1 ======================================
2 Pulse Width Modulation (PWM) interface
3 ======================================
4
5 This provides an overview about the Linux PWM interface
6
7 PWMs are commonly used for controlling LEDs, fans or vibrators in
8 cell phones. PWMs with a fixed purpose have no need implementing
9 the Linux PWM API (although they could). However, PWMs are often
10 found as discrete devices on SoCs which have no fixed purpose. It's
11 up to the board designer to connect them to LEDs or fans. To provide
12 this kind of flexibility the generic PWM API exists.
13
14 Identifying PWMs
15 ----------------
16
17 Users of the legacy PWM API use unique IDs to refer to PWM devices.
18
19 Instead of referring to a PWM device via its unique ID, board setup code
20 should instead register a static mapping that can be used to match PWM
21 consumers 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
35 Using PWMs
36 ----------
37
38 Consumers use the pwm_get() function and pass to it the consumer device or a
39 consumer name. pwm_put() is used to free the PWM device. Managed variants of
40 the getter, devm_pwm_get() and devm_fwnode_pwm_get(), also exist.
41
42 After being requested, a PWM has to be configured using::
43
44 int pwm_apply_might_sleep(struct pwm_device *pwm, struct pwm_state *state);
45
46 This API controls both the PWM period/duty_cycle config and the
47 enable/disable state.
48
49 PWM devices can be used from atomic context, if the PWM does not sleep. You
50 can check if this the case with::
51
52 bool pwm_might_sleep(struct pwm_device *pwm);
53
54 If false, the PWM can also be configured from atomic context with::
55
56 int pwm_apply_atomic(struct pwm_device *pwm, struct pwm_state *state);
57
58 As a consumer, don't rely on the output's state for a disabled PWM. If it's
59 easily possible, drivers are supposed to emit the inactive state, but some
60 drivers cannot. If you rely on getting the inactive state, use .duty_cycle=0,
61 .enabled=true.
62
63 There is also a usage_power setting: If set, the PWM driver is only required to
64 maintain the power output but has more freedom regarding signal form.
65 If supported by the driver, the signal can be optimized, for example to improve
66 EMI by phase shifting the individual channels of a chip.
67
68 The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
69 around pwm_apply_might_sleep() and should not be used if the user wants to change
70 several parameter at once. For example, if you see pwm_config() and
71 pwm_{enable,disable}() calls in the same function, this probably means you
72 should switch to pwm_apply_might_sleep().
73
74 The PWM user API also allows one to query the PWM state that was passed to the
75 last invocation of pwm_apply_might_sleep() using pwm_get_state(). Note this is
76 different to what the driver has actually implemented if the request cannot be
77 satisfied exactly with the hardware in use. There is currently no way for
78 consumers to get the actually implemented settings.
79
80 In addition to the PWM state, the PWM API also exposes PWM arguments, which
81 are the reference PWM config one should use on this PWM.
82 PWM arguments are usually platform-specific and allows the PWM user to only
83 care about dutycycle relatively to the full period (like, duty = 50% of the
84 period). struct pwm_args contains 2 fields (period and polarity) and should
85 be used to set the initial PWM config (usually done in the probe function
86 of the PWM user). PWM arguments are retrieved with pwm_get_args().
87
88 All consumers should really be reconfiguring the PWM upon resume as
89 appropriate. This is the only way to ensure that everything is resumed in
90 the proper order.
91
92 Using PWMs with the sysfs interface
93 -----------------------------------
94
95 If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
96 interface is provided to use the PWMs from userspace. It is exposed at
97 /sys/class/pwm/. Each probed PWM controller/chip will be exported as
98 pwmchipN, where N is the base of the PWM chip. Inside the directory you
99 will find:
100
101 npwm
102 The number of PWM channels this chip supports (read-only).
103
104 export
105 Exports a PWM channel for use with sysfs (write-only).
106
107 unexport
108 Unexports a PWM channel from sysfs (write-only).
109
110 The PWM channels are numbered using a per-chip index from 0 to npwm-1.
111
112 When a PWM channel is exported a pwmX directory will be created in the
113 pwmchipN directory it is associated with, where X is the number of the
114 channel that was exported. The following properties will then be available:
115
116 period
117 The total period of the PWM signal (read/write).
118 Value is in nanoseconds and is the sum of the active and inactive
119 time of the PWM.
120
121 duty_cycle
122 The active time of the PWM signal (read/write).
123 Value is in nanoseconds and must be less than or equal to the period.
124
125 polarity
126 Changes the polarity of the PWM signal (read/write).
127 Writes to this property only work if the PWM chip supports changing
128 the polarity.
129 Value is the string "normal" or "inversed".
130
131 enable
132 Enable/disable the PWM signal (read/write).
133
134 - 0 - disabled
135 - 1 - enabled
136
137 Implementing a PWM driver
138 -------------------------
139
140 Currently there are two ways to implement pwm drivers. Traditionally
141 there only has been the barebone API meaning that each driver has
142 to implement the pwm_*() functions itself. This means that it's impossible
143 to have multiple PWM drivers in the system. For this reason it's mandatory
144 for new drivers to use the generic PWM framework.
145
146 A new PWM controller/chip can be allocated using pwmchip_alloc(), then
147 registered using pwmchip_add() and removed again with pwmchip_remove(). To undo
148 pwmchip_alloc() use pwmchip_put(). pwmchip_add() takes a filled in struct
149 pwm_chip as argument which provides a description of the PWM chip, the number
150 of PWM devices provided by the chip and the chip-specific implementation of the
151 supported PWM operations to the framework.
152
153 When implementing polarity support in a PWM driver, make sure to respect the
154 signal conventions in the PWM framework. By definition, normal polarity
155 characterizes a signal starts high for the duration of the duty cycle and
156 goes low for the remainder of the period. Conversely, a signal with inversed
157 polarity starts low for the duration of the duty cycle and goes high for the
158 remainder of the period.
159
160 Drivers are encouraged to implement ->apply() instead of the legacy
161 ->enable(), ->disable() and ->config() methods. Doing that should provide
162 atomicity in the PWM config workflow, which is required when the PWM controls
163 a critical device (like a regulator).
164
165 The implementation of ->get_state() (a method used to retrieve initial PWM
166 state) is also encouraged for the same reason: letting the PWM user know
167 about the current PWM state would allow him to avoid glitches.
168
169 Drivers should not implement any power management. In other words,
170 consumers should implement it as described in the "Using PWMs" section.
171
172 Locking
173 -------
174
175 The PWM core list manipulations are protected by a mutex, so pwm_get()
176 and pwm_put() may not be called from an atomic context. Currently the
177 PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
178 pwm_config(), so the calling context is currently driver specific. This
179 is an issue derived from the former barebone API and should be fixed soon.
180
181 Helpers
182 -------
183
184 Currently a PWM can only be configured with period_ns and duty_ns. For several
185 use cases freq_hz and duty_percent might be better. Instead of calculating
186 this in your driver please consider adding appropriate helpers to the framework.
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