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[1/2] pwm/doc: Clearify that the pin state after pwm_disable is undefined

Message ID 1423734290-19750-2-git-send-email-u.kleine-koenig@pengutronix.de
State Rejected
Headers show

Commit Message

Uwe Kleine-König Feb. 12, 2015, 9:44 a.m. UTC 
This makes assumptions on the behaviour of the pwm API explicit.

Note that there are drivers that assume that pwm_disable does result in
a stable output matching the last pwm_config; leds-pwm is an example
that is fixed in a followup patch.

On arm/i.MX28 it can really happen that after

	pwm_config(led_dat->pwm, 0, 100);
	pwm_disable(led_dat->pwm);

the output is stuck at 1. That's because the pwm only works with the
newly configured config when a period is over for the previously
configured setting to prevent spikes.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
---
 Documentation/pwm.txt | 5 +++++
 1 file changed, 5 insertions(+)
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Patch

diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt
index ca895fd211e4..abdd21d047ca 100644
--- a/Documentation/pwm.txt
+++ b/Documentation/pwm.txt
@@ -46,6 +46,11 @@  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().
 
+Note that after calling pwm_disable() it's undefined if the output stops in a
+high or low state, even if the duty cycle is set to 0% or 100%. So don't call
+pwm_disable() if there is a need for a specific level. The same applies when
+pwm_enable() was called, but pwm_config() was not.
+
 Using PWMs with the sysfs interface
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