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Linux/Documentation/pwm.txt

  1 Pulse Width Modulation (PWM) interface
  2 
  3 This provides an overview about the Linux PWM interface
  4 
  5 PWMs are commonly used for controlling LEDs, fans or vibrators in
  6 cell phones. PWMs with a fixed purpose have no need implementing
  7 the Linux PWM API (although they could). However, PWMs are often
  8 found as discrete devices on SoCs which have no fixed purpose. It's
  9 up to the board designer to connect them to LEDs or fans. To provide
 10 this kind of flexibility the generic PWM API exists.
 11 
 12 Identifying PWMs
 13 ----------------
 14 
 15 Users of the legacy PWM API use unique IDs to refer to PWM devices.
 16 
 17 Instead of referring to a PWM device via its unique ID, board setup code
 18 should instead register a static mapping that can be used to match PWM
 19 consumers to providers, as given in the following example:
 20 
 21         static struct pwm_lookup board_pwm_lookup[] = {
 22                 PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
 23                            50000, PWM_POLARITY_NORMAL),
 24         };
 25 
 26         static void __init board_init(void)
 27         {
 28                 ...
 29                 pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
 30                 ...
 31         }
 32 
 33 Using PWMs
 34 ----------
 35 
 36 Legacy users can request a PWM device using pwm_request() and free it
 37 after usage with pwm_free().
 38 
 39 New users should use the pwm_get() function and pass to it the consumer
 40 device or a consumer name. pwm_put() is used to free the PWM device. Managed
 41 variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
 42 
 43 After being requested, a PWM has to be configured using:
 44 
 45 int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
 46 
 47 This API controls both the PWM period/duty_cycle config and the
 48 enable/disable state.
 49 
 50 The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
 51 around pwm_apply_state() and should not be used if the user wants to change
 52 several parameter at once. For example, if you see pwm_config() and
 53 pwm_{enable,disable}() calls in the same function, this probably means you
 54 should switch to pwm_apply_state().
 55 
 56 The PWM user API also allows one to query the PWM state with pwm_get_state().
 57 
 58 In addition to the PWM state, the PWM API also exposes PWM arguments, which
 59 are the reference PWM config one should use on this PWM.
 60 PWM arguments are usually platform-specific and allows the PWM user to only
 61 care about dutycycle relatively to the full period (like, duty = 50% of the
 62 period). struct pwm_args contains 2 fields (period and polarity) and should
 63 be used to set the initial PWM config (usually done in the probe function
 64 of the PWM user). PWM arguments are retrieved with pwm_get_args().
 65 
 66 Using PWMs with the sysfs interface
 67 -----------------------------------
 68 
 69 If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
 70 interface is provided to use the PWMs from userspace. It is exposed at
 71 /sys/class/pwm/. Each probed PWM controller/chip will be exported as
 72 pwmchipN, where N is the base of the PWM chip. Inside the directory you
 73 will find:
 74 
 75 npwm - The number of PWM channels this chip supports (read-only).
 76 
 77 export - Exports a PWM channel for use with sysfs (write-only).
 78 
 79 unexport - Unexports a PWM channel from sysfs (write-only).
 80 
 81 The PWM channels are numbered using a per-chip index from 0 to npwm-1.
 82 
 83 When a PWM channel is exported a pwmX directory will be created in the
 84 pwmchipN directory it is associated with, where X is the number of the
 85 channel that was exported. The following properties will then be available:
 86 
 87 period - The total period of the PWM signal (read/write).
 88         Value is in nanoseconds and is the sum of the active and inactive
 89         time of the PWM.
 90 
 91 duty_cycle - The active time of the PWM signal (read/write).
 92         Value is in nanoseconds and must be less than the period.
 93 
 94 polarity - Changes the polarity of the PWM signal (read/write).
 95         Writes to this property only work if the PWM chip supports changing
 96         the polarity. The polarity can only be changed if the PWM is not
 97         enabled. Value is the string "normal" or "inversed".
 98 
 99 enable - Enable/disable the PWM signal (read/write).
100         0 - disabled
101         1 - enabled
102 
103 Implementing a PWM driver
104 -------------------------
105 
106 Currently there are two ways to implement pwm drivers. Traditionally
107 there only has been the barebone API meaning that each driver has
108 to implement the pwm_*() functions itself. This means that it's impossible
109 to have multiple PWM drivers in the system. For this reason it's mandatory
110 for new drivers to use the generic PWM framework.
111 
112 A new PWM controller/chip can be added using pwmchip_add() and removed
113 again with pwmchip_remove(). pwmchip_add() takes a filled in struct
114 pwm_chip as argument which provides a description of the PWM chip, the
115 number of PWM devices provided by the chip and the chip-specific
116 implementation of the supported PWM operations to the framework.
117 
118 When implementing polarity support in a PWM driver, make sure to respect the
119 signal conventions in the PWM framework. By definition, normal polarity
120 characterizes a signal starts high for the duration of the duty cycle and
121 goes low for the remainder of the period. Conversely, a signal with inversed
122 polarity starts low for the duration of the duty cycle and goes high for the
123 remainder of the period.
124 
125 Drivers are encouraged to implement ->apply() instead of the legacy
126 ->enable(), ->disable() and ->config() methods. Doing that should provide
127 atomicity in the PWM config workflow, which is required when the PWM controls
128 a critical device (like a regulator).
129 
130 The implementation of ->get_state() (a method used to retrieve initial PWM
131 state) is also encouraged for the same reason: letting the PWM user know
132 about the current PWM state would allow him to avoid glitches.
133 
134 Locking
135 -------
136 
137 The PWM core list manipulations are protected by a mutex, so pwm_request()
138 and pwm_free() may not be called from an atomic context. Currently the
139 PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
140 pwm_config(), so the calling context is currently driver specific. This
141 is an issue derived from the former barebone API and should be fixed soon.
142 
143 Helpers
144 -------
145 
146 Currently a PWM can only be configured with period_ns and duty_ns. For several
147 use cases freq_hz and duty_percent might be better. Instead of calculating
148 this in your driver please consider adding appropriate helpers to the framework.

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