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

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Date:	Mon, 7 Apr 2014 13:36:53 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Sascha Hauer <s.hauer@...gutronix.de>
Cc:	Lothar Waßmann <LW@...O-electronics.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

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.

Thierry

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