Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 ====================================== 2 Pulse Width Modulation (PWM) interface 3 ====================================== 4 5 This provides an overview about the Linux PWM interface 6 7 PWMs are commonly used for controlling LEDs, fans or vibrators in 8 cell phones. PWMs with a fixed purpose have no need implementing 9 the Linux PWM API (although they could). However, PWMs are often 10 found as discrete devices on SoCs which have no fixed purpose. It's 11 up to the board designer to connect them to LEDs or fans. To provide 12 this kind of flexibility the generic PWM API exists. 13 14 Identifying PWMs 15 ---------------- 16 17 Users of the legacy PWM API use unique IDs to refer to PWM devices. 18 19 Instead of referring to a PWM device via its unique ID, board setup code 20 should instead register a static mapping that can be used to match PWM 21 consumers 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 35 Using PWMs 36 ---------- 37 38 Legacy users can request a PWM device using pwm_request() and free it 39 after usage with pwm_free(). 40 41 New users should use the pwm_get() function and pass to it the consumer 42 device or a consumer name. pwm_put() is used to free the PWM device. Managed 43 variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist. 44 45 After being requested, a PWM has to be configured using:: 46 47 int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state); 48 49 This API controls both the PWM period/duty_cycle config and the 50 enable/disable state. 51 52 The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers 53 around pwm_apply_state() and should not be used if the user wants to change 54 several parameter at once. For example, if you see pwm_config() and 55 pwm_{enable,disable}() calls in the same function, this probably means you 56 should switch to pwm_apply_state(). 57 58 The PWM user API also allows one to query the PWM state with pwm_get_state(). 59 60 In addition to the PWM state, the PWM API also exposes PWM arguments, which 61 are the reference PWM config one should use on this PWM. 62 PWM arguments are usually platform-specific and allows the PWM user to only 63 care about dutycycle relatively to the full period (like, duty = 50% of the 64 period). struct pwm_args contains 2 fields (period and polarity) and should 65 be used to set the initial PWM config (usually done in the probe function 66 of the PWM user). PWM arguments are retrieved with pwm_get_args(). 67 68 Using PWMs with the sysfs interface 69 ----------------------------------- 70 71 If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs 72 interface is provided to use the PWMs from userspace. It is exposed at 73 /sys/class/pwm/. Each probed PWM controller/chip will be exported as 74 pwmchipN, where N is the base of the PWM chip. Inside the directory you 75 will find: 76 77 npwm 78 The number of PWM channels this chip supports (read-only). 79 80 export 81 Exports a PWM channel for use with sysfs (write-only). 82 83 unexport 84 Unexports a PWM channel from sysfs (write-only). 85 86 The PWM channels are numbered using a per-chip index from 0 to npwm-1. 87 88 When a PWM channel is exported a pwmX directory will be created in the 89 pwmchipN directory it is associated with, where X is the number of the 90 channel that was exported. The following properties will then be available: 91 92 period 93 The total period of the PWM signal (read/write). 94 Value is in nanoseconds and is the sum of the active and inactive 95 time of the PWM. 96 97 duty_cycle 98 The active time of the PWM signal (read/write). 99 Value is in nanoseconds and must be less than the period. 100 101 polarity 102 Changes the polarity of the PWM signal (read/write). 103 Writes to this property only work if the PWM chip supports changing 104 the polarity. The polarity can only be changed if the PWM is not 105 enabled. Value is the string "normal" or "inversed". 106 107 enable 108 Enable/disable the PWM signal (read/write). 109 110 - 0 - disabled 111 - 1 - enabled 112 113 Implementing a PWM driver 114 ------------------------- 115 116 Currently there are two ways to implement pwm drivers. Traditionally 117 there only has been the barebone API meaning that each driver has 118 to implement the pwm_*() functions itself. This means that it's impossible 119 to have multiple PWM drivers in the system. For this reason it's mandatory 120 for new drivers to use the generic PWM framework. 121 122 A new PWM controller/chip can be added using pwmchip_add() and removed 123 again with pwmchip_remove(). pwmchip_add() takes a filled in struct 124 pwm_chip as argument which provides a description of the PWM chip, the 125 number of PWM devices provided by the chip and the chip-specific 126 implementation of the supported PWM operations to the framework. 127 128 When implementing polarity support in a PWM driver, make sure to respect the 129 signal conventions in the PWM framework. By definition, normal polarity 130 characterizes a signal starts high for the duration of the duty cycle and 131 goes low for the remainder of the period. Conversely, a signal with inversed 132 polarity starts low for the duration of the duty cycle and goes high for the 133 remainder of the period. 134 135 Drivers are encouraged to implement ->apply() instead of the legacy 136 ->enable(), ->disable() and ->config() methods. Doing that should provide 137 atomicity in the PWM config workflow, which is required when the PWM controls 138 a critical device (like a regulator). 139 140 The implementation of ->get_state() (a method used to retrieve initial PWM 141 state) is also encouraged for the same reason: letting the PWM user know 142 about the current PWM state would allow him to avoid glitches. 143 144 Locking 145 ------- 146 147 The PWM core list manipulations are protected by a mutex, so pwm_request() 148 and pwm_free() may not be called from an atomic context. Currently the 149 PWM core does not enforce any locking to pwm_enable(), pwm_disable() and 150 pwm_config(), so the calling context is currently driver specific. This 151 is an issue derived from the former barebone API and should be fixed soon. 152 153 Helpers 154 ------- 155 156 Currently a PWM can only be configured with period_ns and duty_ns. For several 157 use cases freq_hz and duty_percent might be better. Instead of calculating 158 this in your driver please consider adding appropriate helpers to the framework.