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

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Date:	Tue, 8 Apr 2014 07:02:59 +0200
From:	Lothar Waßmann <LW@...O-electronics.de>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel@...r.kernel.org, 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,

Thierry Reding wrote:
> On Wed, Apr 02, 2014 at 07:53:50AM +0200, Sascha Hauer wrote:
> > On Fri, Mar 28, 2014 at 09:48:58AM +0100, Lothar Waßmann wrote:
> [...]
> > > @@ -183,8 +173,11 @@ static void of_pwmchip_add(struct pwm_chip *chip)
> > >  		return;
> > >  
> > >  	if (!chip->of_xlate) {
> > > -		chip->of_xlate = of_pwm_simple_xlate;
> > > -		chip->of_pwm_n_cells = 2;
> > > +		chip->of_xlate = of_pwm_xlate;
> > > +		if (chip->ops->set_polarity)
> > > +			chip->of_pwm_n_cells = 3;
> > > +		else
> > > +			chip->of_pwm_n_cells = 2;
> > 
> > I think the presence of the set_polarity callback shouldn't influence
> > the number of cells the parser expects. As commented on 2/2 this doesn't
> > actually mean the device actually support polarity inversion.
> 
> How so? A driver should only implement .set_polarity() if it supports
> changing the polarity.
> 
> That said, I agree that the presence of .set_polarity() shouldn't
> determine the number of cells. You could have any number of other flags
> set via the third cell.
> 
> > Also, polarity inversion could still be done in software for hardware
> > that doesn't support it.
> 
> No. You cannot emulate polarity inversion in software.
> 
Why not?

duty_ns = period_ns - duty_ns;


Lothar Waßmann
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