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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	Pulse Width Modulation (PWM) interface
2	
3	This provides an overview about the Linux PWM interface
4	
5	PWMs are commonly used for controlling LEDs, fans or vibrators in
6	cell phones. PWMs with a fixed purpose have no need implementing
7	the Linux PWM API (although they could). However, PWMs are often
8	found as discrete devices on SoCs which have no fixed purpose. It's
9	up to the board designer to connect them to LEDs or fans. To provide
10	this kind of flexibility the generic PWM API exists.
11	
12	Identifying PWMs
13	----------------
14	
15	Users of the legacy PWM API use unique IDs to refer to PWM devices.
16	
17	Instead of referring to a PWM device via its unique ID, board setup code
18	should instead register a static mapping that can be used to match PWM
19	consumers to providers, as given in the following example:
20	
21		static struct pwm_lookup board_pwm_lookup[] = {
22			PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
23				   50000, PWM_POLARITY_NORMAL),
24		};
25	
26		static void __init board_init(void)
27		{
28			...
29			pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
30			...
31		}
32	
33	Using PWMs
34	----------
35	
36	Legacy users can request a PWM device using pwm_request() and free it
37	after usage with pwm_free().
38	
39	New users should use the pwm_get() function and pass to it the consumer
40	device or a consumer name. pwm_put() is used to free the PWM device. Managed
41	variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
42	
43	After being requested, a PWM has to be configured using:
44	
45	int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
46	
47	To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
48	
49	Using PWMs with the sysfs interface
50	-----------------------------------
51	
52	If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
53	interface is provided to use the PWMs from userspace. It is exposed at
54	/sys/class/pwm/. Each probed PWM controller/chip will be exported as
55	pwmchipN, where N is the base of the PWM chip. Inside the directory you
56	will find:
57	
58	npwm - The number of PWM channels this chip supports (read-only).
59	
60	export - Exports a PWM channel for use with sysfs (write-only).
61	
62	unexport - Unexports a PWM channel from sysfs (write-only).
63	
64	The PWM channels are numbered using a per-chip index from 0 to npwm-1.
65	
66	When a PWM channel is exported a pwmX directory will be created in the
67	pwmchipN directory it is associated with, where X is the number of the
68	channel that was exported. The following properties will then be available:
69	
70	period - The total period of the PWM signal (read/write).
71		Value is in nanoseconds and is the sum of the active and inactive
72		time of the PWM.
73	
74	duty_cycle - The active time of the PWM signal (read/write).
75		Value is in nanoseconds and must be less than the period.
76	
77	polarity - Changes the polarity of the PWM signal (read/write).
78		Writes to this property only work if the PWM chip supports changing
79		the polarity. The polarity can only be changed if the PWM is not
80		enabled. Value is the string "normal" or "inversed".
81	
82	enable - Enable/disable the PWM signal (read/write).
83		0 - disabled
84		1 - enabled
85	
86	Implementing a PWM driver
87	-------------------------
88	
89	Currently there are two ways to implement pwm drivers. Traditionally
90	there only has been the barebone API meaning that each driver has
91	to implement the pwm_*() functions itself. This means that it's impossible
92	to have multiple PWM drivers in the system. For this reason it's mandatory
93	for new drivers to use the generic PWM framework.
94	
95	A new PWM controller/chip can be added using pwmchip_add() and removed
96	again with pwmchip_remove(). pwmchip_add() takes a filled in struct
97	pwm_chip as argument which provides a description of the PWM chip, the
98	number of PWM devices provided by the chip and the chip-specific
99	implementation of the supported PWM operations to the framework.
100	
101	When implementing polarity support in a PWM driver, make sure to respect the
102	signal conventions in the PWM framework. By definition, normal polarity
103	characterizes a signal starts high for the duration of the duty cycle and
104	goes low for the remainder of the period. Conversely, a signal with inversed
105	polarity starts low for the duration of the duty cycle and goes high for the
106	remainder of the period.
107	
108	Locking
109	-------
110	
111	The PWM core list manipulations are protected by a mutex, so pwm_request()
112	and pwm_free() may not be called from an atomic context. Currently the
113	PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
114	pwm_config(), so the calling context is currently driver specific. This
115	is an issue derived from the former barebone API and should be fixed soon.
116	
117	Helpers
118	-------
119	
120	Currently a PWM can only be configured with period_ns and duty_ns. For several
121	use cases freq_hz and duty_percent might be better. Instead of calculating
122	this in your driver please consider adding appropriate helpers to the framework.
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