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

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Date:	Wed, 9 Apr 2014 08:04:50 +0200
From:	Lothar Waßmann <LW@...O-electronics.de>
To:	Tim Kryger <tim.kryger@...aro.org>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	"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

Hi,

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.
> 
OK. But for a periodic signal this doesn't make any difference. It's
just a matter of where you set your reference point.
Only if you program the PWM to create a single cycle you would see the
difference. I wonder if this is a real life usecase though.


Lothar Waßmann
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