Root/Documentation/power/power_supply_class.txt

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_MAX_DESIGN, VOLTAGE_MIN_DESIGN - design values for maximal and
88minimal power supply voltages. Maximal/minimal means values of voltages
89when battery considered "full"/"empty" at normal conditions. Yes, there is
90no direct relation between voltage and battery capacity, but some dumb
91batteries use voltage for very approximated calculation of capacity.
92Battery driver also can use this attribute just to inform userspace
93about maximal and minimal voltage thresholds of a given battery.
94
95VOLTAGE_MAX, VOLTAGE_MIN - same as _DESIGN voltage values except that
96these ones should be used if hardware could only guess (measure and
97retain) the thresholds of a given power supply.
98
99CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when
100battery considered full/empty.
101
102ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy.
103
104CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value
105of charge when battery became full/empty". It also could mean "value of
106charge when battery considered full/empty at given conditions (temperature,
107age)". I.e. these attributes represents real thresholds, not design values.
108
109CHARGE_COUNTER - the current charge counter (in µAh). This could easily
110be negative; there is no empty or full value. It is only useful for
111relative, time-based measurements.
112
113ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
114
115CAPACITY - capacity in percents.
116CAPACITY_LEVEL - capacity level. This corresponds to
117POWER_SUPPLY_CAPACITY_LEVEL_*.
118
119TEMP - temperature of the power supply.
120TEMP_AMBIENT - ambient temperature.
121
122TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
123while battery powers a load)
124TIME_TO_FULL - seconds left for battery to be considered full (i.e.
125while battery is charging)
126
127
128Battery <-> external power supply interaction
129~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
130Often power supplies are acting as supplies and supplicants at the same
131time. Batteries are good example. So, batteries usually care if they're
132externally powered or not.
133
134For that case, power supply class implements notification mechanism for
135batteries.
136
137External power supply (AC) lists supplicants (batteries) names in
138"supplied_to" struct member, and each power_supply_changed() call
139issued by external power supply will notify supplicants via
140external_power_changed callback.
141
142
143QA
144~~
145Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
146A: If you cannot find attribute suitable for your driver needs, feel free
147   to add it and send patch along with your driver.
148
149   The attributes available currently are the ones currently provided by the
150   drivers written.
151
152   Good candidates to add in future: model/part#, cycle_time, manufacturer,
153   etc.
154
155
156Q: I have some very specific attribute (e.g. battery color), should I add
157   this attribute to standard ones?
158A: Most likely, no. Such attribute can be placed in the driver itself, if
159   it is useful. Of course, if the attribute in question applicable to
160   large set of batteries, provided by many drivers, and/or comes from
161   some general battery specification/standard, it may be a candidate to
162   be added to the core attribute set.
163
164
165Q: Suppose, my battery monitoring chip/firmware does not provides capacity
166   in percents, but provides charge_{now,full,empty}. Should I calculate
167   percentage capacity manually, inside the driver, and register CAPACITY
168   attribute? The same question about time_to_empty/time_to_full.
169A: Most likely, no. This class is designed to export properties which are
170   directly measurable by the specific hardware available.
171
172   Inferring not available properties using some heuristics or mathematical
173   model is not subject of work for a battery driver. Such functionality
174   should be factored out, and in fact, apm_power, the driver to serve
175   legacy APM API on top of power supply class, uses a simple heuristic of
176   approximating remaining battery capacity based on its charge, current,
177   voltage and so on. But full-fledged battery model is likely not subject
178   for kernel at all, as it would require floating point calculation to deal
179   with things like differential equations and Kalman filters. This is
180   better be handled by batteryd/libbattery, yet to be written.
181

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