Sascha Hauer | 0c2498f | 2011-01-28 09:40:40 +0100 | [diff] [blame] | 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 | |
Thierry Reding | 8138d2d | 2012-03-26 08:42:48 +0200 | [diff] [blame] | 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 | } |
Sascha Hauer | 0c2498f | 2011-01-28 09:40:40 +0100 | [diff] [blame] | 31 | |
| 32 | Using PWMs |
| 33 | ---------- |
| 34 | |
Thierry Reding | 8138d2d | 2012-03-26 08:42:48 +0200 | [diff] [blame] | 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: |
Sascha Hauer | 0c2498f | 2011-01-28 09:40:40 +0100 | [diff] [blame] | 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. |
Thierry Reding | f051c46 | 2011-12-14 11:12:23 +0100 | [diff] [blame] | 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. |
Sascha Hauer | 0c2498f | 2011-01-28 09:40:40 +0100 | [diff] [blame] | 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. |