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