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

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Date:	Tue, 8 Apr 2014 13:43:22 -0700
From:	Tim Kryger <tim.kryger@...aro.org>
To:	Lothar Waßmann <LW@...o-electronics.de>
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

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.

-Tim
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