Commit | Line | Data |
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0c2498f1 SH |
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 | ||
8138d2dd TR |
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 | } | |
0c2498f1 SH |
31 | |
32 | Using PWMs | |
33 | ---------- | |
34 | ||
8138d2dd TR |
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. | |
40 | ||
41 | After being requested a PWM has to be configured using: | |
0c2498f1 SH |
42 | |
43 | int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns); | |
44 | ||
45 | To start/stop toggling the PWM output use pwm_enable()/pwm_disable(). | |
46 | ||
47 | Implementing a PWM driver | |
48 | ------------------------- | |
49 | ||
50 | Currently there are two ways to implement pwm drivers. Traditionally | |
51 | there only has been the barebone API meaning that each driver has | |
52 | to implement the pwm_*() functions itself. This means that it's impossible | |
53 | to have multiple PWM drivers in the system. For this reason it's mandatory | |
54 | for new drivers to use the generic PWM framework. | |
f051c466 TR |
55 | |
56 | A new PWM controller/chip can be added using pwmchip_add() and removed | |
57 | again with pwmchip_remove(). pwmchip_add() takes a filled in struct | |
58 | pwm_chip as argument which provides a description of the PWM chip, the | |
59 | number of PWM devices provider by the chip and the chip-specific | |
60 | implementation of the supported PWM operations to the framework. | |
0c2498f1 SH |
61 | |
62 | Locking | |
63 | ------- | |
64 | ||
65 | The PWM core list manipulations are protected by a mutex, so pwm_request() | |
66 | and pwm_free() may not be called from an atomic context. Currently the | |
67 | PWM core does not enforce any locking to pwm_enable(), pwm_disable() and | |
68 | pwm_config(), so the calling context is currently driver specific. This | |
69 | is an issue derived from the former barebone API and should be fixed soon. | |
70 | ||
71 | Helpers | |
72 | ------- | |
73 | ||
74 | Currently a PWM can only be configured with period_ns and duty_ns. For several | |
75 | use cases freq_hz and duty_percent might be better. Instead of calculating | |
76 | this in your driver please consider adding appropriate helpers to the framework. |