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 48Implementing a PWM driver 49------------------------- 50 51Currently there are two ways to implement pwm drivers. Traditionally 52there only has been the barebone API meaning that each driver has 53to implement the pwm_*() functions itself. This means that it's impossible 54to have multiple PWM drivers in the system. For this reason it's mandatory 55for new drivers to use the generic PWM framework. 56 57A new PWM controller/chip can be added using pwmchip_add() and removed 58again with pwmchip_remove(). pwmchip_add() takes a filled in struct 59pwm_chip as argument which provides a description of the PWM chip, the 60number of PWM devices provider by the chip and the chip-specific 61implementation of the supported PWM operations to the framework. 62 63Locking 64------- 65 66The PWM core list manipulations are protected by a mutex, so pwm_request() 67and pwm_free() may not be called from an atomic context. Currently the 68PWM core does not enforce any locking to pwm_enable(), pwm_disable() and 69pwm_config(), so the calling context is currently driver specific. This 70is an issue derived from the former barebone API and should be fixed soon. 71 72Helpers 73------- 74 75Currently a PWM can only be configured with period_ns and duty_ns. For several 76use cases freq_hz and duty_percent might be better. Instead of calculating 77this in your driver please consider adding appropriate helpers to the framework. 78