linux-kernel - Re: [PATCH] pwm: lpc32xx

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Message-ID: <20121108113353.4fdbb25c@avionic-0020.adnet.avionic-design.de>
Date:	Thu, 8 Nov 2012 11:33:53 +0100
From:	Alban Bedel <alban.bedel@...onic-design.de>
To:	Roland Stigge <stigge@...com.de>
Cc:	Thierry Reding <thierry.reding@...onic-design.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexandre Pereira da Silva <aletes.xgr@...il.com>
Subject: Re: [PATCH] pwm: lpc32xx - Fix the PWM polarity

On Thu, 08 Nov 2012 10:51:35 +0100
Roland Stigge <stigge@...com.de> wrote:

> On 07/11/12 16:25, Alban Bedel wrote:
> > Signed-off-by: Alban Bedel <alban.bedel@...onic-design.de>
> > ---
> >  drivers/pwm/pwm-lpc32xx.c |    6 +++++-
> >  1 files changed, 5 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/pwm/pwm-lpc32xx.c b/drivers/pwm/pwm-lpc32xx.c
> > index adb87f0..0dc278d 100644
> > --- a/drivers/pwm/pwm-lpc32xx.c
> > +++ b/drivers/pwm/pwm-lpc32xx.c
> > @@ -51,7 +51,11 @@ static int lpc32xx_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> >  
> >  	c = 256 * duty_ns;
> >  	do_div(c, period_ns);
> > -	duty_cycles = c;
> > +	if (c == 0)
> > +		c = 256;
> > +	if (c > 255)
> > +		c = 255;
> > +	duty_cycles = 256 - c;
> 
> Except for the range check (for the original c > 255), this results in:
> 
> 	duty_cycles = 256 - c
> 
> except for (c == 0) where
> 
> 	duty_cycles = 1

No it lead to duty_cycles = 0

> which actually is
> 
> 	duty_cycles = (256 - c) - 255
> 
> (think with the original c)
> 
> i.e. nearly a polarity inversion in the case of (c == 0).
> 
> Why is the case (c == 0) so special here? Maybe you can document this,
> if it is really intended?

It is intended, the formular for duty value in the register is:

duty = (256 - 256*duty_ns/period_ns) % 256

But the code avoid the modulo by clamping '256*duty_ns/period_ns' to 1-256.

Perhaps something like:

if (c > 255)
	c = 255;
duty_cycles = (256 - c) % 256;

would be easier to understand.

Alban
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