289 lines
10 KiB
ReStructuredText
289 lines
10 KiB
ReStructuredText
========================
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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/supply/ds2760_battery.c and drivers/power/supply/pda_power.c
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for the 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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+--------------------------------------------------------------------------+
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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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| - `CHARGE_*` |
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| attributes represents capacity in µAh only. |
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| - `ENERGY_*` |
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| attributes represents capacity in µWh only. |
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| - `CAPACITY` |
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| attribute represents capacity in *percents*, from 0 to 100. |
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+--------------------------------------------------------------------------+
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Postfixes:
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_AVG
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*hardware* averaged value, use it if your hardware is really able to
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report averaged values.
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_NOW
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momentary/instantaneous values.
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STATUS
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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
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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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AUTHENTIC
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indicates the power supply (battery or charger) connected
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to the platform is authentic(1) or non authentic(0).
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HEALTH
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represents health of the battery, values corresponds to
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POWER_SUPPLY_HEALTH_*, defined in battery.h.
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VOLTAGE_OCV
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open circuit voltage of the battery.
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VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN
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design values for maximal and minimal power supply voltages.
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Maximal/minimal means values of voltages when battery considered
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"full"/"empty" at normal conditions. Yes, there is no direct relation
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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
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same as _DESIGN voltage values except that these ones should be used
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if hardware could only guess (measure and retain) the thresholds of a
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given power supply.
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VOLTAGE_BOOT
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Reports the voltage measured during boot
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CURRENT_BOOT
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Reports the current measured during boot
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CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN
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design charge values, when battery considered full/empty.
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ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN
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same as above but for energy.
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CHARGE_FULL, CHARGE_EMPTY
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These attributes means "last remembered value of charge when battery
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became full/empty". It also could mean "value of charge when battery
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considered full/empty at given conditions (temperature, age)".
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I.e. these attributes represents real thresholds, not design values.
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ENERGY_FULL, ENERGY_EMPTY
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same as above but for energy.
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CHARGE_COUNTER
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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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PRECHARGE_CURRENT
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the maximum charge current during precharge phase of charge cycle
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(typically 20% of battery capacity).
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CHARGE_TERM_CURRENT
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Charge termination current. The charge cycle terminates when battery
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voltage is above recharge threshold, and charge current is below
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this setting (typically 10% of battery capacity).
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CONSTANT_CHARGE_CURRENT
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constant charge current programmed by charger.
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CONSTANT_CHARGE_CURRENT_MAX
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maximum charge current supported by the power supply object.
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CONSTANT_CHARGE_VOLTAGE
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constant charge voltage programmed by charger.
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CONSTANT_CHARGE_VOLTAGE_MAX
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maximum charge voltage supported by the power supply object.
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INPUT_CURRENT_LIMIT
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input current limit programmed by charger. Indicates
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the current drawn from a charging source.
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INPUT_VOLTAGE_LIMIT
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input voltage limit programmed by charger. Indicates
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the voltage limit from a charging source.
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INPUT_POWER_LIMIT
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input power limit programmed by charger. Indicates
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the power limit from a charging source.
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CHARGE_CONTROL_LIMIT
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current charge control limit setting
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CHARGE_CONTROL_LIMIT_MAX
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maximum charge control limit setting
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CALIBRATE
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battery or coulomb counter calibration status
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CAPACITY
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capacity in percents.
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CAPACITY_ALERT_MIN
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minimum capacity alert value in percents.
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CAPACITY_ALERT_MAX
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maximum capacity alert value in percents.
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CAPACITY_LEVEL
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capacity level. This corresponds to POWER_SUPPLY_CAPACITY_LEVEL_*.
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TEMP
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temperature of the power supply.
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TEMP_ALERT_MIN
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minimum battery temperature alert.
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TEMP_ALERT_MAX
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maximum battery temperature alert.
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TEMP_AMBIENT
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ambient temperature.
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TEMP_AMBIENT_ALERT_MIN
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minimum ambient temperature alert.
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TEMP_AMBIENT_ALERT_MAX
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maximum ambient temperature alert.
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TEMP_MIN
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minimum operatable temperature
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TEMP_MAX
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maximum operatable temperature
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TIME_TO_EMPTY
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seconds left for battery to be considered empty
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(i.e. while battery powers a load)
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TIME_TO_FULL
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seconds left for battery to be considered full
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(i.e. 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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Devicetree battery characteristics
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Drivers should call power_supply_get_battery_info() to obtain battery
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characteristics from a devicetree battery node, defined in
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Documentation/devicetree/bindings/power/supply/battery.txt. This is
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implemented in drivers/power/supply/bq27xxx_battery.c.
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Properties in struct power_supply_battery_info and their counterparts in the
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battery node have names corresponding to elements in enum power_supply_property,
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for naming consistency between sysfs attributes and battery node properties.
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QA
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~~
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Q:
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Where is POWER_SUPPLY_PROP_XYZ attribute?
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A:
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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:
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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:
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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:
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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:
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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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