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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
39device or a consumer name. pwm_put() is used to free the PWM device.
40
41After being requested a PWM has to be configured using:
Sascha Hauer0c2498f2011-01-28 09:40:40 +010042
43int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
44
45To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
46
47Implementing a PWM driver
48-------------------------
49
50Currently there are two ways to implement pwm drivers. Traditionally
51there only has been the barebone API meaning that each driver has
52to implement the pwm_*() functions itself. This means that it's impossible
53to have multiple PWM drivers in the system. For this reason it's mandatory
54for new drivers to use the generic PWM framework.
Thierry Redingf051c462011-12-14 11:12:23 +010055
56A new PWM controller/chip can be added using pwmchip_add() and removed
57again with pwmchip_remove(). pwmchip_add() takes a filled in struct
58pwm_chip as argument which provides a description of the PWM chip, the
59number of PWM devices provider by the chip and the chip-specific
60implementation of the supported PWM operations to the framework.
Sascha Hauer0c2498f2011-01-28 09:40:40 +010061
62Locking
63-------
64
65The PWM core list manipulations are protected by a mutex, so pwm_request()
66and pwm_free() may not be called from an atomic context. Currently the
67PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
68pwm_config(), so the calling context is currently driver specific. This
69is an issue derived from the former barebone API and should be fixed soon.
70
71Helpers
72-------
73
74Currently a PWM can only be configured with period_ns and duty_ns. For several
75use cases freq_hz and duty_percent might be better. Instead of calculating
76this in your driver please consider adding appropriate helpers to the framework.