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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.