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