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.
181