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