linux-kernel - Re: [PATCH 1/2] pwm: sunxi: allow the pwm to finish its pulse before disable

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Date:   Tue, 6 Sep 2016 21:51:49 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Olliver Schinagl <oliver@...inagl.nl>
Cc:     Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Chen-Yu Tsai <wens@...e.org>, linux-pwm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] pwm: sunxi: allow the pwm to finish its pulse before
 disable

On Tue, Sep 06, 2016 at 09:12:56AM +0200, Olliver Schinagl wrote:
> Hi Maxime!,
> 
> On za, 2016-08-27 at 00:19 +0200, Maxime Ripard wrote:
> > On Thu, Aug 25, 2016 at 07:50:10PM +0200, Olliver Schinagl wrote:
> > > 
> > > When we inform the PWM block to stop toggeling the output, we may
> > > end up
> > > in a state where the output is not what we would expect (e.g. not
> > > the
> > > low-pulse) but whatever the output was at when the clock got
> > > disabled.
> > > 
> > > To counter this we have to wait for maximally the time of one whole
> > > period to ensure the pwm hardware was able to finish. Since we
> > > already
> > > told the PWM hardware to disable it self, it will not continue
> > > toggling
> > > but merly finish its current pulse.
> > > 
> > > If a whole period is considered to much, it may be contemplated to
> > > use a
> > > half period + a little bit to ensure we get passed the transition.
> > > 
> > > Signed-off-by: Olliver Schinagl <oliver@...inagl.nl>
> > > ---
> > >  drivers/pwm/pwm-sun4i.c | 11 +++++++++++
> > >  1 file changed, 11 insertions(+)
> > > 
> > > diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> > > index 03a99a5..5e97c8a 100644
> > > --- a/drivers/pwm/pwm-sun4i.c
> > > +++ b/drivers/pwm/pwm-sun4i.c
> > > @@ -8,6 +8,7 @@
> > >  
> > >  #include <linux/bitops.h>
> > >  #include <linux/clk.h>
> > > +#include <linux/delay.h>
> > >  #include <linux/err.h>
> > >  #include <linux/io.h>
> > >  #include <linux/module.h>
> > > @@ -245,6 +246,16 @@ static void sun4i_pwm_disable(struct pwm_chip
> > > *chip, struct pwm_device *pwm)
> > >  	spin_lock(&sun4i_pwm->ctrl_lock);
> > >  	val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
> > >  	val &= ~BIT_CH(PWM_EN, pwm->hwpwm);
> > > +	sun4i_pwm_writel(sun4i_pwm, val, PWM_CTRL_REG);
> > > +	spin_unlock(&sun4i_pwm->ctrl_lock);
> > > +
> > > +	/* Allow for the PWM hardware to finish its last toggle.
> > > The pulse
> > > +	 * may have just started and thus we should wait a full
> > > period.
> > > +	 */
> > > +	ndelay(pwm_get_period(pwm));
> > 
> > Can't that use the ready bit as well?
> It depends whatever is cheaper. If we disable the pwm, we have to
> commit that request to hardware first. Then we have to read back the
> has ready and in the strange situation it is not, wait for it to become
> ready?

If it works like you were suggesting, yes.

> Also, that would mean we would loop in a spin lock, or keep
> setting/clearing an additional spinlock to read the ready bit.

You're using a spin_lock, so it's not that bad, but I was just
suggesting replacing the ndelay.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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