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