1Pulse Width Modulation (PWM) interface
  2
  3This provides an overview about the Linux PWM interface
  4
  5PWMs are commonly used for controlling LEDs, fans or vibrators in
  6cell phones. PWMs with a fixed purpose have no need implementing
  7the Linux PWM API (although they could). However, PWMs are often
  8found as discrete devices on SoCs which have no fixed purpose. It's
  9up to the board designer to connect them to LEDs or fans. To provide
 10this kind of flexibility the generic PWM API exists.
 11
 12Identifying PWMs
 13----------------
 14
 15Users of the legacy PWM API use unique IDs to refer to PWM devices.
 16
 17Instead of referring to a PWM device via its unique ID, board setup code
 18should instead register a static mapping that can be used to match PWM
 19consumers to providers, as given in the following example:
 20
 21	static struct pwm_lookup board_pwm_lookup[] = {
 22		PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
 23			   50000, PWM_POLARITY_NORMAL),
 24	};
 25
 26	static void __init board_init(void)
 27	{
 28		...
 29		pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
 30		...
 31	}
 32
 33Using PWMs
 34----------
 35
 36Legacy users can request a PWM device using pwm_request() and free it
 37after usage with pwm_free().
 38
 39New users should use the pwm_get() function and pass to it the consumer
 40device or a consumer name. pwm_put() is used to free the PWM device. Managed
 41variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
 42
 43After being requested, a PWM has to be configured using:
 44
 45int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
 46
 47To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
 48
 49Using PWMs with the sysfs interface
 50-----------------------------------
 51
 52If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
 53interface is provided to use the PWMs from userspace. It is exposed at
 54/sys/class/pwm/. Each probed PWM controller/chip will be exported as
 55pwmchipN, where N is the base of the PWM chip. Inside the directory you
 56will find:
 57
 58npwm - The number of PWM channels this chip supports (read-only).
 59
 60export - Exports a PWM channel for use with sysfs (write-only).
 61
 62unexport - Unexports a PWM channel from sysfs (write-only).
 63
 64The PWM channels are numbered using a per-chip index from 0 to npwm-1.
 65
 66When a PWM channel is exported a pwmX directory will be created in the
 67pwmchipN directory it is associated with, where X is the number of the
 68channel that was exported. The following properties will then be available:
 69
 70period - The total period of the PWM signal (read/write).
 71	Value is in nanoseconds and is the sum of the active and inactive
 72	time of the PWM.
 73
 74duty_cycle - The active time of the PWM signal (read/write).
 75	Value is in nanoseconds and must be less than the period.
 76
 77polarity - Changes the polarity of the PWM signal (read/write).
 78	Writes to this property only work if the PWM chip supports changing
 79	the polarity. The polarity can only be changed if the PWM is not
 80	enabled. Value is the string "normal" or "inversed".
 81
 82enable - Enable/disable the PWM signal (read/write).
 83	0 - disabled
 84	1 - enabled
 85
 86Implementing a PWM driver
 87-------------------------
 88
 89Currently there are two ways to implement pwm drivers. Traditionally
 90there only has been the barebone API meaning that each driver has
 91to implement the pwm_*() functions itself. This means that it's impossible
 92to have multiple PWM drivers in the system. For this reason it's mandatory
 93for new drivers to use the generic PWM framework.
 94
 95A new PWM controller/chip can be added using pwmchip_add() and removed
 96again with pwmchip_remove(). pwmchip_add() takes a filled in struct
 97pwm_chip as argument which provides a description of the PWM chip, the
 98number of PWM devices provided by the chip and the chip-specific
 99implementation of the supported PWM operations to the framework.
100
101When implementing polarity support in a PWM driver, make sure to respect the
102signal conventions in the PWM framework. By definition, normal polarity
103characterizes a signal starts high for the duration of the duty cycle and
104goes low for the remainder of the period. Conversely, a signal with inversed
105polarity starts low for the duration of the duty cycle and goes high for the
106remainder of the period.
107
108Locking
109-------
110
111The PWM core list manipulations are protected by a mutex, so pwm_request()
112and pwm_free() may not be called from an atomic context. Currently the
113PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
114pwm_config(), so the calling context is currently driver specific. This
115is an issue derived from the former barebone API and should be fixed soon.
116
117Helpers
118-------
119
120Currently a PWM can only be configured with period_ns and duty_ns. For several
121use cases freq_hz and duty_percent might be better. Instead of calculating
122this in your driver please consider adding appropriate helpers to the framework.