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 84AUTHENTIC - indicates the power supply (battery or charger) connected 85to the platform is authentic(1) or non authentic(0). 86 87HEALTH - represents health of the battery, values corresponds to 88POWER_SUPPLY_HEALTH_*, defined in battery.h. 89 90VOLTAGE_OCV - open circuit voltage of the battery. 91 92VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN - design values for maximal and 93minimal power supply voltages. Maximal/minimal means values of voltages 94when battery considered "full"/"empty" at normal conditions. Yes, there is 95no direct relation between voltage and battery capacity, but some dumb 96batteries use voltage for very approximated calculation of capacity. 97Battery driver also can use this attribute just to inform userspace 98about maximal and minimal voltage thresholds of a given battery. 99 100VOLTAGE_MAX, VOLTAGE_MIN - same as _DESIGN voltage values except that 101these ones should be used if hardware could only guess (measure and 102retain) the thresholds of a given power supply. 103 104VOLTAGE_BOOT - Reports the voltage measured during boot 105 106CURRENT_BOOT - Reports the current measured during boot 107 108CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when 109battery considered full/empty. 110 111ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy. 112 113CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value 114of charge when battery became full/empty". It also could mean "value of 115charge when battery considered full/empty at given conditions (temperature, 116age)". I.e. these attributes represents real thresholds, not design values. 117 118CHARGE_COUNTER - the current charge counter (in µAh). This could easily 119be negative; there is no empty or full value. It is only useful for 120relative, time-based measurements. 121 122CONSTANT_CHARGE_CURRENT - constant charge current programmed by charger. 123CONSTANT_CHARGE_CURRENT_MAX - maximum charge current supported by the 124power supply object. 125INPUT_CURRENT_LIMIT - input current limit programmed by charger. Indicates 126the current drawn from a charging source. 127CHARGE_TERM_CURRENT - Charge termination current used to detect the end of charge 128condition. 129 130CALIBRATE - battery or coulomb counter calibration status 131 132CONSTANT_CHARGE_VOLTAGE - constant charge voltage programmed by charger. 133CONSTANT_CHARGE_VOLTAGE_MAX - maximum charge voltage supported by the 134power supply object. 135 136CHARGE_CONTROL_LIMIT - current charge control limit setting 137CHARGE_CONTROL_LIMIT_MAX - maximum charge control limit setting 138 139ENERGY_FULL, ENERGY_EMPTY - same as above but for energy. 140 141CAPACITY - capacity in percents. 142CAPACITY_ALERT_MIN - minimum capacity alert value in percents. 143CAPACITY_ALERT_MAX - maximum capacity alert value in percents. 144CAPACITY_LEVEL - capacity level. This corresponds to 145POWER_SUPPLY_CAPACITY_LEVEL_*. 146 147TEMP - temperature of the power supply. 148TEMP_ALERT_MIN - minimum battery temperature alert. 149TEMP_ALERT_MAX - maximum battery temperature alert. 150TEMP_AMBIENT - ambient temperature. 151TEMP_AMBIENT_ALERT_MIN - minimum ambient temperature alert. 152TEMP_AMBIENT_ALERT_MAX - maximum ambient temperature alert. 153TEMP_MIN - minimum operatable temperature 154TEMP_MAX - maximum operatable temperature 155 156TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e. 157while battery powers a load) 158TIME_TO_FULL - seconds left for battery to be considered full (i.e. 159while battery is charging) 160 161 162Battery <-> external power supply interaction 163~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 164Often power supplies are acting as supplies and supplicants at the same 165time. Batteries are good example. So, batteries usually care if they're 166externally powered or not. 167 168For that case, power supply class implements notification mechanism for 169batteries. 170 171External power supply (AC) lists supplicants (batteries) names in 172"supplied_to" struct member, and each power_supply_changed() call 173issued by external power supply will notify supplicants via 174external_power_changed callback. 175 176 177QA 178~~ 179Q: Where is POWER_SUPPLY_PROP_XYZ attribute? 180A: If you cannot find attribute suitable for your driver needs, feel free 181 to add it and send patch along with your driver. 182 183 The attributes available currently are the ones currently provided by the 184 drivers written. 185 186 Good candidates to add in future: model/part#, cycle_time, manufacturer, 187 etc. 188 189 190Q: I have some very specific attribute (e.g. battery color), should I add 191 this attribute to standard ones? 192A: Most likely, no. Such attribute can be placed in the driver itself, if 193 it is useful. Of course, if the attribute in question applicable to 194 large set of batteries, provided by many drivers, and/or comes from 195 some general battery specification/standard, it may be a candidate to 196 be added to the core attribute set. 197 198 199Q: Suppose, my battery monitoring chip/firmware does not provides capacity 200 in percents, but provides charge_{now,full,empty}. Should I calculate 201 percentage capacity manually, inside the driver, and register CAPACITY 202 attribute? The same question about time_to_empty/time_to_full. 203A: Most likely, no. This class is designed to export properties which are 204 directly measurable by the specific hardware available. 205 206 Inferring not available properties using some heuristics or mathematical 207 model is not subject of work for a battery driver. Such functionality 208 should be factored out, and in fact, apm_power, the driver to serve 209 legacy APM API on top of power supply class, uses a simple heuristic of 210 approximating remaining battery capacity based on its charge, current, 211 voltage and so on. But full-fledged battery model is likely not subject 212 for kernel at all, as it would require floating point calculation to deal 213 with things like differential equations and Kalman filters. This is 214 better be handled by batteryd/libbattery, yet to be written. 215