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Documentation / power / power_supply_class.txt




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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 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/supply/ds2760_battery.c and drivers/power/supply/pda_power.c
43	for the 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	ENERGY_FULL, ENERGY_EMPTY - same as above but for energy.
119	
120	CHARGE_COUNTER - the current charge counter (in µAh).  This could easily
121	be negative; there is no empty or full value.  It is only useful for
122	relative, time-based measurements.
123	
124	PRECHARGE_CURRENT - the maximum charge current during precharge phase
125	of charge cycle (typically 20% of battery capacity).
126	CHARGE_TERM_CURRENT - Charge termination current. The charge cycle
127	terminates when battery voltage is above recharge threshold, and charge
128	current is below this setting (typically 10% of battery capacity).
129	
130	CONSTANT_CHARGE_CURRENT - constant charge current programmed by charger.
131	CONSTANT_CHARGE_CURRENT_MAX - maximum charge current supported by the
132	power supply object.
133	
134	CONSTANT_CHARGE_VOLTAGE - constant charge voltage programmed by charger.
135	CONSTANT_CHARGE_VOLTAGE_MAX - maximum charge voltage supported by the
136	power supply object.
137	
138	INPUT_CURRENT_LIMIT - input current limit programmed by charger. Indicates
139	the current drawn from a charging source.
140	
141	CHARGE_CONTROL_LIMIT - current charge control limit setting
142	CHARGE_CONTROL_LIMIT_MAX - maximum charge control limit setting
143	
144	CALIBRATE - battery or coulomb counter calibration status
145	
146	CAPACITY - capacity in percents.
147	CAPACITY_ALERT_MIN - minimum capacity alert value in percents.
148	CAPACITY_ALERT_MAX - maximum capacity alert value in percents.
149	CAPACITY_LEVEL - capacity level. This corresponds to
150	POWER_SUPPLY_CAPACITY_LEVEL_*.
151	
152	TEMP - temperature of the power supply.
153	TEMP_ALERT_MIN - minimum battery temperature alert.
154	TEMP_ALERT_MAX - maximum battery temperature alert.
155	TEMP_AMBIENT - ambient temperature.
156	TEMP_AMBIENT_ALERT_MIN - minimum ambient temperature alert.
157	TEMP_AMBIENT_ALERT_MAX - maximum ambient temperature alert.
158	TEMP_MIN - minimum operatable temperature
159	TEMP_MAX - maximum operatable temperature
160	
161	TIME_TO_EMPTY - seconds left for battery to be considered empty (i.e.
162	while battery powers a load)
163	TIME_TO_FULL - seconds left for battery to be considered full (i.e.
164	while battery is charging)
165	
166	
167	Battery <-> external power supply interaction
168	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
169	Often power supplies are acting as supplies and supplicants at the same
170	time. Batteries are good example. So, batteries usually care if they're
171	externally powered or not.
172	
173	For that case, power supply class implements notification mechanism for
174	batteries.
175	
176	External power supply (AC) lists supplicants (batteries) names in
177	"supplied_to" struct member, and each power_supply_changed() call
178	issued by external power supply will notify supplicants via
179	external_power_changed callback.
180	
181	
182	Devicetree battery characteristics
183	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
184	Drivers should call power_supply_get_battery_info() to obtain battery
185	characteristics from a devicetree battery node, defined in
186	Documentation/devicetree/bindings/power/supply/battery.txt. This is
187	implemented in drivers/power/supply/bq27xxx_battery.c.
188	
189	Properties in struct power_supply_battery_info and their counterparts in the
190	battery node have names corresponding to elements in enum power_supply_property,
191	for naming consistency between sysfs attributes and battery node properties.
192	
193	
194	QA
195	~~
196	Q: Where is POWER_SUPPLY_PROP_XYZ attribute?
197	A: If you cannot find attribute suitable for your driver needs, feel free
198	   to add it and send patch along with your driver.
199	
200	   The attributes available currently are the ones currently provided by the
201	   drivers written.
202	
203	   Good candidates to add in future: model/part#, cycle_time, manufacturer,
204	   etc.
205	
206	
207	Q: I have some very specific attribute (e.g. battery color), should I add
208	   this attribute to standard ones?
209	A: Most likely, no. Such attribute can be placed in the driver itself, if
210	   it is useful. Of course, if the attribute in question applicable to
211	   large set of batteries, provided by many drivers, and/or comes from
212	   some general battery specification/standard, it may be a candidate to
213	   be added to the core attribute set.
214	
215	
216	Q: Suppose, my battery monitoring chip/firmware does not provides capacity
217	   in percents, but provides charge_{now,full,empty}. Should I calculate
218	   percentage capacity manually, inside the driver, and register CAPACITY
219	   attribute? The same question about time_to_empty/time_to_full.
220	A: Most likely, no. This class is designed to export properties which are
221	   directly measurable by the specific hardware available.
222	
223	   Inferring not available properties using some heuristics or mathematical
224	   model is not subject of work for a battery driver. Such functionality
225	   should be factored out, and in fact, apm_power, the driver to serve
226	   legacy APM API on top of power supply class, uses a simple heuristic of
227	   approximating remaining battery capacity based on its charge, current,
228	   voltage and so on. But full-fledged battery model is likely not subject
229	   for kernel at all, as it would require floating point calculation to deal
230	   with things like differential equations and Kalman filters. This is
231	   better be handled by batteryd/libbattery, yet to be written.
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