1====================================== 2Pulse Width Modulation (PWM) interface 3====================================== 4 5This provides an overview about the Linux PWM interface 6 7PWMs are commonly used for controlling LEDs, fans or vibrators in 8cell phones. PWMs with a fixed purpose have no need implementing 9the Linux PWM API (although they could). However, PWMs are often 10found as discrete devices on SoCs which have no fixed purpose. It's 11up to the board designer to connect them to LEDs or fans. To provide 12this kind of flexibility the generic PWM API exists. 13 14Identifying PWMs 15---------------- 16 17Users of the legacy PWM API use unique IDs to refer to PWM devices. 18 19Instead of referring to a PWM device via its unique ID, board setup code 20should instead register a static mapping that can be used to match PWM 21consumers to providers, as given in the following example:: 22 23 static struct pwm_lookup board_pwm_lookup[] = { 24 PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL, 25 50000, PWM_POLARITY_NORMAL), 26 }; 27 28 static void __init board_init(void) 29 { 30 ... 31 pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup)); 32 ... 33 } 34 35Using PWMs 36---------- 37 38Consumers use the pwm_get() function and pass to it the consumer device or a 39consumer name. pwm_put() is used to free the PWM device. Managed variants of 40the getter, devm_pwm_get() and devm_fwnode_pwm_get(), also exist. 41 42After being requested, a PWM has to be configured using:: 43 44 int pwm_apply_might_sleep(struct pwm_device *pwm, struct pwm_state *state); 45 46This API controls both the PWM period/duty_cycle config and the 47enable/disable state. 48 49PWM devices can be used from atomic context, if the PWM does not sleep. You 50can check if this the case with:: 51 52 bool pwm_might_sleep(struct pwm_device *pwm); 53 54If false, the PWM can also be configured from atomic context with:: 55 56 int pwm_apply_atomic(struct pwm_device *pwm, struct pwm_state *state); 57 58As a consumer, don't rely on the output's state for a disabled PWM. If it's 59easily possible, drivers are supposed to emit the inactive state, but some 60drivers cannot. If you rely on getting the inactive state, use .duty_cycle=0, 61.enabled=true. 62 63There is also a usage_power setting: If set, the PWM driver is only required to 64maintain the power output but has more freedom regarding signal form. 65If supported by the driver, the signal can be optimized, for example to improve 66EMI by phase shifting the individual channels of a chip. 67 68The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers 69around pwm_apply_might_sleep() and should not be used if the user wants to change 70several parameter at once. For example, if you see pwm_config() and 71pwm_{enable,disable}() calls in the same function, this probably means you 72should switch to pwm_apply_might_sleep(). 73 74The PWM user API also allows one to query the PWM state that was passed to the 75last invocation of pwm_apply_might_sleep() using pwm_get_state(). Note this is 76different to what the driver has actually implemented if the request cannot be 77satisfied exactly with the hardware in use. There is currently no way for 78consumers to get the actually implemented settings. 79 80In addition to the PWM state, the PWM API also exposes PWM arguments, which 81are the reference PWM config one should use on this PWM. 82PWM arguments are usually platform-specific and allows the PWM user to only 83care about dutycycle relatively to the full period (like, duty = 50% of the 84period). struct pwm_args contains 2 fields (period and polarity) and should 85be used to set the initial PWM config (usually done in the probe function 86of the PWM user). PWM arguments are retrieved with pwm_get_args(). 87 88All consumers should really be reconfiguring the PWM upon resume as 89appropriate. This is the only way to ensure that everything is resumed in 90the proper order. 91 92Using PWMs with the sysfs interface 93----------------------------------- 94 95If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs 96interface is provided to use the PWMs from userspace. It is exposed at 97/sys/class/pwm/. Each probed PWM controller/chip will be exported as 98pwmchipN, where N is the base of the PWM chip. Inside the directory you 99will find: 100 101 npwm 102 The number of PWM channels this chip supports (read-only). 103 104 export 105 Exports a PWM channel for use with sysfs (write-only). 106 107 unexport 108 Unexports a PWM channel from sysfs (write-only). 109 110The PWM channels are numbered using a per-chip index from 0 to npwm-1. 111 112When a PWM channel is exported a pwmX directory will be created in the 113pwmchipN directory it is associated with, where X is the number of the 114channel that was exported. The following properties will then be available: 115 116 period 117 The total period of the PWM signal (read/write). 118 Value is in nanoseconds and is the sum of the active and inactive 119 time of the PWM. 120 121 duty_cycle 122 The active time of the PWM signal (read/write). 123 Value is in nanoseconds and must be less than or equal to the period. 124 125 polarity 126 Changes the polarity of the PWM signal (read/write). 127 Writes to this property only work if the PWM chip supports changing 128 the polarity. 129 Value is the string "normal" or "inversed". 130 131 enable 132 Enable/disable the PWM signal (read/write). 133 134 - 0 - disabled 135 - 1 - enabled 136 137Implementing a PWM driver 138------------------------- 139 140Currently there are two ways to implement pwm drivers. Traditionally 141there only has been the barebone API meaning that each driver has 142to implement the pwm_*() functions itself. This means that it's impossible 143to have multiple PWM drivers in the system. For this reason it's mandatory 144for new drivers to use the generic PWM framework. 145 146A new PWM controller/chip can be added using pwmchip_add() and removed 147again with pwmchip_remove(). pwmchip_add() takes a filled in struct 148pwm_chip as argument which provides a description of the PWM chip, the 149number of PWM devices provided by the chip and the chip-specific 150implementation of the supported PWM operations to the framework. 151 152When implementing polarity support in a PWM driver, make sure to respect the 153signal conventions in the PWM framework. By definition, normal polarity 154characterizes a signal starts high for the duration of the duty cycle and 155goes low for the remainder of the period. Conversely, a signal with inversed 156polarity starts low for the duration of the duty cycle and goes high for the 157remainder of the period. 158 159Drivers are encouraged to implement ->apply() instead of the legacy 160->enable(), ->disable() and ->config() methods. Doing that should provide 161atomicity in the PWM config workflow, which is required when the PWM controls 162a critical device (like a regulator). 163 164The implementation of ->get_state() (a method used to retrieve initial PWM 165state) is also encouraged for the same reason: letting the PWM user know 166about the current PWM state would allow him to avoid glitches. 167 168Drivers should not implement any power management. In other words, 169consumers should implement it as described in the "Using PWMs" section. 170 171Locking 172------- 173 174The PWM core list manipulations are protected by a mutex, so pwm_get() 175and pwm_put() may not be called from an atomic context. Currently the 176PWM core does not enforce any locking to pwm_enable(), pwm_disable() and 177pwm_config(), so the calling context is currently driver specific. This 178is an issue derived from the former barebone API and should be fixed soon. 179 180Helpers 181------- 182 183Currently a PWM can only be configured with period_ns and duty_ns. For several 184use cases freq_hz and duty_percent might be better. Instead of calculating 185this in your driver please consider adding appropriate helpers to the framework. 186