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Documentation / power / power_supply_class.txt




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Based on kernel version 3.19. Page generated on 2015-02-13 21:22 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	VOLTAGE_BOOT - Reports the voltage measured during boot
105	
106	CURRENT_BOOT - Reports the current measured during boot
107	
108	CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when
109	battery considered full/empty.
110	
111	ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy.
112	
113	CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value
114	of charge when battery became full/empty". It also could mean "value of
115	charge when battery considered full/empty at given conditions (temperature,
116	age)". I.e. these attributes represents real thresholds, not design values.
117	
118	CHARGE_COUNTER - the current charge counter (in µAh).  This could easily
119	be negative; there is no empty or full value.  It is only useful for
120	relative, time-based measurements.
121	
122	CONSTANT_CHARGE_CURRENT - constant charge current programmed by charger.
123	CONSTANT_CHARGE_CURRENT_MAX - maximum charge current supported by the
124	power supply object.
125	INPUT_CURRENT_LIMIT - input current limit programmed by charger. Indicates
126	the current drawn from a charging source.
127	CHARGE_TERM_CURRENT - Charge termination current used to detect the end of charge
128	condition.
129	
130	CALIBRATE - battery or coulomb counter calibration status
131	
132	CONSTANT_CHARGE_VOLTAGE - constant charge voltage programmed by charger.
133	CONSTANT_CHARGE_VOLTAGE_MAX - maximum charge voltage supported by the
134	power supply object.
135	
136	CHARGE_CONTROL_LIMIT - current charge control limit setting
137	CHARGE_CONTROL_LIMIT_MAX - maximum charge control limit setting
138	
139	ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
140	
141	CAPACITY - capacity in percents.
142	CAPACITY_ALERT_MIN - minimum capacity alert value in percents.
143	CAPACITY_ALERT_MAX - maximum capacity alert value in percents.
144	CAPACITY_LEVEL - capacity level. This corresponds to
145	POWER_SUPPLY_CAPACITY_LEVEL_*.
146	
147	TEMP - temperature of the power supply.
148	TEMP_ALERT_MIN - minimum battery temperature alert.
149	TEMP_ALERT_MAX - maximum battery temperature alert.
150	TEMP_AMBIENT - ambient temperature.
151	TEMP_AMBIENT_ALERT_MIN - minimum ambient temperature alert.
152	TEMP_AMBIENT_ALERT_MAX - maximum ambient temperature alert.
153	TEMP_MIN - minimum operatable temperature
154	TEMP_MAX - maximum operatable temperature
155	
156	TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
157	while battery powers a load)
158	TIME_TO_FULL - seconds left for battery to be considered full (i.e.
159	while battery is charging)
160	
161	
162	Battery <-> external power supply interaction
163	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
164	Often power supplies are acting as supplies and supplicants at the same
165	time. Batteries are good example. So, batteries usually care if they're
166	externally powered or not.
167	
168	For that case, power supply class implements notification mechanism for
169	batteries.
170	
171	External power supply (AC) lists supplicants (batteries) names in
172	"supplied_to" struct member, and each power_supply_changed() call
173	issued by external power supply will notify supplicants via
174	external_power_changed callback.
175	
176	
177	QA
178	~~
179	Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
180	A: If you cannot find attribute suitable for your driver needs, feel free
181	   to add it and send patch along with your driver.
182	
183	   The attributes available currently are the ones currently provided by the
184	   drivers written.
185	
186	   Good candidates to add in future: model/part#, cycle_time, manufacturer,
187	   etc.
188	
189	
190	Q: I have some very specific attribute (e.g. battery color), should I add
191	   this attribute to standard ones?
192	A: Most likely, no. Such attribute can be placed in the driver itself, if
193	   it is useful. Of course, if the attribute in question applicable to
194	   large set of batteries, provided by many drivers, and/or comes from
195	   some general battery specification/standard, it may be a candidate to
196	   be added to the core attribute set.
197	
198	
199	Q: Suppose, my battery monitoring chip/firmware does not provides capacity
200	   in percents, but provides charge_{now,full,empty}. Should I calculate
201	   percentage capacity manually, inside the driver, and register CAPACITY
202	   attribute? The same question about time_to_empty/time_to_full.
203	A: Most likely, no. This class is designed to export properties which are
204	   directly measurable by the specific hardware available.
205	
206	   Inferring not available properties using some heuristics or mathematical
207	   model is not subject of work for a battery driver. Such functionality
208	   should be factored out, and in fact, apm_power, the driver to serve
209	   legacy APM API on top of power supply class, uses a simple heuristic of
210	   approximating remaining battery capacity based on its charge, current,
211	   voltage and so on. But full-fledged battery model is likely not subject
212	   for kernel at all, as it would require floating point calculation to deal
213	   with things like differential equations and Kalman filters. This is
214	   better be handled by batteryd/libbattery, yet to be written.
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