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4a11b59d AV |
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/ds2760_battery.c and drivers/power/pda_power.c for the | |
43 | 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 | HEALTH - represents health of the battery, values corresponds to | |
80 | POWER_SUPPLY_HEALTH_*, defined in battery.h. | |
81 | ||
82 | VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN - design values for maximal and | |
83 | minimal power supply voltages. Maximal/minimal means values of voltages | |
84 | when battery considered "full"/"empty" at normal conditions. Yes, there is | |
85 | no direct relation between voltage and battery capacity, but some dumb | |
86 | batteries use voltage for very approximated calculation of capacity. | |
87 | Battery driver also can use this attribute just to inform userspace | |
88 | about maximal and minimal voltage thresholds of a given battery. | |
89 | ||
c7cc930f DB |
90 | VOLTAGE_MAX, VOLTAGE_MIN - same as _DESIGN voltage values except that |
91 | these ones should be used if hardware could only guess (measure and | |
92 | retain) the thresholds of a given power supply. | |
93 | ||
4a11b59d AV |
94 | CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when |
95 | battery considered full/empty. | |
96 | ||
97 | ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy. | |
98 | ||
99 | CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value | |
100 | of charge when battery became full/empty". It also could mean "value of | |
101 | charge when battery considered full/empty at given conditions (temperature, | |
102 | age)". I.e. these attributes represents real thresholds, not design values. | |
103 | ||
8e552c36 AS |
104 | CHARGE_COUNTER - the current charge counter (in µAh). This could easily |
105 | be negative; there is no empty or full value. It is only useful for | |
106 | relative, time-based measurements. | |
107 | ||
4a11b59d AV |
108 | ENERGY_FULL, ENERGY_EMPTY - same as above but for energy. |
109 | ||
110 | CAPACITY - capacity in percents. | |
4a11b59d AV |
111 | |
112 | TEMP - temperature of the power supply. | |
113 | TEMP_AMBIENT - ambient temperature. | |
114 | ||
115 | TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e. | |
116 | while battery powers a load) | |
117 | TIME_TO_FULL - seconds left for battery to be considered full (i.e. | |
118 | while battery is charging) | |
119 | ||
120 | ||
121 | Battery <-> external power supply interaction | |
122 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
123 | Often power supplies are acting as supplies and supplicants at the same | |
124 | time. Batteries are good example. So, batteries usually care if they're | |
125 | externally powered or not. | |
126 | ||
127 | For that case, power supply class implements notification mechanism for | |
128 | batteries. | |
129 | ||
130 | External power supply (AC) lists supplicants (batteries) names in | |
131 | "supplied_to" struct member, and each power_supply_changed() call | |
132 | issued by external power supply will notify supplicants via | |
133 | external_power_changed callback. | |
134 | ||
135 | ||
136 | QA | |
137 | ~~ | |
138 | Q: Where is POWER_SUPPLY_PROP_XYZ attribute? | |
139 | A: If you cannot find attribute suitable for your driver needs, feel free | |
140 | to add it and send patch along with your driver. | |
141 | ||
142 | The attributes available currently are the ones currently provided by the | |
143 | drivers written. | |
144 | ||
145 | Good candidates to add in future: model/part#, cycle_time, manufacturer, | |
146 | etc. | |
147 | ||
148 | ||
149 | Q: I have some very specific attribute (e.g. battery color), should I add | |
150 | this attribute to standard ones? | |
151 | A: Most likely, no. Such attribute can be placed in the driver itself, if | |
152 | it is useful. Of course, if the attribute in question applicable to | |
153 | large set of batteries, provided by many drivers, and/or comes from | |
154 | some general battery specification/standard, it may be a candidate to | |
155 | be added to the core attribute set. | |
156 | ||
157 | ||
158 | Q: Suppose, my battery monitoring chip/firmware does not provides capacity | |
159 | in percents, but provides charge_{now,full,empty}. Should I calculate | |
160 | percentage capacity manually, inside the driver, and register CAPACITY | |
161 | attribute? The same question about time_to_empty/time_to_full. | |
162 | A: Most likely, no. This class is designed to export properties which are | |
163 | directly measurable by the specific hardware available. | |
164 | ||
165 | Inferring not available properties using some heuristics or mathematical | |
166 | model is not subject of work for a battery driver. Such functionality | |
167 | should be factored out, and in fact, apm_power, the driver to serve | |
168 | legacy APM API on top of power supply class, uses a simple heuristic of | |
169 | approximating remaining battery capacity based on its charge, current, | |
170 | voltage and so on. But full-fledged battery model is likely not subject | |
171 | for kernel at all, as it would require floating point calculation to deal | |
172 | with things like differential equations and Kalman filters. This is | |
173 | better be handled by batteryd/libbattery, yet to be written. |