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Documentation / power / power_supply_class.txt




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Based on kernel version 3.2. Page generated on 2012-01-05 23:29 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	HEALTH - represents health of the battery, values corresponds to
85	POWER_SUPPLY_HEALTH_*, defined in battery.h.
86	
87	VOLTAGE_MAX_DESIGN, VOLTAGE_MIN_DESIGN - design values for maximal and
88	minimal power supply voltages. Maximal/minimal means values of voltages
89	when battery considered "full"/"empty" at normal conditions. Yes, there is
90	no direct relation between voltage and battery capacity, but some dumb
91	batteries use voltage for very approximated calculation of capacity.
92	Battery driver also can use this attribute just to inform userspace
93	about maximal and minimal voltage thresholds of a given battery.
94	
95	VOLTAGE_MAX, VOLTAGE_MIN - same as _DESIGN voltage values except that
96	these ones should be used if hardware could only guess (measure and
97	retain) the thresholds of a given power supply.
98	
99	CHARGE_FULL_DESIGN, CHARGE_EMPTY_DESIGN - design charge values, when
100	battery considered full/empty.
101	
102	ENERGY_FULL_DESIGN, ENERGY_EMPTY_DESIGN - same as above but for energy.
103	
104	CHARGE_FULL, CHARGE_EMPTY - These attributes means "last remembered value
105	of charge when battery became full/empty". It also could mean "value of
106	charge when battery considered full/empty at given conditions (temperature,
107	age)". I.e. these attributes represents real thresholds, not design values.
108	
109	CHARGE_COUNTER - the current charge counter (in µAh).  This could easily
110	be negative; there is no empty or full value.  It is only useful for
111	relative, time-based measurements.
112	
113	ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
114	
115	CAPACITY - capacity in percents.
116	CAPACITY_LEVEL - capacity level. This corresponds to
117	POWER_SUPPLY_CAPACITY_LEVEL_*.
118	
119	TEMP - temperature of the power supply.
120	TEMP_AMBIENT - ambient temperature.
121	
122	TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
123	while battery powers a load)
124	TIME_TO_FULL - seconds left for battery to be considered full (i.e.
125	while battery is charging)
126	
127	
128	Battery <-> external power supply interaction
129	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
130	Often power supplies are acting as supplies and supplicants at the same
131	time. Batteries are good example. So, batteries usually care if they're
132	externally powered or not.
133	
134	For that case, power supply class implements notification mechanism for
135	batteries.
136	
137	External power supply (AC) lists supplicants (batteries) names in
138	"supplied_to" struct member, and each power_supply_changed() call
139	issued by external power supply will notify supplicants via
140	external_power_changed callback.
141	
142	
143	QA
144	~~
145	Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
146	A: 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	
156	Q: I have some very specific attribute (e.g. battery color), should I add
157	   this attribute to standard ones?
158	A: 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	
165	Q: 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.
169	A: 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.
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