Using PWMs
Consumers 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() and devm_fwnode_pwm_get(), also exist.
After being requested, a PWM has to be configured using:
int pwm_apply_might_sleep(struct pwm_device *pwm, struct pwm_state *state);
This API controls both the PWM period/duty_cycle config and the
enable/disable state.
PWM devices can be used from atomic context, if the PWM does not sleep. You
can check if this the case with:
bool pwm_might_sleep(struct pwm_device *pwm);
If false, the PWM can also be configured from atomic context with:
int pwm_apply_atomic(struct pwm_device *pwm, struct pwm_state *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_might_sleep() 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_might_sleep().
The PWM user API also allows one to query the PWM state that was passed to the
last invocation of pwm_apply_might_sleep() 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.
https://docs.kernel.org/driver-api/pwm.html#:~:text=Consumers%20use%20the%20pwm_get%20%28%29%20function%20and%20pass,%28%29%20is%20used%20to%20free%20the%20PWM%20device.
