Documentation/power_supply_class.txt

   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 CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when
  91 battery considered full/empty.
  92
  93 ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy.
  94
  95 CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value
  96 of charge when battery became full/empty". It also could mean "value of
  97 charge when battery considered full/empty at given conditions (temperature,
  98 age)". I.e. these attributes represents real thresholds, not design values.
  99
 100 ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
 101
 102 CAPACITY - capacity in percents.
 103 CAPACITY_LEVEL - capacity level. This corresponds to
 104 POWER_SUPPLY_CAPACITY_LEVEL_*.
 105
 106 TEMP - temperature of the power supply.
 107 TEMP_AMBIENT - ambient temperature.
 108
 109 TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
 110 while battery powers a load)
 111 TIME_TO_FULL - seconds left for battery to be considered full (i.e.
 112 while battery is charging)
 113
 114
 115 Battery <-> external power supply interaction
 116 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 117 Often power supplies are acting as supplies and supplicants at the same
 118 time. Batteries are good example. So, batteries usually care if they're
 119 externally powered or not.
 120
 121 For that case, power supply class implements notification mechanism for
 122 batteries.
 123
 124 External power supply (AC) lists supplicants (batteries) names in
 125 "supplied_to" struct member, and each power_supply_changed() call
 126 issued by external power supply will notify supplicants via
 127 external_power_changed callback.
 128
 129
 130 QA
 131 ~~
 132 Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
 133 A: If you cannot find attribute suitable for your driver needs, feel free
 134    to add it and send patch along with your driver.
 135
 136    The attributes available currently are the ones currently provided by the
 137    drivers written.
 138
 139    Good candidates to add in future: model/part#, cycle_time, manufacturer,
 140    etc.
 141
 142
 143 Q: I have some very specific attribute (e.g. battery color), should I add
 144    this attribute to standard ones?
 145 A: Most likely, no. Such attribute can be placed in the driver itself, if
 146    it is useful. Of course, if the attribute in question applicable to
 147    large set of batteries, provided by many drivers, and/or comes from
 148    some general battery specification/standard, it may be a candidate to
 149    be added to the core attribute set.
 150
 151
 152 Q: Suppose, my battery monitoring chip/firmware does not provides capacity
 153    in percents, but provides charge_{now,full,empty}. Should I calculate
 154    percentage capacity manually, inside the driver, and register CAPACITY
 155    attribute? The same question about time_to_empty/time_to_full.
 156 A: Most likely, no. This class is designed to export properties which are
 157    directly measurable by the specific hardware available.
 158
 159    Inferring not available properties using some heuristics or mathematical
 160    model is not subject of work for a battery driver. Such functionality
 161    should be factored out, and in fact, apm_power, the driver to serve
 162    legacy APM API on top of power supply class, uses a simple heuristic of
 163    approximating remaining battery capacity based on its charge, current,
 164    voltage and so on. But full-fledged battery model is likely not subject
 165    for kernel at all, as it would require floating point calculation to deal
 166    with things like differential equations and Kalman filters. This is
 167    better be handled by batteryd/libbattery, yet to be written.