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pwm: Update documentation 

Update the PWM subsystem documentation to reflect the atomic PWM
changes.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
This commit is contained in:
Boris Brezillon 2016-04-14 21:17:42 +02:00 committed by Thierry Reding
parent 5ec803edcb
commit a07136fdcf
1 changed files with 28 additions and 2 deletions
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30
Documentation/pwm.txt
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@ -42,9 +42,26 @@ variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
After being requested, a PWM has to be configured using:
int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
To start/stop toggling the PWM output use pwm_enable()/pwm_disable().
This API controls both the PWM period/duty_cycle config and the
enable/disable state.
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 with pwm_get_state().
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().
Using PWMs with the sysfs interface
-----------------------------------
@ -105,6 +122,15 @@ 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.
Locking
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