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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	======================================
2	Pulse Width Modulation (PWM) interface
3	======================================
4	
5	This provides an overview about the Linux PWM interface
6	
7	PWMs are commonly used for controlling LEDs, fans or vibrators in
8	cell phones. PWMs with a fixed purpose have no need implementing
9	the Linux PWM API (although they could). However, PWMs are often
10	found as discrete devices on SoCs which have no fixed purpose. It's
11	up to the board designer to connect them to LEDs or fans. To provide
12	this kind of flexibility the generic PWM API exists.
13	
14	Identifying PWMs
15	----------------
16	
17	Users of the legacy PWM API use unique IDs to refer to PWM devices.
18	
19	Instead of referring to a PWM device via its unique ID, board setup code
20	should instead register a static mapping that can be used to match PWM
21	consumers to providers, as given in the following example::
22	
23		static struct pwm_lookup board_pwm_lookup[] = {
24			PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
25				   50000, PWM_POLARITY_NORMAL),
26		};
27	
28		static void __init board_init(void)
29		{
30			...
31			pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
32			...
33		}
34	
35	Using PWMs
36	----------
37	
38	Legacy users can request a PWM device using pwm_request() and free it
39	after usage with pwm_free().
40	
41	New users should use the pwm_get() function and pass to it the consumer
42	device or a consumer name. pwm_put() is used to free the PWM device. Managed
43	variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
44	
45	After being requested, a PWM has to be configured using::
46	
47		int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
48	
49	This API controls both the PWM period/duty_cycle config and the
50	enable/disable state.
51	
52	The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
53	around pwm_apply_state() and should not be used if the user wants to change
54	several parameter at once. For example, if you see pwm_config() and
55	pwm_{enable,disable}() calls in the same function, this probably means you
56	should switch to pwm_apply_state().
57	
58	The PWM user API also allows one to query the PWM state with pwm_get_state().
59	
60	In addition to the PWM state, the PWM API also exposes PWM arguments, which
61	are the reference PWM config one should use on this PWM.
62	PWM arguments are usually platform-specific and allows the PWM user to only
63	care about dutycycle relatively to the full period (like, duty = 50% of the
64	period). struct pwm_args contains 2 fields (period and polarity) and should
65	be used to set the initial PWM config (usually done in the probe function
66	of the PWM user). PWM arguments are retrieved with pwm_get_args().
67	
68	Using PWMs with the sysfs interface
69	-----------------------------------
70	
71	If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
72	interface is provided to use the PWMs from userspace. It is exposed at
73	/sys/class/pwm/. Each probed PWM controller/chip will be exported as
74	pwmchipN, where N is the base of the PWM chip. Inside the directory you
75	will find:
76	
77	  npwm
78	    The number of PWM channels this chip supports (read-only).
79	
80	  export
81	    Exports a PWM channel for use with sysfs (write-only).
82	
83	  unexport
84	   Unexports a PWM channel from sysfs (write-only).
85	
86	The PWM channels are numbered using a per-chip index from 0 to npwm-1.
87	
88	When a PWM channel is exported a pwmX directory will be created in the
89	pwmchipN directory it is associated with, where X is the number of the
90	channel that was exported. The following properties will then be available:
91	
92	  period
93	    The total period of the PWM signal (read/write).
94	    Value is in nanoseconds and is the sum of the active and inactive
95	    time of the PWM.
96	
97	  duty_cycle
98	    The active time of the PWM signal (read/write).
99	    Value is in nanoseconds and must be less than the period.
100	
101	  polarity
102	    Changes the polarity of the PWM signal (read/write).
103	    Writes to this property only work if the PWM chip supports changing
104	    the polarity. The polarity can only be changed if the PWM is not
105	    enabled. Value is the string "normal" or "inversed".
106	
107	  enable
108	    Enable/disable the PWM signal (read/write).
109	
110		- 0 - disabled
111		- 1 - enabled
112	
113	Implementing a PWM driver
114	-------------------------
115	
116	Currently there are two ways to implement pwm drivers. Traditionally
117	there only has been the barebone API meaning that each driver has
118	to implement the pwm_*() functions itself. This means that it's impossible
119	to have multiple PWM drivers in the system. For this reason it's mandatory
120	for new drivers to use the generic PWM framework.
121	
122	A new PWM controller/chip can be added using pwmchip_add() and removed
123	again with pwmchip_remove(). pwmchip_add() takes a filled in struct
124	pwm_chip as argument which provides a description of the PWM chip, the
125	number of PWM devices provided by the chip and the chip-specific
126	implementation of the supported PWM operations to the framework.
127	
128	When implementing polarity support in a PWM driver, make sure to respect the
129	signal conventions in the PWM framework. By definition, normal polarity
130	characterizes a signal starts high for the duration of the duty cycle and
131	goes low for the remainder of the period. Conversely, a signal with inversed
132	polarity starts low for the duration of the duty cycle and goes high for the
133	remainder of the period.
134	
135	Drivers are encouraged to implement ->apply() instead of the legacy
136	->enable(), ->disable() and ->config() methods. Doing that should provide
137	atomicity in the PWM config workflow, which is required when the PWM controls
138	a critical device (like a regulator).
139	
140	The implementation of ->get_state() (a method used to retrieve initial PWM
141	state) is also encouraged for the same reason: letting the PWM user know
142	about the current PWM state would allow him to avoid glitches.
143	
144	Locking
145	-------
146	
147	The PWM core list manipulations are protected by a mutex, so pwm_request()
148	and pwm_free() may not be called from an atomic context. Currently the
149	PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
150	pwm_config(), so the calling context is currently driver specific. This
151	is an issue derived from the former barebone API and should be fixed soon.
152	
153	Helpers
154	-------
155	
156	Currently a PWM can only be configured with period_ns and duty_ns. For several
157	use cases freq_hz and duty_percent might be better. Instead of calculating
158	this in your driver please consider adding appropriate helpers to the framework.
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