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. One
goal of the new PWM framework is to get rid of this global namespace.

Using PWMs
----------

A PWM can be requested using pwm_request() and freed after usage with
pwm_free(). After being requested a PWM has to be configured using

int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);

To start/stop toggling the PWM output use pwm_enable()/pwm_disable().

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 provider by the chip and the chip-specific
implementation of the supported PWM operations to the framework.

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.