About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / pwm.txt




Custom Search

Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 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		};
24	
25		static void __init board_init(void)
26		{
27			...
28			pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
29			...
30		}
31	
32	Using PWMs
33	----------
34	
35	Legacy users can request a PWM device using pwm_request() and free it
36	after usage with pwm_free().
37	
38	New users should use the pwm_get() function and pass to it the consumer
39	device or a consumer name. pwm_put() is used to free the PWM device. Managed
40	variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
41	
42	After being requested, a PWM has to be configured using:
43	
44	int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
45	
46	To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
47	
48	Using PWMs with the sysfs interface
49	-----------------------------------
50	
51	If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
52	interface is provided to use the PWMs from userspace. It is exposed at
53	/sys/class/pwm/. Each probed PWM controller/chip will be exported as
54	pwmchipN, where N is the base of the PWM chip. Inside the directory you
55	will find:
56	
57	npwm - The number of PWM channels this chip supports (read-only).
58	
59	export - Exports a PWM channel for use with sysfs (write-only).
60	
61	unexport - Unexports a PWM channel from sysfs (write-only).
62	
63	The PWM channels are numbered using a per-chip index from 0 to npwm-1.
64	
65	When a PWM channel is exported a pwmX directory will be created in the
66	pwmchipN directory it is associated with, where X is the number of the
67	channel that was exported. The following properties will then be available:
68	
69	period - The total period of the PWM signal (read/write).
70		Value is in nanoseconds and is the sum of the active and inactive
71		time of the PWM.
72	
73	duty_cycle - The active time of the PWM signal (read/write).
74		Value is in nanoseconds and must be less than the period.
75	
76	polarity - Changes the polarity of the PWM signal (read/write).
77		Writes to this property only work if the PWM chip supports changing
78		the polarity. The polarity can only be changed if the PWM is not
79		enabled. Value is the string "normal" or "inversed".
80	
81	enable - Enable/disable the PWM signal (read/write).
82		0 - disabled
83		1 - enabled
84	
85	Implementing a PWM driver
86	-------------------------
87	
88	Currently there are two ways to implement pwm drivers. Traditionally
89	there only has been the barebone API meaning that each driver has
90	to implement the pwm_*() functions itself. This means that it's impossible
91	to have multiple PWM drivers in the system. For this reason it's mandatory
92	for new drivers to use the generic PWM framework.
93	
94	A new PWM controller/chip can be added using pwmchip_add() and removed
95	again with pwmchip_remove(). pwmchip_add() takes a filled in struct
96	pwm_chip as argument which provides a description of the PWM chip, the
97	number of PWM devices provided by the chip and the chip-specific
98	implementation of the supported PWM operations to the framework.
99	
100	Locking
101	-------
102	
103	The PWM core list manipulations are protected by a mutex, so pwm_request()
104	and pwm_free() may not be called from an atomic context. Currently the
105	PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
106	pwm_config(), so the calling context is currently driver specific. This
107	is an issue derived from the former barebone API and should be fixed soon.
108	
109	Helpers
110	-------
111	
112	Currently a PWM can only be configured with period_ns and duty_ns. For several
113	use cases freq_hz and duty_percent might be better. Instead of calculating
114	this in your driver please consider adding appropriate helpers to the framework.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.