Documentation / power / power_supply_class.txt

Custom Search

Based on kernel version 3.9. Page generated on 2013-05-02 23:12 EST.

	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/ds2760_battery.c and drivers/power/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.
	
	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.
	
	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.
	
	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.
	
	CHARGE_CONTROL_LIMIT - current charge control limit setting
	CHARGE_CONTROL_LIMIT_MAX - maximum charge control limit setting
	
	ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
	
	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 value in milli centigrade.
	TEMP_ALERT_MAX - maximum battery temperature alert value in milli centigrade.
	TEMP_AMBIENT - ambient temperature.
	TEMP_AMBIENT_ALERT_MIN - minimum ambient temperature alert value in milli centigrade.
	TEMP_AMBIENT_ALERT_MAX - maximum ambient temperature alert value in milli centigrade.
	
	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.
	
	
	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.

• [ power ]
• 00-INDEX
• apm-acpi.txt
• basic-pm-debugging.txt
• charger-manager.txt
• devices.txt
• drivers-testing.txt
• freezing-of-tasks.txt
• interface.txt
• notifiers.txt
• opp.txt
• pci.txt
• pm_qos_interface.txt
• power_supply_class.txt
• [ regulator ]
• runtime_pm.txt
• s2ram.txt
• states.txt
• suspend-and-cpuhotplug.txt
• swsusp-and-swap-files.txt
• swsusp-dmcrypt.txt
• swsusp.txt
• tricks.txt
• userland-swsusp.txt
• video.txt
• video_extension.txt
•