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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.

HEALTH - represents health of the battery, values corresponds to

POWER_SUPPLY_HEALTH_*, defined in battery.h.

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.

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.

CAPACITY - capacity in percents.

CAPACITY_LEVEL - capacity level. This corresponds to

POWER_SUPPLY_CAPACITY_LEVEL_*.

TEMP - temperature of the power supply.

TEMP_AMBIENT - ambient 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.

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.