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diff --git a/Documentation/power/power_supply_class.rst b/Documentation/power/power_supply_class.rst new file mode 100644 index 000000000000..3f2c3fe38a61 --- /dev/null +++ b/Documentation/power/power_supply_class.rst @@ -0,0 +1,282 @@ +======================== +Linux power supply class +======================== + +Synopsis +~~~~~~~~ +Power supply class used to represent battery, UPS, AC or DC power supply +properties to user-space. + +It defines core set of attributes, which should be applicable to (almost) +every power supply out there. Attributes are available via sysfs and uevent +interfaces. + +Each attribute has well defined meaning, up to unit of measure used. While +the attributes provided are believed to be universally applicable to any +power supply, specific monitoring hardware may not be able to provide them +all, so any of them may be skipped. + +Power supply class is extensible, and allows to define drivers own attributes. +The core attribute set is subject to the standard Linux evolution (i.e. +if it will be found that some attribute is applicable to many power supply +types or their drivers, it can be added to the core set). + +It also integrates with LED framework, for the purpose of providing +typically expected feedback of battery charging/fully charged status and +AC/USB power supply online status. (Note that specific details of the +indication (including whether to use it at all) are fully controllable by +user and/or specific machine defaults, per design principles of LED +framework). + + +Attributes/properties +~~~~~~~~~~~~~~~~~~~~~ +Power supply class has predefined set of attributes, this eliminates code +duplication across drivers. Power supply class insist on reusing its +predefined attributes *and* their units. + +So, userspace gets predictable set of attributes and their units for any +kind of power supply, and can process/present them to a user in consistent +manner. Results for different power supplies and machines are also directly +comparable. + +See drivers/power/supply/ds2760_battery.c and drivers/power/supply/pda_power.c +for the example how to declare and handle attributes. + + +Units +~~~~~ +Quoting include/linux/power_supply.h: + + All voltages, currents, charges, energies, time and temperatures in µV, + µA, µAh, µWh, seconds and tenths of degree Celsius unless otherwise + stated. It's driver's job to convert its raw values to units in which + this class operates. + + +Attributes/properties detailed +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + ++--------------------------------------------------------------------------+ +| **Charge/Energy/Capacity - how to not confuse** | ++--------------------------------------------------------------------------+ +| **Because both "charge" (µAh) and "energy" (µWh) represents "capacity" | +| of battery, this class distinguish these terms. Don't mix them!** | +| | +| - `CHARGE_*` | +| attributes represents capacity in µAh only. | +| - `ENERGY_*` | +| attributes represents capacity in µWh only. | +| - `CAPACITY` | +| attribute represents capacity in *percents*, from 0 to 100. | ++--------------------------------------------------------------------------+ + +Postfixes: + +_AVG + *hardware* averaged value, use it if your hardware is really able to + report averaged values. +_NOW + momentary/instantaneous values. + +STATUS + this attribute represents operating status (charging, full, + discharging (i.e. powering a load), etc.). This corresponds to + `BATTERY_STATUS_*` values, as defined in battery.h. + +CHARGE_TYPE + batteries can typically charge at different rates. + This defines trickle and fast charges. For batteries that + are already charged or discharging, 'n/a' can be displayed (or + 'unknown', if the status is not known). + +AUTHENTIC + indicates the power supply (battery or charger) connected + to the platform is authentic(1) or non authentic(0). + +HEALTH + represents health of the battery, values corresponds to + POWER_SUPPLY_HEALTH_*, defined in battery.h. + +VOLTAGE_OCV + open circuit voltage of the battery. + +VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN + design values for maximal and minimal power supply voltages. + Maximal/minimal means values of voltages when battery considered + "full"/"empty" at normal conditions. Yes, there is no direct relation + between voltage and battery capacity, but some dumb + batteries use voltage for very approximated calculation of capacity. + Battery driver also can use this attribute just to inform userspace + about maximal and minimal voltage thresholds of a given battery. + +VOLTAGE_MAX, VOLTAGE_MIN + same as _DESIGN voltage values except that these ones should be used + if hardware could only guess (measure and retain) the thresholds of a + given power supply. + +VOLTAGE_BOOT + Reports the voltage measured during boot + +CURRENT_BOOT + Reports the current measured during boot + +CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN + design charge values, when battery considered full/empty. + +ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN + same as above but for energy. + +CHARGE_FULL, CHARGE_EMPTY + These attributes means "last remembered value of charge when battery + became full/empty". It also could mean "value