linux-kernel - Re: pwm: atmel: Fix disabling of PWM channels

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Date:	Mon, 13 Jun 2016 12:31:58 +0200
From:	Guillermo Rodriguez Garcia <guille.rodriguez@...il.com>
To:	linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: pwm: atmel: Fix disabling of PWM channels

(resending to MLs for the record -- previous message bounced)

El viernes, 10 de junio de 2016, Alexandre Belloni
<alexandre.belloni@...e-electrons.com> escribió:
>
> Hi,
>
> On 13/05/2016 at 13:09:37 +0200, Guillermo Rodriguez wrote :
> > When disabling a PWM channel, the PWM clock was being stopped
> > immediately after writing to PWM_DIS. As a result, the disabling
> > of the PWM channel did not complete properly, and the PWM output
> > might be left at the wrong level.
> >
> > Fix this by waiting for the channel to be effectively disabled
> > (by checking the PWM_SR register) before disabling the clock.
> >
> > Signed-off-by: Guillermo Rodriguez <guille.rodriguez@...il.com>
> > ---
> >  drivers/pwm/pwm-atmel.c |   10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/drivers/pwm/pwm-atmel.c b/drivers/pwm/pwm-atmel.c
> > index 0e4bd4e..a714434 100644
> > --- a/drivers/pwm/pwm-atmel.c
> > +++ b/drivers/pwm/pwm-atmel.c
> > @@ -271,6 +271,16 @@ static void atmel_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm)
> >       mutex_unlock(&atmel_pwm->isr_lock);
> >       atmel_pwm_writel(atmel_pwm, PWM_DIS, 1 << pwm->hwpwm);
> >
> > +     /*
> > +      * Wait for the PWM channel disable operation to be effective before
> > +      * stopping the clock.
> > +      */
> > +     timeout = jiffies + 2 * HZ;
> > +     while ((atmel_pwm_readl(atmel_pwm, PWM_SR)& (1 << pwm->hwpwm)) &&
> > +            time_before(jiffies, timeout)) {
> > +             usleep_range(10, 100);
> > +     }
> > +
>
> While this seems good, can you tell on which SoC you observed that? I'd
> like to understand whether this is only on v1 or v2 or both.

I observed it on a sama5d3, however this article from atmel's KB would
seem to suggest that it is not specific to this SoC:

http://atmel.force.com/support/pkb_mobile#/articles/en_US/FAQ/Why-PWM-Channel-Polarity-Inversion

Guillermo