pwm.rst (7488B)
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(), devm_of_pwm_get(), 44devm_fwnode_pwm_get(), also exist. 45 46After being requested, a PWM has to be configured using:: 47 48 int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state); 49 50This API controls both the PWM period/duty_cycle config and the 51enable/disable state. 52 53As a consumer, don't rely on the output's state for a disabled PWM. If it's 54easily possible, drivers are supposed to emit the inactive state, but some 55drivers cannot. If you rely on getting the inactive state, use .duty_cycle=0, 56.enabled=true. 57 58There is also a usage_power setting: If set, the PWM driver is only required to 59maintain the power output but has more freedom regarding signal form. 60If supported by the driver, the signal can be optimized, for example to improve 61EMI by phase shifting the individual channels of a chip. 62 63The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers 64around pwm_apply_state() and should not be used if the user wants to change 65several parameter at once. For example, if you see pwm_config() and 66pwm_{enable,disable}() calls in the same function, this probably means you 67should switch to pwm_apply_state(). 68 69The PWM user API also allows one to query the PWM state that was passed to the 70last invocation of pwm_apply_state() using pwm_get_state(). Note this is 71different to what the driver has actually implemented if the request cannot be 72satisfied exactly with the hardware in use. There is currently no way for 73consumers to get the actually implemented settings. 74 75In addition to the PWM state, the PWM API also exposes PWM arguments, which 76are the reference PWM config one should use on this PWM. 77PWM arguments are usually platform-specific and allows the PWM user to only 78care about dutycycle relatively to the full period (like, duty = 50% of the 79period). struct pwm_args contains 2 fields (period and polarity) and should 80be used to set the initial PWM config (usually done in the probe function 81of the PWM user). PWM arguments are retrieved with pwm_get_args(). 82 83All consumers should really be reconfiguring the PWM upon resume as 84appropriate. This is the only way to ensure that everything is resumed in 85the proper order. 86 87Using PWMs with the sysfs interface 88----------------------------------- 89 90If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs 91interface is provided to use the PWMs from userspace. It is exposed at 92/sys/class/pwm/. Each probed PWM controller/chip will be exported as 93pwmchipN, where N is the base of the PWM chip. Inside the directory you 94will find: 95 96 npwm 97 The number of PWM channels this chip supports (read-only). 98 99 export 100 Exports a PWM channel for use with sysfs (write-only). 101 102 unexport 103 Unexports a PWM channel from sysfs (write-only). 104 105The PWM channels are numbered using a per-chip index from 0 to npwm-1. 106 107When a PWM channel is exported a pwmX directory will be created in the 108pwmchipN directory it is associated with, where X is the number of the 109channel that was exported. The following properties will then be available: 110 111 period 112 The total period of the PWM signal (read/write). 113 Value is in nanoseconds and is the sum of the active and inactive 114 time of the PWM. 115 116 duty_cycle 117 The active time of the PWM signal (read/write). 118 Value is in nanoseconds and must be less than the period. 119 120 polarity 121 Changes the polarity of the PWM signal (read/write). 122 Writes to this property only work if the PWM chip supports changing 123 the polarity. The polarity can only be changed if the PWM is not 124 enabled. Value is the string "normal" or "inversed". 125 126 enable 127 Enable/disable the PWM signal (read/write). 128 129 - 0 - disabled 130 - 1 - enabled 131 132Implementing a PWM driver 133------------------------- 134 135Currently there are two ways to implement pwm drivers. Traditionally 136there only has been the barebone API meaning that each driver has 137to implement the pwm_*() functions itself. This means that it's impossible 138to have multiple PWM drivers in the system. For this reason it's mandatory 139for new drivers to use the generic PWM framework. 140 141A new PWM controller/chip can be added using pwmchip_add() and removed 142again with pwmchip_remove(). pwmchip_add() takes a filled in struct 143pwm_chip as argument which provides a description of the PWM chip, the 144number of PWM devices provided by the chip and the chip-specific 145implementation of the supported PWM operations to the framework. 146 147When implementing polarity support in a PWM driver, make sure to respect the 148signal conventions in the PWM framework. By definition, normal polarity 149characterizes a signal starts high for the duration of the duty cycle and 150goes low for the remainder of the period. Conversely, a signal with inversed 151polarity starts low for the duration of the duty cycle and goes high for the 152remainder of the period. 153 154Drivers are encouraged to implement ->apply() instead of the legacy 155->enable(), ->disable() and ->config() methods. Doing that should provide 156atomicity in the PWM config workflow, which is required when the PWM controls 157a critical device (like a regulator). 158 159The implementation of ->get_state() (a method used to retrieve initial PWM 160state) is also encouraged for the same reason: letting the PWM user know 161about the current PWM state would allow him to avoid glitches. 162 163Drivers should not implement any power management. In other words, 164consumers should implement it as described in the "Using PWMs" section. 165 166Locking 167------- 168 169The PWM core list manipulations are protected by a mutex, so pwm_request() 170and pwm_free() may not be called from an atomic context. Currently the 171PWM core does not enforce any locking to pwm_enable(), pwm_disable() and 172pwm_config(), so the calling context is currently driver specific. This 173is an issue derived from the former barebone API and should be fixed soon. 174 175Helpers 176------- 177 178Currently a PWM can only be configured with period_ns and duty_ns. For several 179use cases freq_hz and duty_percent might be better. Instead of calculating 180this in your driver please consider adding appropriate helpers to the framework.