linux/Documentation/power/power_supply_class.txt
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   1Linux power supply class
   2========================
   3
   4Synopsis
   5~~~~~~~~
   6Power supply class used to represent battery, UPS, AC or DC power supply
   7properties to user-space.
   8
   9It defines core set of attributes, which should be applicable to (almost)
  10every power supply out there. Attributes are available via sysfs and uevent
  11interfaces.
  12
  13Each attribute has well defined meaning, up to unit of measure used. While
  14the attributes provided are believed to be universally applicable to any
  15power supply, specific monitoring hardware may not be able to provide them
  16all, so any of them may be skipped.
  17
  18Power supply class is extensible, and allows to define drivers own attributes.
  19The core attribute set is subject to the standard Linux evolution (i.e.
  20if it will be found that some attribute is applicable to many power supply
  21types or their drivers, it can be added to the core set).
  22
  23It also integrates with LED framework, for the purpose of providing
  24typically expected feedback of battery charging/fully charged status and
  25AC/USB power supply online status. (Note that specific details of the
  26indication (including whether to use it at all) are fully controllable by
  27user and/or specific machine defaults, per design principles of LED
  28framework).
  29
  30
  31Attributes/properties
  32~~~~~~~~~~~~~~~~~~~~~
  33Power supply class has predefined set of attributes, this eliminates code
  34duplication across drivers. Power supply class insist on reusing its
  35predefined attributes *and* their units.
  36
  37So, userspace gets predictable set of attributes and their units for any
  38kind of power supply, and can process/present them to a user in consistent
  39manner. Results for different power supplies and machines are also directly
  40comparable.
  41
  42See drivers/power/ds2760_battery.c and drivers/power/pda_power.c for the
  43example how to declare and handle attributes.
  44
  45
  46Units
  47~~~~~
  48Quoting 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
  56Attributes/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
  70Postfixes:
  71_AVG - *hardware* averaged value, use it if your hardware is really able to
  72report averaged values.
  73_NOW - momentary/instantaneous values.
  74
  75STATUS - this attribute represents operating status (charging, full,
  76discharging (i.e. powering a load), etc.). This corresponds to
  77BATTERY_STATUS_* values, as defined in battery.h.
  78
  79CHARGE_TYPE - batteries can typically charge at different rates.
  80This defines trickle and fast charges.  For batteries that
  81are already charged or discharging, 'n/a' can be displayed (or
  82'unknown', if the status is not known).
  83
  84HEALTH - represents health of the battery, values corresponds to
  85POWER_SUPPLY_HEALTH_*, defined in battery.h.
  86
  87VOLTAGE_OCV - open circuit voltage of the battery.
  88
  89VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN - design values for maximal and
  90minimal power supply voltages. Maximal/minimal means values of voltages
  91when battery considered "full"/"empty" at normal conditions. Yes, there is
  92no direct relation between voltage and battery capacity, but some dumb
  93batteries use voltage for very approximated calculation of capacity.
  94Battery driver also can use this attribute just to inform userspace
  95about maximal and minimal voltage thresholds of a given battery.
  96
  97VOLTAGE_MAX, VOLTAGE_MIN - same as _DESIGN voltage values except that
  98these ones should be used if hardware could only guess (measure and
  99retain) the thresholds of a given power supply.
 100
 101CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when
 102battery considered full/empty.
 103
 104ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy.
 105
 106CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value
 107of charge when battery became full/empty". It also could mean "value of
 108charge when battery considered full/empty at given conditions (temperature,
 109age)". I.e. these attributes represents real thresholds, not design values.
 110
 111CHARGE_COUNTER - the current charge counter (in µAh).  This could easily
 112be negative; there is no empty or full value.  It is only useful for
 113relative, time-based measurements.
 114
 115CONSTANT_CHARGE_CURRENT - constant charge current programmed by charger.
 116
 117CONSTANT_CHARGE_VOLTAGE - constant charge voltage programmed by charger.
 118
 119ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
 120
 121CAPACITY - capacity in percents.
 122CAPACITY_ALERT_MIN - minimum capacity alert value in percents.
 123CAPACITY_ALERT_MAX - maximum capacity alert value in percents.
 124CAPACITY_LEVEL - capacity level. This corresponds to
 125POWER_SUPPLY_CAPACITY_LEVEL_*.
 126
 127TEMP - temperature of the power supply.
 128TEMP_ALERT_MIN - minimum battery temperature alert value in milli centigrade.
 129TEMP_ALERT_MAX - maximum battery temperature alert value in milli centigrade.
 130TEMP_AMBIENT - ambient temperature.
 131TEMP_AMBIENT_ALERT_MIN - minimum ambient temperature alert value in milli centigrade.
 132TEMP_AMBIENT_ALERT_MAX - maximum ambient temperature alert value in milli centigrade.
 133
 134TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
 135while battery powers a load)
 136TIME_TO_FULL - seconds left for battery to be considered full (i.e.
 137while battery is charging)
 138
 139
 140Battery <-> external power supply interaction
 141~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 142Often power supplies are acting as supplies and supplicants at the same
 143time. Batteries are good example. So, batteries usually care if they're
 144externally powered or not.
 145
 146For that case, power supply class implements notification mechanism for
 147batteries.
 148
 149External power supply (AC) lists supplicants (batteries) names in
 150"supplied_to" struct member, and each power_supply_changed() call
 151issued by external power supply will notify supplicants via
 152external_power_changed callback.
 153
 154
 155QA
 156~~
 157Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
 158A: If you cannot find attribute suitable for your driver needs, feel free
 159   to add it and send patch along with your driver.
 160
 161   The attributes available currently are the ones currently provided by the
 162   drivers written.
 163
 164   Good candidates to add in future: model/part#, cycle_time, manufacturer,
 165   etc.
 166
 167
 168Q: I have some very specific attribute (e.g. battery color), should I add
 169   this attribute to standard ones?
 170A: Most likely, no. Such attribute can be placed in the driver itself, if
 171   it is useful. Of course, if the attribute in question applicable to
 172   large set of batteries, provided by many drivers, and/or comes from
 173   some general battery specification/standard, it may be a candidate to
 174   be added to the core attribute set.
 175
 176
 177Q: Suppose, my battery monitoring chip/firmware does not provides capacity
 178   in percents, but provides charge_{now,full,empty}. Should I calculate
 179   percentage capacity manually, inside the driver, and register CAPACITY
 180   attribute? The same question about time_to_empty/time_to_full.
 181A: Most likely, no. This class is designed to export properties which are
 182   directly measurable by the specific hardware available.
 183
 184   Inferring not available properties using some heuristics or mathematical
 185   model is not subject of work for a battery driver. Such functionality
 186   should be factored out, and in fact, apm_power, the driver to serve
 187   legacy APM API on top of power supply class, uses a simple heuristic of
 188   approximating remaining battery capacity based on its charge, current,
 189   voltage and so on. But full-fledged battery model is likely not subject
 190   for kernel at all, as it would require floating point calculation to deal
 191   with things like differential equations and Kalman filters. This is
 192   better be handled by batteryd/libbattery, yet to be written.
 193
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