linux-kernel - Re: [PATCHv3 1/3] pwm: make the PWM

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Date:	Wed, 9 Apr 2014 08:12:09 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Tim Kryger <tim.kryger@...aro.org>
Cc:	Lothar Waßmann <LW@...o-electronics.de>,
	Thierry Reding <thierry.reding@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux PWM List <linux-pwm@...r.kernel.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	Sascha Hauer <kernel@...gutronix.de>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCHv3 1/3] pwm: make the PWM_POLARITY flag in DTB optional

On Tue, Apr 08, 2014 at 01:43:22PM -0700, Tim Kryger wrote:
> On Mon, Apr 7, 2014 at 10:02 PM, Lothar Waßmann <LW@...o-electronics.de> wrote:
> > Thierry Reding wrote:
> 
> >> No. You cannot emulate polarity inversion in software.
> >>
> > Why not?
> >
> > duty_ns = period_ns - duty_ns;
> 
> Since I made the same mistake, I will pass along the pointer Thierry gave me.
> 
> In include/linux/pwm.h the second difference for an inverted signal is
> described.
> 
> /**
>  * enum pwm_polarity - polarity of a PWM signal
>  * @PWM_POLARITY_NORMAL: a high signal for the duration of the duty-
>  * cycle, followed by a low signal for the remainder of the pulse
>  * period
>  * @PWM_POLARITY_INVERSED: a low signal for the duration of the duty-
>  * cycle, followed by a high signal for the remainder of the pulse
>  * period
>  */
> enum pwm_polarity {
> PWM_POLARITY_NORMAL,
> PWM_POLARITY_INVERSED,
> };
> 
> Of course, I suspect not all PWM hardware respects this definition of
> inverted output.
> 
> Either way, hacking the duty in software certainly would get the
> high/low order wrong.

This only relevant if you have some reference signal the PWM must be
relative to, for example if you combine multiple PWMs for motor control.
For PWMs used for backlight or beepers a signal inversion in software is
perfectly fine. And I also think that it makes sense to put it once into
the framework instead of bothering all consumer drivers with the
inversion.

Sascha

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