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 }; 24 25 static void __init board_init(void) 26 { 27 ... 28 pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup)); 29 ... 30 } 31 32Using PWMs 33---------- 34 35Legacy 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. Managed 40variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist. 41 42After being requested a PWM has to be configured using: 43 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 48Using 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 85Implementing 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. 93 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 97number of PWM devices provider by the chip and the chip-specific 98implementation of the supported PWM operations to the framework. 99 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. 115