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Documentation / power / power_supply_class.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 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/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	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	CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when
105	battery considered full/empty.
106	
107	ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy.
108	
109	CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value
110	of charge when battery became full/empty". It also could mean "value of
111	charge when battery considered full/empty at given conditions (temperature,
112	age)". I.e. these attributes represents real thresholds, not design values.
113	
114	CHARGE_COUNTER - the current charge counter (in µAh).  This could easily
115	be negative; there is no empty or full value.  It is only useful for
116	relative, time-based measurements.
117	
118	CONSTANT_CHARGE_CURRENT - constant charge current programmed by charger.
119	CONSTANT_CHARGE_CURRENT_MAX - maximum charge current supported by the
120	power supply object.
121	
122	CONSTANT_CHARGE_VOLTAGE - constant charge voltage programmed by charger.
123	CONSTANT_CHARGE_VOLTAGE_MAX - maximum charge voltage supported by the
124	power supply object.
125	
126	CHARGE_CONTROL_LIMIT - current charge control limit setting
127	CHARGE_CONTROL_LIMIT_MAX - maximum charge control limit setting
128	
129	ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
130	
131	CAPACITY - capacity in percents.
132	CAPACITY_ALERT_MIN - minimum capacity alert value in percents.
133	CAPACITY_ALERT_MAX - maximum capacity alert value in percents.
134	CAPACITY_LEVEL - capacity level. This corresponds to
135	POWER_SUPPLY_CAPACITY_LEVEL_*.
136	
137	TEMP - temperature of the power supply.
138	TEMP_ALERT_MIN - minimum battery temperature alert.
139	TEMP_ALERT_MAX - maximum battery temperature alert.
140	TEMP_AMBIENT - ambient temperature.
141	TEMP_AMBIENT_ALERT_MIN - minimum ambient temperature alert.
142	TEMP_AMBIENT_ALERT_MAX - maximum ambient temperature alert.
143	
144	TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
145	while battery powers a load)
146	TIME_TO_FULL - seconds left for battery to be considered full (i.e.
147	while battery is charging)
148	
149	
150	Battery <-> external power supply interaction
151	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
152	Often power supplies are acting as supplies and supplicants at the same
153	time. Batteries are good example. So, batteries usually care if they're
154	externally powered or not.
155	
156	For that case, power supply class implements notification mechanism for
157	batteries.
158	
159	External power supply (AC) lists supplicants (batteries) names in
160	"supplied_to" struct member, and each power_supply_changed() call
161	issued by external power supply will notify supplicants via
162	external_power_changed callback.
163	
164	
165	QA
166	~~
167	Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
168	A: If you cannot find attribute suitable for your driver needs, feel free
169	   to add it and send patch along with your driver.
170	
171	   The attributes available currently are the ones currently provided by the
172	   drivers written.
173	
174	   Good candidates to add in future: model/part#, cycle_time, manufacturer,
175	   etc.
176	
177	
178	Q: I have some very specific attribute (e.g. battery color), should I add
179	   this attribute to standard ones?
180	A: Most likely, no. Such attribute can be placed in the driver itself, if
181	   it is useful. Of course, if the attribute in question applicable to
182	   large set of batteries, provided by many drivers, and/or comes from
183	   some general battery specification/standard, it may be a candidate to
184	   be added to the core attribute set.
185	
186	
187	Q: Suppose, my battery monitoring chip/firmware does not provides capacity
188	   in percents, but provides charge_{now,full,empty}. Should I calculate
189	   percentage capacity manually, inside the driver, and register CAPACITY
190	   attribute? The same question about time_to_empty/time_to_full.
191	A: Most likely, no. This class is designed to export properties which are
192	   directly measurable by the specific hardware available.
193	
194	   Inferring not available properties using some heuristics or mathematical
195	   model is not subject of work for a battery driver. Such functionality
196	   should be factored out, and in fact, apm_power, the driver to serve
197	   legacy APM API on top of power supply class, uses a simple heuristic of
198	   approximating remaining battery capacity based on its charge, current,
199	   voltage and so on. But full-fledged battery model is likely not subject
200	   for kernel at all, as it would require floating point calculation to deal
201	   with things like differential equations and Kalman filters. This is
202	   better be handled by batteryd/libbattery, yet to be written.
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