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0001 Pulse Width Modulation (PWM) interface
0002 
0003 This provides an overview about the Linux PWM interface
0004 
0005 PWMs are commonly used for controlling LEDs, fans or vibrators in
0006 cell phones. PWMs with a fixed purpose have no need implementing
0007 the Linux PWM API (although they could). However, PWMs are often
0008 found as discrete devices on SoCs which have no fixed purpose. It's
0009 up to the board designer to connect them to LEDs or fans. To provide
0010 this kind of flexibility the generic PWM API exists.
0011 
0012 Identifying PWMs
0013 ----------------
0014 
0015 Users of the legacy PWM API use unique IDs to refer to PWM devices.
0016 
0017 Instead of referring to a PWM device via its unique ID, board setup code
0018 should instead register a static mapping that can be used to match PWM
0019 consumers to providers, as given in the following example:
0020 
0021         static struct pwm_lookup board_pwm_lookup[] = {
0022                 PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
0023                            50000, PWM_POLARITY_NORMAL),
0024         };
0025 
0026         static void __init board_init(void)
0027         {
0028                 ...
0029                 pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
0030                 ...
0031         }
0032 
0033 Using PWMs
0034 ----------
0035 
0036 Legacy users can request a PWM device using pwm_request() and free it
0037 after usage with pwm_free().
0038 
0039 New users should use the pwm_get() function and pass to it the consumer
0040 device or a consumer name. pwm_put() is used to free the PWM device. Managed
0041 variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
0042 
0043 After being requested, a PWM has to be configured using:
0044 
0045 int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
0046 
0047 This API controls both the PWM period/duty_cycle config and the
0048 enable/disable state.
0049 
0050 The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
0051 around pwm_apply_state() and should not be used if the user wants to change
0052 several parameter at once. For example, if you see pwm_config() and
0053 pwm_{enable,disable}() calls in the same function, this probably means you
0054 should switch to pwm_apply_state().
0055 
0056 The PWM user API also allows one to query the PWM state with pwm_get_state().
0057 
0058 In addition to the PWM state, the PWM API also exposes PWM arguments, which
0059 are the reference PWM config one should use on this PWM.
0060 PWM arguments are usually platform-specific and allows the PWM user to only
0061 care about dutycycle relatively to the full period (like, duty = 50% of the
0062 period). struct pwm_args contains 2 fields (period and polarity) and should
0063 be used to set the initial PWM config (usually done in the probe function
0064 of the PWM user). PWM arguments are retrieved with pwm_get_args().
0065 
0066 Using PWMs with the sysfs interface
0067 -----------------------------------
0068 
0069 If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
0070 interface is provided to use the PWMs from userspace. It is exposed at
0071 /sys/class/pwm/. Each probed PWM controller/chip will be exported as
0072 pwmchipN, where N is the base of the PWM chip. Inside the directory you
0073 will find:
0074 
0075 npwm - The number of PWM channels this chip supports (read-only).
0076 
0077 export - Exports a PWM channel for use with sysfs (write-only).
0078 
0079 unexport - Unexports a PWM channel from sysfs (write-only).
0080 
0081 The PWM channels are numbered using a per-chip index from 0 to npwm-1.
0082 
0083 When a PWM channel is exported a pwmX directory will be created in the
0084 pwmchipN directory it is associated with, where X is the number of the
0085 channel that was exported. The following properties will then be available:
0086 
0087 period - The total period of the PWM signal (read/write).
0088         Value is in nanoseconds and is the sum of the active and inactive
0089         time of the PWM.
0090 
0091 duty_cycle - The active time of the PWM signal (read/write).
0092         Value is in nanoseconds and must be less than the period.
0093 
0094 polarity - Changes the polarity of the PWM signal (read/write).
0095         Writes to this property only work if the PWM chip supports changing
0096         the polarity. The polarity can only be changed if the PWM is not
0097         enabled. Value is the string "normal" or "inversed".
0098 
0099 enable - Enable/disable the PWM signal (read/write).
0100         0 - disabled
0101         1 - enabled
0102 
0103 Implementing a PWM driver
0104 -------------------------
0105 
0106 Currently there are two ways to implement pwm drivers. Traditionally
0107 there only has been the barebone API meaning that each driver has
0108 to implement the pwm_*() functions itself. This means that it's impossible
0109 to have multiple PWM drivers in the system. For this reason it's mandatory
0110 for new drivers to use the generic PWM framework.
0111 
0112 A new PWM controller/chip can be added using pwmchip_add() and removed
0113 again with pwmchip_remove(). pwmchip_add() takes a filled in struct
0114 pwm_chip as argument which provides a description of the PWM chip, the
0115 number of PWM devices provided by the chip and the chip-specific
0116 implementation of the supported PWM operations to the framework.
0117 
0118 When implementing polarity support in a PWM driver, make sure to respect the
0119 signal conventions in the PWM framework. By definition, normal polarity
0120 characterizes a signal starts high for the duration of the duty cycle and
0121 goes low for the remainder of the period. Conversely, a signal with inversed
0122 polarity starts low for the duration of the duty cycle and goes high for the
0123 remainder of the period.
0124 
0125 Drivers are encouraged to implement ->apply() instead of the legacy
0126 ->enable(), ->disable() and ->config() methods. Doing that should provide
0127 atomicity in the PWM config workflow, which is required when the PWM controls
0128 a critical device (like a regulator).
0129 
0130 The implementation of ->get_state() (a method used to retrieve initial PWM
0131 state) is also encouraged for the same reason: letting the PWM user know
0132 about the current PWM state would allow him to avoid glitches.
0133 
0134 Locking
0135 -------
0136 
0137 The PWM core list manipulations are protected by a mutex, so pwm_request()
0138 and pwm_free() may not be called from an atomic context. Currently the
0139 PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
0140 pwm_config(), so the calling context is currently driver specific. This
0141 is an issue derived from the former barebone API and should be fixed soon.
0142 
0143 Helpers
0144 -------
0145 
0146 Currently a PWM can only be configured with period_ns and duty_ns. For several
0147 use cases freq_hz and duty_percent might be better. Instead of calculating
0148 this in your driver please consider adding appropriate helpers to the framework.