181 lines
7.7 KiB
Plaintext
181 lines
7.7 KiB
Plaintext
Linux power supply class
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========================
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Synopsis
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~~~~~~~~
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Power supply class used to represent battery, UPS, AC or DC power supply
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properties to user-space.
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It defines core set of attributes, which should be applicable to (almost)
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every power supply out there. Attributes are available via sysfs and uevent
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interfaces.
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Each attribute has well defined meaning, up to unit of measure used. While
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the attributes provided are believed to be universally applicable to any
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power supply, specific monitoring hardware may not be able to provide them
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all, so any of them may be skipped.
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Power supply class is extensible, and allows to define drivers own attributes.
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The core attribute set is subject to the standard Linux evolution (i.e.
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if it will be found that some attribute is applicable to many power supply
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types or their drivers, it can be added to the core set).
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It also integrates with LED framework, for the purpose of providing
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typically expected feedback of battery charging/fully charged status and
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AC/USB power supply online status. (Note that specific details of the
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indication (including whether to use it at all) are fully controllable by
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user and/or specific machine defaults, per design principles of LED
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framework).
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Attributes/properties
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~~~~~~~~~~~~~~~~~~~~~
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Power supply class has predefined set of attributes, this eliminates code
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duplication across drivers. Power supply class insist on reusing its
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predefined attributes *and* their units.
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So, userspace gets predictable set of attributes and their units for any
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kind of power supply, and can process/present them to a user in consistent
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manner. Results for different power supplies and machines are also directly
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comparable.
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See drivers/power/ds2760_battery.c and drivers/power/pda_power.c for the
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example how to declare and handle attributes.
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Units
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~~~~~
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Quoting include/linux/power_supply.h:
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All voltages, currents, charges, energies, time and temperatures in µV,
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µA, µAh, µWh, seconds and tenths of degree Celsius unless otherwise
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stated. It's driver's job to convert its raw values to units in which
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this class operates.
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Attributes/properties detailed
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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~ ~ ~ ~ ~ ~ ~ Charge/Energy/Capacity - how to not confuse ~ ~ ~ ~ ~ ~ ~
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~ ~
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~ Because both "charge" (µAh) and "energy" (µWh) represents "capacity" ~
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~ of battery, this class distinguish these terms. Don't mix them! ~
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~ ~
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~ CHARGE_* attributes represents capacity in µAh only. ~
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~ ENERGY_* attributes represents capacity in µWh only. ~
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~ CAPACITY attribute represents capacity in *percents*, from 0 to 100. ~
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~ ~
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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Postfixes:
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_AVG - *hardware* averaged value, use it if your hardware is really able to
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report averaged values.
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_NOW - momentary/instantaneous values.
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STATUS - this attribute represents operating status (charging, full,
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discharging (i.e. powering a load), etc.). This corresponds to
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BATTERY_STATUS_* values, as defined in battery.h.
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CHARGE_TYPE - batteries can typically charge at different rates.
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This defines trickle and fast charges. For batteries that
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are already charged or discharging, 'n/a' can be displayed (or
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'unknown', if the status is not known).
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HEALTH - represents health of the battery, values corresponds to
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POWER_SUPPLY_HEALTH_*, defined in battery.h.
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VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN - design values for maximal and
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minimal power supply voltages. Maximal/minimal means values of voltages
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when battery considered "full"/"empty" at normal conditions. Yes, there is
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no direct relation between voltage and battery capacity, but some dumb
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batteries use voltage for very approximated calculation of capacity.
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Battery driver also can use this attribute just to inform userspace
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about maximal and minimal voltage thresholds of a given battery.
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VOLTAGE_MAX, VOLTAGE_MIN - same as _DESIGN voltage values except that
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these ones should be used if hardware could only guess (measure and
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retain) the thresholds of a given power supply.
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CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when
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battery considered full/empty.
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ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy.
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CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value
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of charge when battery became full/empty". It also could mean "value of
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charge when battery considered full/empty at given conditions (temperature,
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age)". I.e. these attributes represents real thresholds, not design values.
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CHARGE_COUNTER - the current charge counter (in µAh). This could easily
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be negative; there is no empty or full value. It is only useful for
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relative, time-based measurements.
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ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
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CAPACITY - capacity in percents.
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CAPACITY_LEVEL - capacity level. This corresponds to
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POWER_SUPPLY_CAPACITY_LEVEL_*.
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TEMP - temperature of the power supply.
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TEMP_AMBIENT - ambient temperature.
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TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
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while battery powers a load)
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TIME_TO_FULL - seconds left for battery to be considered full (i.e.
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while battery is charging)
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Battery <-> external power supply interaction
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Often power supplies are acting as supplies and supplicants at the same
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time. Batteries are good example. So, batteries usually care if they're
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externally powered or not.
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For that case, power supply class implements notification mechanism for
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batteries.
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External power supply (AC) lists supplicants (batteries) names in
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"supplied_to" struct member, and each power_supply_changed() call
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issued by external power supply will notify supplicants via
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external_power_changed callback.
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QA
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~~
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Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
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A: If you cannot find attribute suitable for your driver needs, feel free
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to add it and send patch along with your driver.
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The attributes available currently are the ones currently provided by the
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drivers written.
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Good candidates to add in future: model/part#, cycle_time, manufacturer,
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etc.
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Q: I have some very specific attribute (e.g. battery color), should I add
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this attribute to standard ones?
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A: Most likely, no. Such attribute can be placed in the driver itself, if
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it is useful. Of course, if the attribute in question applicable to
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large set of batteries, provided by many drivers, and/or comes from
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some general battery specification/standard, it may be a candidate to
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be added to the core attribute set.
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Q: Suppose, my battery monitoring chip/firmware does not provides capacity
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in percents, but provides charge_{now,full,empty}. Should I calculate
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percentage capacity manually, inside the driver, and register CAPACITY
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attribute? The same question about time_to_empty/time_to_full.
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A: Most likely, no. This class is designed to export properties which are
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directly measurable by the specific hardware available.
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Inferring not available properties using some heuristics or mathematical
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model is not subject of work for a battery driver. Such functionality
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should be factored out, and in fact, apm_power, the driver to serve
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legacy APM API on top of power supply class, uses a simple heuristic of
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approximating remaining battery capacity based on its charge, current,
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voltage and so on. But full-fledged battery model is likely not subject
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for kernel at all, as it would require floating point calculation to deal
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with things like differential equations and Kalman filters. This is
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better be handled by batteryd/libbattery, yet to be written.
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