" (µAh) and "energy" (µWh) represents "capacity" \|
	\| of battery, this class distinguish these terms. Don't mix them!** \|
	\| \|
	\| - `CHARGE_*` \|
	\| attributes represents capacity in µAh only. \|
	\| - `ENERGY_*` \|
	\| attributes represents capacity in µWh only. \|
	\| - `CAPACITY` \|
	\| attribute represents capacity in percents, from 0 to 100. \|
	+--------------------------------------------------------------------------+

	Postfixes:

	_AVG
	hardware averaged value, use it if your hardware is really able to
	report averaged values.
	_NOW
	momentary/instantaneous values.

	STATUS
	this attribute represents operating status (charging, full,
	discharging (i.e. powering a load), etc.). This corresponds to
	`BATTERY_STATUS_*` values, as defined in battery.h.

	CHARGE_TYPE
	batteries can typically charge at different rates.
	This defines trickle and fast charges. For batteries that
	are already charged or discharging, 'n/a' can be displayed (or
	'unknown', if the status is not known).

	AUTHENTIC
	indicates the power supply (battery or charger) connected
	to the platform is authentic(1) or non authentic(0).

	HEALTH
	represents health of the battery, values corresponds to
	POWER_SUPPLY_HEALTH_*, defined in battery.h.

	VOLTAGE_OCV
	open circuit voltage of the battery.

	VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN
	design values for maximal and minimal power supply voltages.
	Maximal/minimal means values of voltages when battery considered
	"full"/"empty" at normal conditions. Yes, there is no direct relation
	between voltage and battery capacity, but some dumb
	batteries use voltage for very approximated calculation of capacity.
	Battery driver also can use this attribute just to inform userspace
	about maximal and minimal voltage thresholds of a given battery.

	VOLTAGE_MAX, VOLTAGE_MIN
	same as _DESIGN voltage values except that these ones should be used
	if hardware could only guess (measure and retain) the thresholds of a
	given power supply.

	VOLTAGE_BOOT
	Reports the voltage measured during boot

	CURRENT_BOOT
	Reports the current measured during boot

	CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN
	design charge values, when battery considered full/empty.

	ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN
	same as above but for energy.

	CHARGE_FULL, CHARGE_EMPTY
	These attributes means "last remembered value of charge when battery
	became full/empty". It also could mean "value of charge when battery
	considered full/empty at given conditions (temperature, age)".
	I.e. these attributes represents real thresholds, not design values.

	ENERGY_FULL, ENERGY_EMPTY
	same as above but for energy.

	CHARGE_COUNTER
	the current charge counter (in µAh). This could easily
	be negative; there is no empty or full value. It is only useful for
	relative, time-based measurements.

	PRECHARGE_CURRENT
	the maximum charge current during precharge phase of charge cycle
	(typically 20% of battery capacity).

	CHARGE_TERM_CURRENT
	Charge termination current. The charge cycle terminates when battery
	voltage is above recharge threshold, and charge current is below
	this setting (typically 10% of battery capacity).

	CONSTANT_CHARGE_CURRENT
	constant charge current programmed by charger.


	CONSTANT_CHARGE_CURRENT_MAX
	maximum charge current supported by the power supply object.

	CONSTANT_CHARGE_VOLTAGE
	constant charge voltage programmed by charger.
	CONSTANT_CHARGE_VOLTAGE_MAX
	maximum charge voltage supported by the power supply object.

	INPUT_CURRENT_LIMIT
	input current limit programmed by charger. Indicates
	the current drawn from a charging source.
	INPUT_VOLTAGE_LIMIT
	input voltage limit programmed by charger. Indicates
	the voltage limit from a charging source.
	INPUT_POWER_LIMIT
	input power limit programmed by charger. Indicates
	the power limit from a charging source.

	CHARGE_CONTROL_LIMIT
	current charge control limit setting
	CHARGE_CONTROL_LIMIT_MAX
	maximum charge control limit setting

	CALIBRATE
	battery or coulomb counter calibration status

	CAPACITY
	capacity in percents.
	CAPACITY_ALERT_MIN
	minimum capacity alert value in percents.
	CAPACITY_ALERT_MAX
	maximum capacity alert value in percents.
	CAPACITY_LEVEL
	capacity level. This corresponds to POWER_SUPPLY_CAPACITY_LEVEL_*.

	TEMP
	temperature of the power supply.
	TEMP_ALERT_MIN
	minimum battery temperature alert.
	TEMP_ALERT_MAX
	maximum battery temperature alert.
	TEMP_AMBIENT
	ambient temperature.
	TEMP_AMBIENT_ALERT_MIN
	minimum ambient temperature alert.
	TEMP_AMBIENT_ALERT_MAX
	maximum ambient temperature alert.
	TEMP_MIN
	minimum operatable temperature
	TEMP_MAX
	maximum operatable temperature

	TIME_TO_EMPTY
	seconds left for battery to be considered empty
	(i.e. while battery powers a load)
	TIME_TO_FULL
	seconds left for battery to be considered full
	(i.e. while battery is charging)


	Battery <-> external power supply interaction
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Often power supplies are acting as supplies and supplicants at the same
	time. Batteries are good example. So, batteries usually care if they're
	externally powered or not.

	For that case, power supply class implements notification mechanism for
	batteries.

	External power supply (AC) lists supplicants (batteries) names in
	"supplied_to" struct member, and each power_supply_changed() call
	issued by external power supply will notify supplicants via
	external_power_changed callback.


	Devicetree battery characteristics
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Drivers should call power_supply_get_battery_info() to obtain battery
	characteristics from a devicetree battery node, defined in
	Documentation/devicetree/bindings/power/supply/battery.txt. This is
	implemented in drivers/power/supply/bq27xxx_battery.c.

	Properties in struct power_supply_battery_info and their counterparts in the
	battery node have names corresponding to elements in enum power_supply_property,
	for naming consistency between sysfs attributes and battery node properties.


	QA
	~~

	Q:
	Where is POWER_SUPPLY_PROP_XYZ attribute?
	A:
	If you cannot find attribute suitable for your driver needs, feel free
	to add it and send patch along with your driver.

	The attributes available currently are the ones currently provided by the
	drivers written.

	Good candidates to add in future: model/part#, cycle_time, manufacturer,
	etc.


	Q:
	I have some very specific attribute (e.g. battery color), should I add
	this attribute to standard ones?
	A:
	Most likely, no. Such attribute can be placed in the driver itself, if
	it is useful. Of course, if the attribute in question applicable to
	large set of batteries, provided by many drivers, and/or comes from
	some general battery specification/standard, it may be a candidate to
	be added to the core attribute set.


	Q:
	Suppose, my battery monitoring chip/firmware does not provides capacity
	in percents, but provides charge_{now,full,empty}. Should I calculate
	percentage capacity manually, inside the driver, and register CAPACITY
	attribute? The same question about time_to_empty/time_to_full.
	A:
	Most likely, no. This class is designed to export properties which are
	directly measurable by the specific hardware available.

	Inferring not available properties using some heuristics or mathematical
	model is not subject of work for a battery driver. Such functionality
	should be factored out, and in fact, apm_power, the driver to serve
	legacy APM API on top of power supply class, uses a simple heuristic of
	approximating remaining battery capacity based on its charge, current,
	voltage and so on. But full-fledged battery model is likely not subject
	for kernel at all, as it would require floating point calculation to deal
	with things like differential equations and Kalman filters. This is
	better be handled by batteryd/libbattery, yet to be written.