The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/Documentation/power_supply_class.txt

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    1 Linux power supply class
    2 ========================
    3 
    4 Synopsis
    5 ~~~~~~~~
    6 Power supply class used to represent battery, UPS, AC or DC power supply
    7 properties to user-space.
    8 
    9 It defines core set of attributes, which should be applicable to (almost)
   10 every power supply out there. Attributes are available via sysfs and uevent
   11 interfaces.
   12 
   13 Each attribute has well defined meaning, up to unit of measure used. While
   14 the attributes provided are believed to be universally applicable to any
   15 power supply, specific monitoring hardware may not be able to provide them
   16 all, so any of them may be skipped.
   17 
   18 Power supply class is extensible, and allows to define drivers own attributes.
   19 The core attribute set is subject to the standard Linux evolution (i.e.
   20 if it will be found that some attribute is applicable to many power supply
   21 types or their drivers, it can be added to the core set).
   22 
   23 It also integrates with LED framework, for the purpose of providing
   24 typically expected feedback of battery charging/fully charged status and
   25 AC/USB power supply online status. (Note that specific details of the
   26 indication (including whether to use it at all) are fully controllable by
   27 user and/or specific machine defaults, per design principles of LED
   28 framework).
   29 
   30 
   31 Attributes/properties
   32 ~~~~~~~~~~~~~~~~~~~~~
   33 Power supply class has predefined set of attributes, this eliminates code
   34 duplication across drivers. Power supply class insist on reusing its
   35 predefined attributes *and* their units.
   36 
   37 So, userspace gets predictable set of attributes and their units for any
   38 kind of power supply, and can process/present them to a user in consistent
   39 manner. Results for different power supplies and machines are also directly
   40 comparable.
   41 
   42 See drivers/power/ds2760_battery.c and drivers/power/pda_power.c for the
   43 example how to declare and handle attributes.
   44 
   45 
   46 Units
   47 ~~~~~
   48 Quoting include/linux/power_supply.h:
   49 
   50   All voltages, currents, charges, energies, time and temperatures in µV,
   51   µA, µAh, µWh, seconds and tenths of degree Celsius unless otherwise
   52   stated. It's driver's job to convert its raw values to units in which
   53   this class operates.
   54 
   55 
   56 Attributes/properties detailed
   57 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   58 
   59 ~ ~ ~ ~ ~ ~ ~  Charge/Energy/Capacity - how to not confuse  ~ ~ ~ ~ ~ ~ ~
   60 ~                                                                       ~
   61 ~ Because both "charge" (µAh) and "energy" (µWh) represents "capacity"  ~
   62 ~ of battery, this class distinguish these terms. Don't mix them!       ~
   63 ~                                                                       ~
   64 ~ CHARGE_* attributes represents capacity in µAh only.                  ~
   65 ~ ENERGY_* attributes represents capacity in µWh only.                  ~
   66 ~ CAPACITY attribute represents capacity in *percents*, from 0 to 100.  ~
   67 ~                                                                       ~
   68 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
   69 
   70 Postfixes:
   71 _AVG - *hardware* averaged value, use it if your hardware is really able to
   72 report averaged values.
   73 _NOW - momentary/instantaneous values.
   74 
   75 STATUS - this attribute represents operating status (charging, full,
   76 discharging (i.e. powering a load), etc.). This corresponds to
   77 BATTERY_STATUS_* values, as defined in battery.h.
   78 
   79 HEALTH - represents health of the battery, values corresponds to
   80 POWER_SUPPLY_HEALTH_*, defined in battery.h.
   81 
   82 VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN - design values for maximal and
   83 minimal power supply voltages. Maximal/minimal means values of voltages
   84 when battery considered "full"/"empty" at normal conditions. Yes, there is
   85 no direct relation between voltage and battery capacity, but some dumb
   86 batteries use voltage for very approximated calculation of capacity.
   87 Battery driver also can use this attribute just to inform userspace
   88 about maximal and minimal voltage thresholds of a given battery.
   89 
   90 VOLTAGE_MAX, VOLTAGE_MIN - same as _DESIGN voltage values except that
   91 these ones should be used if hardware could only guess (measure and
   92 retain) the thresholds of a given power supply.
   93 
   94 CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when
   95 battery considered full/empty.
   96 
   97 ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy.
   98 
   99 CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value
  100 of charge when battery became full/empty". It also could mean "value of
  101 charge when battery considered full/empty at given conditions (temperature,
  102 age)". I.e. these attributes represents real thresholds, not design values.
  103 
  104 ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
  105 
  106 CAPACITY - capacity in percents.
  107 
  108 TEMP - temperature of the power supply.
  109 TEMP_AMBIENT - ambient temperature.
  110 
  111 TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
  112 while battery powers a load)
  113 TIME_TO_FULL - seconds left for battery to be considered full (i.e.
  114 while battery is charging)
  115 
  116 
  117 Battery <-> external power supply interaction
  118 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  119 Often power supplies are acting as supplies and supplicants at the same
  120 time. Batteries are good example. So, batteries usually care if they're
  121 externally powered or not.
  122 
  123 For that case, power supply class implements notification mechanism for
  124 batteries.
  125 
  126 External power supply (AC) lists supplicants (batteries) names in
  127 "supplied_to" struct member, and each power_supply_changed() call
  128 issued by external power supply will notify supplicants via
  129 external_power_changed callback.
  130 
  131 
  132 QA
  133 ~~
  134 Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
  135 A: If you cannot find attribute suitable for your driver needs, feel free
  136    to add it and send patch along with your driver.
  137 
  138    The attributes available currently are the ones currently provided by the
  139    drivers written.
  140 
  141    Good candidates to add in future: model/part#, cycle_time, manufacturer,
  142    etc.
  143 
  144 
  145 Q: I have some very specific attribute (e.g. battery color), should I add
  146    this attribute to standard ones?
  147 A: Most likely, no. Such attribute can be placed in the driver itself, if
  148    it is useful. Of course, if the attribute in question applicable to
  149    large set of batteries, provided by many drivers, and/or comes from
  150    some general battery specification/standard, it may be a candidate to
  151    be added to the core attribute set.
  152 
  153 
  154 Q: Suppose, my battery monitoring chip/firmware does not provides capacity
  155    in percents, but provides charge_{now,full,empty}. Should I calculate
  156    percentage capacity manually, inside the driver, and register CAPACITY
  157    attribute? The same question about time_to_empty/time_to_full.
  158 A: Most likely, no. This class is designed to export properties which are
  159    directly measurable by the specific hardware available.
  160 
  161    Inferring not available properties using some heuristics or mathematical
  162    model is not subject of work for a battery driver. Such functionality
  163    should be factored out, and in fact, apm_power, the driver to serve
  164    legacy APM API on top of power supply class, uses a simple heuristic of
  165    approximating remaining battery capacity based on its charge, current,
  166    voltage and so on. But full-fledged battery model is likely not subject
  167    for kernel at all, as it would require floating point calculation to deal
  168    with things like differential equations and Kalman filters. This is
  169    better be handled by batteryd/libbattery, yet to be written.

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