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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:47 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_apply_state(struct pwm_device *pwm, struct pwm_state *state);
46	
47	This API controls both the PWM period/duty_cycle config and the
48	enable/disable state.
49	
50	The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
51	around pwm_apply_state() and should not be used if the user wants to change
52	several parameter at once. For example, if you see pwm_config() and
53	pwm_{enable,disable}() calls in the same function, this probably means you
54	should switch to pwm_apply_state().
55	
56	The PWM user API also allows one to query the PWM state with pwm_get_state().
57	
58	In addition to the PWM state, the PWM API also exposes PWM arguments, which
59	are the reference PWM config one should use on this PWM.
60	PWM arguments are usually platform-specific and allows the PWM user to only
61	care about dutycycle relatively to the full period (like, duty = 50% of the
62	period). struct pwm_args contains 2 fields (period and polarity) and should
63	be used to set the initial PWM config (usually done in the probe function
64	of the PWM user). PWM arguments are retrieved with pwm_get_args().
65	
66	Using PWMs with the sysfs interface
67	-----------------------------------
68	
69	If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
70	interface is provided to use the PWMs from userspace. It is exposed at
71	/sys/class/pwm/. Each probed PWM controller/chip will be exported as
72	pwmchipN, where N is the base of the PWM chip. Inside the directory you
73	will find:
74	
75	npwm - The number of PWM channels this chip supports (read-only).
76	
77	export - Exports a PWM channel for use with sysfs (write-only).
78	
79	unexport - Unexports a PWM channel from sysfs (write-only).
80	
81	The PWM channels are numbered using a per-chip index from 0 to npwm-1.
82	
83	When a PWM channel is exported a pwmX directory will be created in the
84	pwmchipN directory it is associated with, where X is the number of the
85	channel that was exported. The following properties will then be available:
86	
87	period - The total period of the PWM signal (read/write).
88		Value is in nanoseconds and is the sum of the active and inactive
89		time of the PWM.
90	
91	duty_cycle - The active time of the PWM signal (read/write).
92		Value is in nanoseconds and must be less than the period.
93	
94	polarity - Changes the polarity of the PWM signal (read/write).
95		Writes to this property only work if the PWM chip supports changing
96		the polarity. The polarity can only be changed if the PWM is not
97		enabled. Value is the string "normal" or "inversed".
98	
99	enable - Enable/disable the PWM signal (read/write).
100		0 - disabled
101		1 - enabled
102	
103	Implementing a PWM driver
104	-------------------------
105	
106	Currently there are two ways to implement pwm drivers. Traditionally
107	there only has been the barebone API meaning that each driver has
108	to implement the pwm_*() functions itself. This means that it's impossible
109	to have multiple PWM drivers in the system. For this reason it's mandatory
110	for new drivers to use the generic PWM framework.
111	
112	A new PWM controller/chip can be added using pwmchip_add() and removed
113	again with pwmchip_remove(). pwmchip_add() takes a filled in struct
114	pwm_chip as argument which provides a description of the PWM chip, the
115	number of PWM devices provided by the chip and the chip-specific
116	implementation of the supported PWM operations to the framework.
117	
118	When implementing polarity support in a PWM driver, make sure to respect the
119	signal conventions in the PWM framework. By definition, normal polarity
120	characterizes a signal starts high for the duration of the duty cycle and
121	goes low for the remainder of the period. Conversely, a signal with inversed
122	polarity starts low for the duration of the duty cycle and goes high for the
123	remainder of the period.
124	
125	Drivers are encouraged to implement ->apply() instead of the legacy
126	->enable(), ->disable() and ->config() methods. Doing that should provide
127	atomicity in the PWM config workflow, which is required when the PWM controls
128	a critical device (like a regulator).
129	
130	The implementation of ->get_state() (a method used to retrieve initial PWM
131	state) is also encouraged for the same reason: letting the PWM user know
132	about the current PWM state would allow him to avoid glitches.
133	
134	Locking
135	-------
136	
137	The PWM core list manipulations are protected by a mutex, so pwm_request()
138	and pwm_free() may not be called from an atomic context. Currently the
139	PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
140	pwm_config(), so the calling context is currently driver specific. This
141	is an issue derived from the former barebone API and should be fixed soon.
142	
143	Helpers
144	-------
145	
146	Currently a PWM can only be configured with period_ns and duty_ns. For several
147	use cases freq_hz and duty_percent might be better. Instead of calculating
148	this in your driver please consider adding appropriate helpers to the framework.
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