Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
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/supply/ds2760_battery.c and drivers/power/supply/pda_power.c 43 for the 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 CHARGE_TYPE - batteries can typically charge at different rates. 80 This defines trickle and fast charges. For batteries that 81 are already charged or discharging, 'n/a' can be displayed (or 82 'unknown', if the status is not known). 83 84 AUTHENTIC - indicates the power supply (battery or charger) connected 85 to the platform is authentic(1) or non authentic(0). 86 87 HEALTH - represents health of the battery, values corresponds to 88 POWER_SUPPLY_HEALTH_*, defined in battery.h. 89 90 VOLTAGE_OCV - open circuit voltage of the battery. 91 92 VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN - design values for maximal and 93 minimal power supply voltages. Maximal/minimal means values of voltages 94 when battery considered "full"/"empty" at normal conditions. Yes, there is 95 no direct relation between voltage and battery capacity, but some dumb 96 batteries use voltage for very approximated calculation of capacity. 97 Battery driver also can use this attribute just to inform userspace 98 about maximal and minimal voltage thresholds of a given battery. 99 100 VOLTAGE_MAX, VOLTAGE_MIN - same as _DESIGN voltage values except that 101 these ones should be used if hardware could only guess (measure and 102 retain) the thresholds of a given power supply. 103 104 VOLTAGE_BOOT - Reports the voltage measured during boot 105 106 CURRENT_BOOT - Reports the current measured during boot 107 108 CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when 109 battery considered full/empty. 110 111 ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy. 112 113 CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value 114 of charge when battery became full/empty". It also could mean "value of 115 charge when battery considered full/empty at given conditions (temperature, 116 age)". I.e. these attributes represents real thresholds, not design values. 117 118 ENERGY_FULL, ENERGY_EMPTY - same as above but for energy. 119 120 CHARGE_COUNTER - the current charge counter (in µAh). This could easily 121 be negative; there is no empty or full value. It is only useful for 122 relative, time-based measurements. 123 124 PRECHARGE_CURRENT - the maximum charge current during precharge phase 125 of charge cycle (typically 20% of battery capacity). 126 CHARGE_TERM_CURRENT - Charge termination current. The charge cycle 127 terminates when battery voltage is above recharge threshold, and charge 128 current is below this setting (typically 10% of battery capacity). 129 130 CONSTANT_CHARGE_CURRENT - constant charge current programmed by charger. 131 CONSTANT_CHARGE_CURRENT_MAX - maximum charge current supported by the 132 power supply object. 133 134 CONSTANT_CHARGE_VOLTAGE - constant charge voltage programmed by charger. 135 CONSTANT_CHARGE_VOLTAGE_MAX - maximum charge voltage supported by the 136 power supply object. 137 138 INPUT_CURRENT_LIMIT - input current limit programmed by charger. Indicates 139 the current drawn from a charging source. 140 141 CHARGE_CONTROL_LIMIT - current charge control limit setting 142 CHARGE_CONTROL_LIMIT_MAX - maximum charge control limit setting 143 144 CALIBRATE - battery or coulomb counter calibration status 145 146 CAPACITY - capacity in percents. 147 CAPACITY_ALERT_MIN - minimum capacity alert value in percents. 148 CAPACITY_ALERT_MAX - maximum capacity alert value in percents. 149 CAPACITY_LEVEL - capacity level. This corresponds to 150 POWER_SUPPLY_CAPACITY_LEVEL_*. 151 152 TEMP - temperature of the power supply. 153 TEMP_ALERT_MIN - minimum battery temperature alert. 154 TEMP_ALERT_MAX - maximum battery temperature alert. 155 TEMP_AMBIENT - ambient temperature. 156 TEMP_AMBIENT_ALERT_MIN - minimum ambient temperature alert. 157 TEMP_AMBIENT_ALERT_MAX - maximum ambient temperature alert. 158 TEMP_MIN - minimum operatable temperature 159 TEMP_MAX - maximum operatable temperature 160 161 TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e. 162 while battery powers a load) 163 TIME_TO_FULL - seconds left for battery to be considered full (i.e. 164 while battery is charging) 165 166 167 Battery <-> external power supply interaction 168 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 169 Often power supplies are acting as supplies and supplicants at the same 170 time. Batteries are good example. So, batteries usually care if they're 171 externally powered or not. 172 173 For that case, power supply class implements notification mechanism for 174 batteries. 175 176 External power supply (AC) lists supplicants (batteries) names in 177 "supplied_to" struct member, and each power_supply_changed() call 178 issued by external power supply will notify supplicants via 179 external_power_changed callback. 180 181 182 Devicetree battery characteristics 183 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 184 Drivers should call power_supply_get_battery_info() to obtain battery 185 characteristics from a devicetree battery node, defined in 186 Documentation/devicetree/bindings/power/supply/battery.txt. This is 187 implemented in drivers/power/supply/bq27xxx_battery.c. 188 189 Properties in struct power_supply_battery_info and their counterparts in the 190 battery node have names corresponding to elements in enum power_supply_property, 191 for naming consistency between sysfs attributes and battery node properties. 192 193 194 QA 195 ~~ 196 Q: Where is POWER_SUPPLY_PROP_XYZ attribute? 197 A: If you cannot find attribute suitable for your driver needs, feel free 198 to add it and send patch along with your driver. 199 200 The attributes available currently are the ones currently provided by the 201 drivers written. 202 203 Good candidates to add in future: model/part#, cycle_time, manufacturer, 204 etc. 205 206 207 Q: I have some very specific attribute (e.g. battery color), should I add 208 this attribute to standard ones? 209 A: Most likely, no. Such attribute can be placed in the driver itself, if 210 it is useful. Of course, if the attribute in question applicable to 211 large set of batteries, provided by many drivers, and/or comes from 212 some general battery specification/standard, it may be a candidate to 213 be added to the core attribute set. 214 215 216 Q: Suppose, my battery monitoring chip/firmware does not provides capacity 217 in percents, but provides charge_{now,full,empty}. Should I calculate 218 percentage capacity manually, inside the driver, and register CAPACITY 219 attribute? The same question about time_to_empty/time_to_full. 220 A: Most likely, no. This class is designed to export properties which are 221 directly measurable by the specific hardware available. 222 223 Inferring not available properties using some heuristics or mathematical 224 model is not subject of work for a battery driver. Such functionality 225 should be factored out, and in fact, apm_power, the driver to serve 226 legacy APM API on top of power supply class, uses a simple heuristic of 227 approximating remaining battery capacity based on its charge, current, 228 voltage and so on. But full-fledged battery model is likely not subject 229 for kernel at all, as it would require floating point calculation to deal 230 with things like differential equations and Kalman filters. This is 231 better be handled by batteryd/libbattery, yet to be written.