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 ======================================
 Pulse Width Modulation (PWM) interface
 ======================================
 
 This provides an overview about the Linux PWM interface
 
 PWMs are commonly used for controlling LEDs, fans or vibrators in
 cell phones. PWMs with a fixed purpose have no need implementing
 the Linux PWM API (although they could). However, PWMs are often
 found as discrete devices on SoCs which have no fixed purpose. It's
 up to the board designer to connect them to LEDs or fans. To provide
 this kind of flexibility the generic PWM API exists.
 
 Identifying PWMs
 ----------------
 
 Users of the legacy PWM API use unique IDs to refer to PWM devices.
 
 Instead of referring to a PWM device via its unique ID, board setup code
 should instead register a static mapping that can be used to match PWM
 consumers to providers, as given in the following example::
 
         static struct pwm_lookup board_pwm_lookup[] = {
                 PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
                            50000, PWM_POLARITY_NORMAL),
         };
 
         static void __init board_init(void)
         {
                 ...
                 pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
                 ...
         }
 
 Using PWMs
 ----------
 
 Legacy users can request a PWM device using pwm_request() and free it
 after usage with pwm_free().
 
 New users should use the pwm_get() function and pass to it the consumer
 device or a consumer name. pwm_put() is used to free the PWM device. Managed
 variants of the getter, devm_pwm_get(), devm_of_pwm_get(),
 devm_fwnode_pwm_get(), also exist.
 
 After being requested, a PWM has to be configured using::
 
         int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
 
 This API controls both the PWM period/duty_cycle config and the
 enable/disable state.
 
 As a consumer, don't rely on the output's state for a disabled PWM. If it's
 easily possible, drivers are supposed to emit the inactive state, but some
 drivers cannot. If you rely on getting the inactive state, use .duty_cycle=0,
 .enabled=true.
 
 There is also a usage_power setting: If set, the PWM driver is only required to
 maintain the power output but has more freedom regarding signal form.
 If supported by the driver, the signal can be optimized, for example to improve
 EMI by phase shifting the individual channels of a chip.
 
 The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
 around pwm_apply_state() and should not be used if the user wants to change
 several parameter at once. For example, if you see pwm_config() and
 pwm_{enable,disable}() calls in the same function, this probably means you
 should switch to pwm_apply_state().
 
 The PWM user API also allows one to query the PWM state that was passed to the
 last invocation of pwm_apply_state() using pwm_get_state(). Note this is
 different to what the driver has actually implemented if the request cannot be
 satisfied exactly with the hardware in use. There is currently no way for
 consumers to get the actually implemented settings.
 
 In addition to the PWM state, the PWM API also exposes PWM arguments, which
 are the reference PWM config one should use on this PWM.
 PWM arguments are usually platform-specific and allows the PWM user to only
 care about dutycycle relatively to the full period (like, duty = 50% of the
 period). struct pwm_args contains 2 fields (period and polarity) and should
 be used to set the initial PWM config (usually done in the probe function
 of the PWM user). PWM arguments are retrieved with pwm_get_args().
 
 All consumers should really be reconfiguring the PWM upon resume as
 appropriate. This is the only way to ensure that everything is resumed in
 the proper order.
 
 Using PWMs with the sysfs interface
 -----------------------------------
 
 If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
 interface is provided to use the PWMs from userspace. It is exposed at
 /sys/class/pwm/. Each probed PWM controller/chip will be exported as
 pwmchipN, where N is the base of the PWM chip. Inside the directory you
 will find:
 
   npwm
     The number of PWM channels this chip supports (read-only).
 
   export
     Exports a PWM channel for use with sysfs (write-only).
 
   unexport
    Unexports a PWM channel from sysfs (write-only).
 
 The PWM channels are numbered using a per-chip index from 0 to npwm-1.
 
 When a PWM channel is exported a pwmX directory will be created in the
 pwmchipN directory it is associated with, where X is the number of the
 channel that was exported. The following properties will then be available:
 
   period
     The total period of the PWM signal (read/write).
     Value is in nanoseconds and is the sum of the active and inactive
     time of the PWM.
 
   duty_cycle
     The active time of the PWM signal (read/write).
     Value is in nanoseconds and must be less than the period.
 
   polarity
     Changes the polarity of the PWM signal (read/write).
     Writes to this property only work if the PWM chip supports changing
     the polarity. The polarity can only be changed if the PWM is not
     enabled. Value is the string "normal" or "inversed".
 
   enable
     Enable/disable the PWM signal (read/write).
 
         - 0 - disabled
         - 1 - enabled
 
 Implementing a PWM driver
 -------------------------
 
 Currently there are two ways to implement pwm drivers. Traditionally
 there only has been the barebone API meaning that each driver has
 to implement the pwm_*() functions itself. This means that it's impossible
 to have multiple PWM drivers in the system. For this reason it's mandatory
 for new drivers to use the generic PWM framework.
 
 A new PWM controller/chip can be added using pwmchip_add() and removed
 again with pwmchip_remove(). pwmchip_add() takes a filled in struct
 pwm_chip as argument which provides a description of the PWM chip, the
 number of PWM devices provided by the chip and the chip-specific
 implementation of the supported PWM operations to the framework.
 
 When implementing polarity support in a PWM driver, make sure to respect the
 signal conventions in the PWM framework. By definition, normal polarity
 characterizes a signal starts high for the duration of the duty cycle and
 goes low for the remainder of the period. Conversely, a signal with inversed
 polarity starts low for the duration of the duty cycle and goes high for the
 remainder of the period.
 
 Drivers are encouraged to implement ->apply() instead of the legacy
 ->enable(), ->disable() and ->config() methods. Doing that should provide
 atomicity in the PWM config workflow, which is required when the PWM controls
 a critical device (like a regulator).
 
 The implementation of ->get_state() (a method used to retrieve initial PWM
 state) is also encouraged for the same reason: letting the PWM user know
 about the current PWM state would allow him to avoid glitches.
 
 Drivers should not implement any power management. In other words,
 consumers should implement it as described in the "Using PWMs" section.
 
 Locking
 -------
 
 The PWM core list manipulations are protected by a mutex, so pwm_request()
 and pwm_free() may not be called from an atomic context. Currently the
 PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
 pwm_config(), so the calling context is currently driver specific. This
 is an issue derived from the former barebone API and should be fixed soon.
 
 Helpers
 -------
 
 Currently a PWM can only be configured with period_ns and duty_ns. For several
 use cases freq_hz and duty_percent might be better. Instead of calculating
 this in your driver please consider adding appropriate helpers to the framework.

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