of charge when battery + considered full/empty at given conditions (temperature, age)". + I.e. these attributes represents real thresholds, not design values. + +ENERGY_FULL, ENERGY_EMPTY + same as above but for energy. + +CHARGE_COUNTER + the current charge counter (in µAh). This could easily + be negative; there is no empty or full value. It is only useful for + relative, time-based measurements. + +PRECHARGE_CURRENT + the maximum charge current during precharge phase of charge cycle + (typically 20% of battery capacity). + +CHARGE_TERM_CURRENT + Charge termination current. The charge cycle terminates when battery + voltage is above recharge threshold, and charge current is below + this setting (typically 10% of battery capacity). + +CONSTANT_CHARGE_CURRENT + constant charge current programmed by charger. + + +CONSTANT_CHARGE_CURRENT_MAX + maximum charge current supported by the power supply object. + +CONSTANT_CHARGE_VOLTAGE + constant charge voltage programmed by charger. +CONSTANT_CHARGE_VOLTAGE_MAX + maximum charge voltage supported by the power supply object. + +INPUT_CURRENT_LIMIT + input current limit programmed by charger. Indicates + the current drawn from a charging source. + +CHARGE_CONTROL_LIMIT + current charge control limit setting +CHARGE_CONTROL_LIMIT_MAX + maximum charge control limit setting + +CALIBRATE + battery or coulomb counter calibration status + +CAPACITY + capacity in percents. +CAPACITY_ALERT_MIN + minimum capacity alert value in percents. +CAPACITY_ALERT_MAX + maximum capacity alert value in percents. +CAPACITY_LEVEL + capacity level. This corresponds to POWER_SUPPLY_CAPACITY_LEVEL_*. + +TEMP + temperature of the power supply. +TEMP_ALERT_MIN + minimum battery temperature alert. +TEMP_ALERT_MAX + maximum battery temperature alert. +TEMP_AMBIENT + ambient temperature. +TEMP_AMBIENT_ALERT_MIN + minimum ambient temperature alert. +TEMP_AMBIENT_ALERT_MAX + maximum ambient temperature alert. +TEMP_MIN + minimum operatable temperature +TEMP_MAX + maximum operatable temperature + +TIME_TO_EMPTY + seconds left for battery to be considered empty + (i.e. while battery powers a load) +TIME_TO_FULL + seconds left for battery to be considered full + (i.e. while battery is charging) + + +Battery <-> external power supply interaction +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Often power supplies are acting as supplies and supplicants at the same +time. Batteries are good example. So, batteries usually care if they're +externally powered or not. + +For that case, power supply class implements notification mechanism for +batteries. + +External power supply (AC) lists supplicants (batteries) names in +"supplied_to" struct member, and each power_supply_changed() call +issued by external power supply will notify supplicants via +external_power_changed callback. + + +Devicetree battery characteristics +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Drivers should call power_supply_get_battery_info() to obtain battery +characteristics from a devicetree battery node, defined in +Documentation/devicetree/bindings/power/supply/battery.txt. This is +implemented in drivers/power/supply/bq27xxx_battery.c. + +Properties in struct power_supply_battery_info and their counterparts in the +battery node have names corresponding to elements in enum power_supply_property, +for naming consistency between sysfs attributes and battery node properties. + + +QA +~~ + +Q: + Where is POWER_SUPPLY_PROP_XYZ attribute? +A: + If you cannot find attribute suitable for your driver needs, feel free + to add it and send patch along with your driver. + + The attributes available currently are the ones currently provided by the + drivers written. + + Good candidates to add in future: model/part#, cycle_time, manufacturer, + etc. + + +Q: + I have some very specific attribute (e.g. battery color), should I add + this attribute to standard ones? +A: + Most likely, no. Such attribute can be placed in the driver itself, if + it is useful. Of course, if the attribute in question applicable to + large set of batteries, provided by many drivers, and/or comes from + some general battery specification/standard, it may be a candidate to + be added to the core attribute set. + + +Q: + Suppose, my battery monitoring chip/firmware does not provides capacity + in percents, but provides charge_{now,full,empty}. Should I calculate + percentage capacity manually, inside the driver, and register CAPACITY + attribute? The same question about time_to_empty/time_to_full. +A: + Most likely, no. This class is designed to export properties which are + directly measurable by the specific hardware available. + + Inferring not available properties using some heuristics or mathematical + model is not subject of work for a battery driver. Such functionality + should be factored out, and in fact, apm_power, the driver to serve + legacy APM API on top of power supply class, uses a simple heuristic of + approximating remaining battery capacity based on its charge, current, + voltage and so on. But full-fledged battery model is likely not subject + for kernel at all, as it would require floating point calculation to deal + with things like differential equations and Kalman filters. This is + better be handled by batteryd/libbattery, yet to be written. |