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() and devm_fwnode_pwm_get(), 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
52As a consumer, don't rely on the output's state for a disabled PWM. If it's
53easily possible, drivers are supposed to emit the inactive state, but some
54drivers cannot. If you rely on getting the inactive state, use .duty_cycle=0,
55.enabled=true.
56
57There is also a usage_power setting: If set, the PWM driver is only required to
58maintain the power output but has more freedom regarding signal form.
59If supported by the driver, the signal can be optimized, for example to improve
60EMI by phase shifting the individual channels of a chip.
61
62The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
63around pwm_apply_state() and should not be used if the user wants to change
64several parameter at once. For example, if you see pwm_config() and
65pwm_{enable,disable}() calls in the same function, this probably means you
66should switch to pwm_apply_state().
67
68The PWM user API also allows one to query the PWM state that was passed to the
69last invocation of pwm_apply_state() using pwm_get_state(). Note this is
70different to what the driver has actually implemented if the request cannot be
71satisfied exactly with the hardware in use. There is currently no way for
72consumers to get the actually implemented settings.
73
74In addition to the PWM state, the PWM API also exposes PWM arguments, which
75are the reference PWM config one should use on this PWM.
76PWM arguments are usually platform-specific and allows the PWM user to only
77care about dutycycle relatively to the full period (like, duty = 50% of the
78period). struct pwm_args contains 2 fields (period and polarity) and should
79be used to set the initial PWM config (usually done in the probe function
80of the PWM user). PWM arguments are retrieved with pwm_get_args().
81
82All consumers should really be reconfiguring the PWM upon resume as
83appropriate. This is the only way to ensure that everything is resumed in
84the proper order.
85
86Using PWMs with the sysfs interface
87-----------------------------------
88
89If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
90interface is provided to use the PWMs from userspace. It is exposed at
91/sys/class/pwm/. Each probed PWM controller/chip will be exported as
92pwmchipN, where N is the base of the PWM chip. Inside the directory you
93will find:
94
95  npwm
96    The number of PWM channels this chip supports (read-only).
97
98  export
99    Exports a PWM channel for use with sysfs (write-only).
100
101  unexport
102   Unexports a PWM channel from sysfs (write-only).
103
104The PWM channels are numbered using a per-chip index from 0 to npwm-1.
105
106When a PWM channel is exported a pwmX directory will be created in the
107pwmchipN directory it is associated with, where X is the number of the
108channel that was exported. The following properties will then be available:
109
110  period
111    The total period of the PWM signal (read/write).
112    Value is in nanoseconds and is the sum of the active and inactive
113    time of the PWM.
114
115  duty_cycle
116    The active time of the PWM signal (read/write).
117    Value is in nanoseconds and must be less than the period.
118
119  polarity
120    Changes the polarity of the PWM signal (read/write).
121    Writes to this property only work if the PWM chip supports changing
122    the polarity. The polarity can only be changed if the PWM is not
123    enabled. Value is the string "normal" or "inversed".
124
125  enable
126    Enable/disable the PWM signal (read/write).
127
128	- 0 - disabled
129	- 1 - enabled
130
131Implementing a PWM driver
132-------------------------
133
134Currently there are two ways to implement pwm drivers. Traditionally
135there only has been the barebone API meaning that each driver has
136to implement the pwm_*() functions itself. This means that it's impossible
137to have multiple PWM drivers in the system. For this reason it's mandatory
138for new drivers to use the generic PWM framework.
139
140A new PWM controller/chip can be added using pwmchip_add() and removed
141again with pwmchip_remove(). pwmchip_add() takes a filled in struct
142pwm_chip as argument which provides a description of the PWM chip, the
143number of PWM devices provided by the chip and the chip-specific
144implementation of the supported PWM operations to the framework.
145
146When implementing polarity support in a PWM driver, make sure to respect the
147signal conventions in the PWM framework. By definition, normal polarity
148characterizes a signal starts high for the duration of the duty cycle and
149goes low for the remainder of the period. Conversely, a signal with inversed
150polarity starts low for the duration of the duty cycle and goes high for the
151remainder of the period.
152
153Drivers are encouraged to implement ->apply() instead of the legacy
154->enable(), ->disable() and ->config() methods. Doing that should provide
155atomicity in the PWM config workflow, which is required when the PWM controls
156a critical device (like a regulator).
157
158The implementation of ->get_state() (a method used to retrieve initial PWM
159state) is also encouraged for the same reason: letting the PWM user know
160about the current PWM state would allow him to avoid glitches.
161
162Drivers should not implement any power management. In other words,
163consumers should implement it as described in the "Using PWMs" section.
164
165Locking
166-------
167
168The PWM core list manipulations are protected by a mutex, so pwm_request()
169and pwm_free() may not be called from an atomic context. Currently the
170PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
171pwm_config(), so the calling context is currently driver specific. This
172is an issue derived from the former barebone API and should be fixed soon.
173
174Helpers
175-------
176
177Currently a PWM can only be configured with period_ns and duty_ns. For several
178use cases freq_hz and duty_percent might be better. Instead of calculating
179this in your driver please consider adding appropriate helpers to the framework.
180