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 these functions, devm_pwm_get() and devm_pwm_put(), 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
 52The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
 53around pwm_apply_state() and should not be used if the user wants to change
 54several parameter at once. For example, if you see pwm_config() and
 55pwm_{enable,disable}() calls in the same function, this probably means you
 56should switch to pwm_apply_state().
 57
 58The PWM user API also allows one to query the PWM state with pwm_get_state().
 59
 60In addition to the PWM state, the PWM API also exposes PWM arguments, which
 61are the reference PWM config one should use on this PWM.
 62PWM arguments are usually platform-specific and allows the PWM user to only
 63care about dutycycle relatively to the full period (like, duty = 50% of the
 64period). struct pwm_args contains 2 fields (period and polarity) and should
 65be used to set the initial PWM config (usually done in the probe function
 66of the PWM user). PWM arguments are retrieved with pwm_get_args().
 67
 68Using PWMs with the sysfs interface
 69-----------------------------------
 70
 71If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
 72interface 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
 74pwmchipN, where N is the base of the PWM chip. Inside the directory you
 75will 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
 86The PWM channels are numbered using a per-chip index from 0 to npwm-1.
 87
 88When a PWM channel is exported a pwmX directory will be created in the
 89pwmchipN directory it is associated with, where X is the number of the
 90channel 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
113Implementing a PWM driver
114-------------------------
115
116Currently there are two ways to implement pwm drivers. Traditionally
117there only has been the barebone API meaning that each driver has
118to implement the pwm_*() functions itself. This means that it's impossible
119to have multiple PWM drivers in the system. For this reason it's mandatory
120for new drivers to use the generic PWM framework.
121
122A new PWM controller/chip can be added using pwmchip_add() and removed
123again with pwmchip_remove(). pwmchip_add() takes a filled in struct
124pwm_chip as argument which provides a description of the PWM chip, the
125number of PWM devices provided by the chip and the chip-specific
126implementation of the supported PWM operations to the framework.
127
128When implementing polarity support in a PWM driver, make sure to respect the
129signal conventions in the PWM framework. By definition, normal polarity
130characterizes a signal starts high for the duration of the duty cycle and
131goes low for the remainder of the period. Conversely, a signal with inversed
132polarity starts low for the duration of the duty cycle and goes high for the
133remainder of the period.
134
135Drivers are encouraged to implement ->apply() instead of the legacy
136->enable(), ->disable() and ->config() methods. Doing that should provide
137atomicity in the PWM config workflow, which is required when the PWM controls
138a critical device (like a regulator).
139
140The implementation of ->get_state() (a method used to retrieve initial PWM
141state) is also encouraged for the same reason: letting the PWM user know
142about the current PWM state would allow him to avoid glitches.
143
144Locking
145-------
146
147The PWM core list manipulations are protected by a mutex, so pwm_request()
148and pwm_free() may not be called from an atomic context. Currently the
149PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
150pwm_config(), so the calling context is currently driver specific. This
151is an issue derived from the former barebone API and should be fixed soon.
152
153Helpers
154-------
155
156Currently a PWM can only be configured with period_ns and duty_ns. For several
157use cases freq_hz and duty_percent might be better. Instead of calculating
158this in your driver please consider adding appropriate helpers to the framework.