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Sascha Hauer0c2498f2011-01-28 09:40:40 +01001Pulse 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
Thierry Reding8138d2d2012-03-26 08:42:48 +020015Users 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 };
24
25 static void __init board_init(void)
26 {
27 ...
28 pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
29 ...
30 }
Sascha Hauer0c2498f2011-01-28 09:40:40 +010031
32Using PWMs
33----------
34
Thierry Reding8138d2d2012-03-26 08:42:48 +020035Legacy users can request a PWM device using pwm_request() and free it
36after usage with pwm_free().
37
38New users should use the pwm_get() function and pass to it the consumer
Alexandre Courbot63543162012-08-01 19:20:58 +090039device or a consumer name. pwm_put() is used to free the PWM device. Managed
40variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
Thierry Reding8138d2d2012-03-26 08:42:48 +020041
Sachin Kamat702e3042013-10-24 14:13:47 +053042After being requested, a PWM has to be configured using:
Sascha Hauer0c2498f2011-01-28 09:40:40 +010043
44int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
45
46To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
47
H Hartley Sweeten76abbdde2013-06-11 10:38:59 -070048Using PWMs with the sysfs interface
49-----------------------------------
50
51If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
52interface 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
54pwmchipN, where N is the base of the PWM chip. Inside the directory you
55will find:
56
57npwm - The number of PWM channels this chip supports (read-only).
58
59export - Exports a PWM channel for use with sysfs (write-only).
60
61unexport - Unexports a PWM channel from sysfs (write-only).
62
63The PWM channels are numbered using a per-chip index from 0 to npwm-1.
64
65When a PWM channel is exported a pwmX directory will be created in the
66pwmchipN directory it is associated with, where X is the number of the
67channel that was exported. The following properties will then be available:
68
69period - 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
73duty_cycle - The active time of the PWM signal (read/write).
74 Value is in nanoseconds and must be less than the period.
75
76polarity - 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
81enable - Enable/disable the PWM signal (read/write).
82 0 - disabled
83 1 - enabled
84
Sascha Hauer0c2498f2011-01-28 09:40:40 +010085Implementing a PWM driver
86-------------------------
87
88Currently there are two ways to implement pwm drivers. Traditionally
89there only has been the barebone API meaning that each driver has
90to implement the pwm_*() functions itself. This means that it's impossible
91to have multiple PWM drivers in the system. For this reason it's mandatory
92for new drivers to use the generic PWM framework.
Thierry Redingf051c462011-12-14 11:12:23 +010093
94A new PWM controller/chip can be added using pwmchip_add() and removed
95again with pwmchip_remove(). pwmchip_add() takes a filled in struct
96pwm_chip as argument which provides a description of the PWM chip, the
Sachin Kamat702e3042013-10-24 14:13:47 +053097number of PWM devices provided by the chip and the chip-specific
Thierry Redingf051c462011-12-14 11:12:23 +010098implementation of the supported PWM operations to the framework.
Sascha Hauer0c2498f2011-01-28 09:40:40 +010099
100Locking
101-------
102
103The PWM core list manipulations are protected by a mutex, so pwm_request()
104and pwm_free() may not be called from an atomic context. Currently the
105PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
106pwm_config(), so the calling context is currently driver specific. This
107is an issue derived from the former barebone API and should be fixed soon.
108
109Helpers
110-------
111
112Currently a PWM can only be configured with period_ns and duty_ns. For several
113use cases freq_hz and duty_percent might be better. Instead of calculating
114this in your driver please consider adding appropriate helpers to the framework.