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