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2 Pulse Width Modulation (PWM) interface
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5 This provides an overview about the Linux PWM interface
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.
17 Users of the legacy PWM API use unique IDs to refer to PWM devices.
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::
23 static struct pwm_lookup board_pwm_lookup[] = {
24 PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
25 50000, PWM_POLARITY_NORMAL),
28 static void __init board_init(void)
31 pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
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.
42 After being requested, a PWM has to be configured using::
44 int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
46 This API controls both the PWM period/duty_cycle config and the
49 As a consumer, don't rely on the output's state for a disabled PWM. If it's
50 easily possible, drivers are supposed to emit the inactive state, but some
51 drivers cannot. If you rely on getting the inactive state, use .duty_cycle=0,
54 There is also a usage_power setting: If set, the PWM driver is only required to
55 maintain the power output but has more freedom regarding signal form.
56 If supported by the driver, the signal can be optimized, for example to improve
57 EMI by phase shifting the individual channels of a chip.
59 The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
60 around pwm_apply_state() and should not be used if the user wants to change
61 several parameter at once. For example, if you see pwm_config() and
62 pwm_{enable,disable}() calls in the same function, this probably means you
63 should switch to pwm_apply_state().
65 The PWM user API also allows one to query the PWM state that was passed to the
66 last invocation of pwm_apply_state() using pwm_get_state(). Note this is
67 different to what the driver has actually implemented if the request cannot be
68 satisfied exactly with the hardware in use. There is currently no way for
69 consumers to get the actually implemented settings.
71 In addition to the PWM state, the PWM API also exposes PWM arguments, which
72 are the reference PWM config one should use on this PWM.
73 PWM arguments are usually platform-specific and allows the PWM user to only
74 care about dutycycle relatively to the full period (like, duty = 50% of the
75 period). struct pwm_args contains 2 fields (period and polarity) and should
76 be used to set the initial PWM config (usually done in the probe function
77 of the PWM user). PWM arguments are retrieved with pwm_get_args().
79 All consumers should really be reconfiguring the PWM upon resume as
80 appropriate. This is the only way to ensure that everything is resumed in
83 Using PWMs with the sysfs interface
84 -----------------------------------
86 If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
87 interface 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
89 pwmchipN, where N is the base of the PWM chip. Inside the directory you
93 The number of PWM channels this chip supports (read-only).
96 Exports a PWM channel for use with sysfs (write-only).
99 Unexports a PWM channel from sysfs (write-only).
101 The PWM channels are numbered using a per-chip index from 0 to npwm-1.
103 When a PWM channel is exported a pwmX directory will be created in the
104 pwmchipN directory it is associated with, where X is the number of the
105 channel that was exported. The following properties will then be available:
108 The total period of the PWM signal (read/write).
109 Value is in nanoseconds and is the sum of the active and inactive
113 The active time of the PWM signal (read/write).
114 Value is in nanoseconds and must be less than the period.
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".
123 Enable/disable the PWM signal (read/write).
128 Implementing a PWM driver
129 -------------------------
131 Currently there are two ways to implement pwm drivers. Traditionally
132 there only has been the barebone API meaning that each driver has
133 to implement the pwm_*() functions itself. This means that it's impossible
134 to have multiple PWM drivers in the system. For this reason it's mandatory
135 for new drivers to use the generic PWM framework.
137 A new PWM controller/chip can be added using pwmchip_add() and removed
138 again with pwmchip_remove(). pwmchip_add() takes a filled in struct
139 pwm_chip as argument which provides a description of the PWM chip, the
140 number of PWM devices provided by the chip and the chip-specific
141 implementation of the supported PWM operations to the framework.
143 When implementing polarity support in a PWM driver, make sure to respect the
144 signal conventions in the PWM framework. By definition, normal polarity
145 characterizes a signal starts high for the duration of the duty cycle and
146 goes low for the remainder of the period. Conversely, a signal with inversed
147 polarity starts low for the duration of the duty cycle and goes high for the
148 remainder of the period.
150 Drivers are encouraged to implement ->apply() instead of the legacy
151 ->enable(), ->disable() and ->config() methods. Doing that should provide
152 atomicity in the PWM config workflow, which is required when the PWM controls
153 a critical device (like a regulator).
155 The implementation of ->get_state() (a method used to retrieve initial PWM
156 state) is also encouraged for the same reason: letting the PWM user know
157 about the current PWM state would allow him to avoid glitches.
159 Drivers should not implement any power management. In other words,
160 consumers should implement it as described in the "Using PWMs" section.
165 The PWM core list manipulations are protected by a mutex, so pwm_get()
166 and pwm_put() may not be called from an atomic context. Currently the
167 PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
168 pwm_config(), so the calling context is currently driver specific. This
169 is an issue derived from the former barebone API and should be fixed soon.
174 Currently a PWM can only be configured with period_ns and duty_ns. For several
175 use cases freq_hz and duty_percent might be better. Instead of calculating
176 this in your driver please consider adding appropriate helpers to the framework.