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
38Consumers use the pwm_get() function and pass to it the consumer device or a
39consumer name. pwm_put() is used to free the PWM device. Managed variants of
40the getter, devm_pwm_get() and devm_fwnode_pwm_get(), also exist.
41
42After being requested, a PWM has to be configured using::
43
44	int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
45
46This API controls both the PWM period/duty_cycle config and the
47enable/disable state.
48
49As a consumer, don't rely on the output's state for a disabled PWM. If it's
50easily possible, drivers are supposed to emit the inactive state, but some
51drivers cannot. If you rely on getting the inactive state, use .duty_cycle=0,
52.enabled=true.
53
54There is also a usage_power setting: If set, the PWM driver is only required to
55maintain the power output but has more freedom regarding signal form.
56If supported by the driver, the signal can be optimized, for example to improve
57EMI by phase shifting the individual channels of a chip.
58
59The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
60around pwm_apply_state() and should not be used if the user wants to change
61several parameter at once. For example, if you see pwm_config() and
62pwm_{enable,disable}() calls in the same function, this probably means you
63should switch to pwm_apply_state().
64
65The PWM user API also allows one to query the PWM state that was passed to the
66last invocation of pwm_apply_state() using pwm_get_state(). Note this is
67different to what the driver has actually implemented if the request cannot be
68satisfied exactly with the hardware in use. There is currently no way for
69consumers to get the actually implemented settings.
70
71In addition to the PWM state, the PWM API also exposes PWM arguments, which
72are the reference PWM config one should use on this PWM.
73PWM arguments are usually platform-specific and allows the PWM user to only
74care about dutycycle relatively to the full period (like, duty = 50% of the
75period). struct pwm_args contains 2 fields (period and polarity) and should
76be used to set the initial PWM config (usually done in the probe function
77of the PWM user). PWM arguments are retrieved with pwm_get_args().
78
79All consumers should really be reconfiguring the PWM upon resume as
80appropriate. This is the only way to ensure that everything is resumed in
81the proper order.
82
83Using PWMs with the sysfs interface
84-----------------------------------
85
86If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
87interface is provided to use the PWMs from userspace. It is exposed at
88/sys/class/pwm/. Each probed PWM controller/chip will be exported as
89pwmchipN, where N is the base of the PWM chip. Inside the directory you
90will find:
91
92  npwm
93    The number of PWM channels this chip supports (read-only).
94
95  export
96    Exports a PWM channel for use with sysfs (write-only).
97
98  unexport
99   Unexports a PWM channel from sysfs (write-only).
100
101The PWM channels are numbered using a per-chip index from 0 to npwm-1.
102
103When a PWM channel is exported a pwmX directory will be created in the
104pwmchipN directory it is associated with, where X is the number of the
105channel that was exported. The following properties will then be available:
106
107  period
108    The total period of the PWM signal (read/write).
109    Value is in nanoseconds and is the sum of the active and inactive
110    time of the PWM.
111
112  duty_cycle
113    The active time of the PWM signal (read/write).
114    Value is in nanoseconds and must be less than the period.
115
116  polarity
117    Changes the polarity of the PWM signal (read/write).
118    Writes to this property only work if the PWM chip supports changing
119    the polarity. The polarity can only be changed if the PWM is not
120    enabled. Value is the string "normal" or "inversed".
121
122  enable
123    Enable/disable the PWM signal (read/write).
124
125	- 0 - disabled
126	- 1 - enabled
127
128Implementing a PWM driver
129-------------------------
130
131Currently there are two ways to implement pwm drivers. Traditionally
132there only has been the barebone API meaning that each driver has
133to implement the pwm_*() functions itself. This means that it's impossible
134to have multiple PWM drivers in the system. For this reason it's mandatory
135for new drivers to use the generic PWM framework.
136
137A new PWM controller/chip can be added using pwmchip_add() and removed
138again with pwmchip_remove(). pwmchip_add() takes a filled in struct
139pwm_chip as argument which provides a description of the PWM chip, the
140number of PWM devices provided by the chip and the chip-specific
141implementation of the supported PWM operations to the framework.
142
143When implementing polarity support in a PWM driver, make sure to respect the
144signal conventions in the PWM framework. By definition, normal polarity
145characterizes a signal starts high for the duration of the duty cycle and
146goes low for the remainder of the period. Conversely, a signal with inversed
147polarity starts low for the duration of the duty cycle and goes high for the
148remainder of the period.
149
150Drivers are encouraged to implement ->apply() instead of the legacy
151->enable(), ->disable() and ->config() methods. Doing that should provide
152atomicity in the PWM config workflow, which is required when the PWM controls
153a critical device (like a regulator).
154
155The implementation of ->get_state() (a method used to retrieve initial PWM
156state) is also encouraged for the same reason: letting the PWM user know
157about the current PWM state would allow him to avoid glitches.
158
159Drivers should not implement any power management. In other words,
160consumers should implement it as described in the "Using PWMs" section.
161
162Locking
163-------
164
165The PWM core list manipulations are protected by a mutex, so pwm_get()
166and pwm_put() may not be called from an atomic context. Currently the
167PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
168pwm_config(), so the calling context is currently driver specific. This
169is an issue derived from the former barebone API and should be fixed soon.
170
171Helpers
172-------
173
174Currently a PWM can only be configured with period_ns and duty_ns. For several
175use cases freq_hz and duty_percent might be better. Instead of calculating
176this in your driver please consider adding appropriate helpers to the framework.
177