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