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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
79HEALTH - represents health of the battery, values corresponds to
80POWER_SUPPLY_HEALTH_*, defined in battery.h.
81
82VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN - design values for maximal and
83minimal power supply voltages. Maximal/minimal means values of voltages
84when battery considered "full"/"empty" at normal conditions. Yes, there is
85no direct relation between voltage and battery capacity, but some dumb
86batteries use voltage for very approximated calculation of capacity.
87Battery driver also can use this attribute just to inform userspace
88about maximal and minimal voltage thresholds of a given battery.
89
90VOLTAGE_MAX, VOLTAGE_MIN - same as _DESIGN voltage values except that
91these ones should be used if hardware could only guess (measure and
92retain) the thresholds of a given power supply.
93
94CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when
95battery considered full/empty.
96
97ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy.
98
99CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value
100of charge when battery became full/empty". It also could mean "value of
101charge when battery considered full/empty at given conditions (temperature,
102age)". I.e. these attributes represents real thresholds, not design values.
103
104ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
105
106CAPACITY - capacity in percents.
107
108TEMP - temperature of the power supply.
109TEMP_AMBIENT - ambient temperature.
110
111TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
112while battery powers a load)
113TIME_TO_FULL - seconds left for battery to be considered full (i.e.
114while battery is charging)
115
116
117Battery <-> external power supply interaction
118~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
119Often power supplies are acting as supplies and supplicants at the same
120time. Batteries are good example. So, batteries usually care if they're
121externally powered or not.
122
123For that case, power supply class implements notification mechanism for
124batteries.
125
126External power supply (AC) lists supplicants (batteries) names in
127"supplied_to" struct member, and each power_supply_changed() call
128issued by external power supply will notify supplicants via
129external_power_changed callback.
130
131
132QA
133~~
134Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
135A: If you cannot find attribute suitable for your driver needs, feel free
136 to add it and send patch along with your driver.
137
138 The attributes available currently are the ones currently provided by the
139 drivers written.
140
141 Good candidates to add in future: model/part#, cycle_time, manufacturer,
142 etc.
143
144
145Q: I have some very specific attribute (e.g. battery color), should I add
146 this attribute to standard ones?
147A: Most likely, no. Such attribute can be placed in the driver itself, if
148 it is useful. Of course, if the attribute in question applicable to
149 large set of batteries, provided by many drivers, and/or comes from
150 some general battery specification/standard, it may be a candidate to
151 be added to the core attribute set.
152
153
154Q: Suppose, my battery monitoring chip/firmware does not provides capacity
155 in percents, but provides charge_{now,full,empty}. Should I calculate
156 percentage capacity manually, inside the driver, and register CAPACITY
157 attribute? The same question about time_to_empty/time_to_full.
158A: Most likely, no. This class is designed to export properties which are
159 directly measurable by the specific hardware available.
160
161 Inferring not available properties using some heuristics or mathematical
162 model is not subject of work for a battery driver. Such functionality
163 should be factored out, and in fact, apm_power, the driver to serve
164 legacy APM API on top of power supply class, uses a simple heuristic of
165 approximating remaining battery capacity based on its charge, current,
166 voltage and so on. But full-fledged battery model is likely not subject
167 for kernel at all, as it would require floating point calculation to deal
168 with things like differential equations and Kalman filters. This is
169 better be handled by batteryd/libbattery, yet to be written.