1Pulse Width Modulation (PWM) interface
2
3This provides an overview about the Linux PWM interface
4
5PWMs are commonly used for controlling LEDs, fans or vibrators in
6cell phones. PWMs with a fixed purpose have no need implementing
7the Linux PWM API (although they could). However, PWMs are often
8found as discrete devices on SoCs which have no fixed purpose. It's
9up to the board designer to connect them to LEDs or fans. To provide
10this kind of flexibility the generic PWM API exists.
11
12Identifying PWMs
13----------------
14
15Users of the legacy PWM API use unique IDs to refer to PWM devices.
16
17Instead of referring to a PWM device via its unique ID, board setup code
18should instead register a static mapping that can be used to match PWM
19consumers 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
33Using PWMs
34----------
35
36Legacy users can request a PWM device using pwm_request() and free it
37after usage with pwm_free().
38
39New users should use the pwm_get() function and pass to it the consumer
40device or a consumer name. pwm_put() is used to free the PWM device. Managed
41variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
42
43After being requested, a PWM has to be configured using:
44
45int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
46
47To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
48
49Using PWMs with the sysfs interface
50-----------------------------------
51
52If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
53interface 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
55pwmchipN, where N is the base of the PWM chip. Inside the directory you
56will find:
57
58npwm - The number of PWM channels this chip supports (read-only).
59
60export - Exports a PWM channel for use with sysfs (write-only).
61
62unexport - Unexports a PWM channel from sysfs (write-only).
63
64The PWM channels are numbered using a per-chip index from 0 to npwm-1.
65
66When a PWM channel is exported a pwmX directory will be created in the
67pwmchipN directory it is associated with, where X is the number of the
68channel that was exported. The following properties will then be available:
69
70period - 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
74duty_cycle - The active time of the PWM signal (read/write).
75	Value is in nanoseconds and must be less than the period.
76
77polarity - 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
82enable - Enable/disable the PWM signal (read/write).
83	0 - disabled
84	1 - enabled
85
86Implementing a PWM driver
87-------------------------
88
89Currently there are two ways to implement pwm drivers. Traditionally
90there only has been the barebone API meaning that each driver has
91to implement the pwm_*() functions itself. This means that it's impossible
92to have multiple PWM drivers in the system. For this reason it's mandatory
93for new drivers to use the generic PWM framework.
94
95A new PWM controller/chip can be added using pwmchip_add() and removed
96again with pwmchip_remove(). pwmchip_add() takes a filled in struct
97pwm_chip as argument which provides a description of the PWM chip, the
98number of PWM devices provided by the chip and the chip-specific
99implementation of the supported PWM operations to the framework.
100
101When implementing polarity support in a PWM driver, make sure to respect the
102signal conventions in the PWM framework. By definition, normal polarity
103characterizes a signal starts high for the duration of the duty cycle and
104goes low for the remainder of the period. Conversely, a signal with inversed
105polarity starts low for the duration of the duty cycle and goes high for the
106remainder of the period.
107
108Locking
109-------
110
111The PWM core list manipulations are protected by a mutex, so pwm_request()
112and pwm_free() may not be called from an atomic context. Currently the
113PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
114pwm_config(), so the calling context is currently driver specific. This
115is an issue derived from the former barebone API and should be fixed soon.
116
117Helpers
118-------
119
120Currently a PWM can only be configured with period_ns and duty_ns. For several
121use cases freq_hz and duty_percent might be better. Instead of calculating
122this in your driver please consider adding appropriate helpers to the framework.
123