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