linux-kernel - Re: [PATCH 1/2] PWM: let of_xlate handlers check args count

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Message-ID: <20140124074103.GJ16215@pengutronix.de>
Date:	Fri, 24 Jan 2014 08:41:03 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Lothar Waßmann <LW@...O-electronics.de>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Mark Rutland <mark.rutland@....com>, linux-pwm@...r.kernel.org,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Rob Landley <rob@...dley.net>,
	Kumar Gala <galak@...eaurora.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] PWM: let of_xlate handlers check args count

On Fri, Jan 24, 2014 at 06:42:54AM +0100, Lothar Waßmann wrote:
> Hi,
> 
> > Okay, this works, but there's a problem with pwm-leds.
> > 
> > When the duty cycle is set to zero (when you set the brightness to zero)
> > pwm-leds decides to disable the PWM after configuring it.  This causes
> > the PWM output to be driven low, causing the LED to go to maximum
> > brightness.
> > 
> > So, using the inversion at PWM level doesn't work.
> > 
> The problem is that the driver calls pwm_disable() when the duty cycle is 0.
> This sets the PWM output low independent from the output polarity setting.
> 
> > To make this work correctly, we really need pwm-leds to do the inversion
> > rather than setting the inversion bit in hardware.
> > 
> The same holds for the pwm-backlight driver.
> 
> The easiest fix would be not to call pwm_disable() even for a zero duty
> cycle.

IMO that's the right thing to do anyway due to the different PWM
hardware controllers we have. I'm thinking about the following patch
for some time.

Sascha

--------------------8<--------------------------

>From 9ebbc3d72c71bd97d7fc4458f60ae3ecd5876984 Mon Sep 17 00:00:00 2001
From: Sascha Hauer <s.hauer@...gutronix.de>
Date: Fri, 24 Jan 2014 08:34:16 +0100
Subject: [PATCH] PWM: Document disabled PWM output as undefined

When disabled PWM hardware reacts differently. Some controllers
just stop with their current value, others produce a constant high
or low output, sometimes depending on the output inversion bit. Update
the documentation to reflect that and request from the PWM consumer
drivers to set a constant high/low value with duty cycles of 0/100%
instead of disabling the PWM.

Signed-off-by: Sascha Hauer <s.hauer@...gutronix.de>
---
 Documentation/pwm.txt | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt
index 93cb979..b7e8a31 100644
--- a/Documentation/pwm.txt
+++ b/Documentation/pwm.txt
@@ -44,6 +44,8 @@ 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().
+The output of a disabled PWM is undefined. Set the duty cycle to 100%
+for a constant high output and to 0 for constant low output.
 
 Using PWMs with the sysfs interface
 -----------------------------------
-- 
1.8.5.2

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