From: Olliver Schinagl <oliver@schinagl.nl>
To: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>,
Thierry Reding <thierry.reding@gmail.com>,
Chen-Yu Tsai <wens@csie.org>,
linux-pwm@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] pwm: sunxi: allow the pwm to finish its pulse before disable
Date: Fri, 09 Sep 2016 11:01:08 +0200 [thread overview]
Message-ID: <1473411668.731.75.camel@schinagl.nl> (raw)
In-Reply-To: <20160906195149.GJ9040@lukather>
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On di, 2016-09-06 at 21:51 +0200, Maxime Ripard wrote:
> 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@schinagl.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.
If you say the spin_lock + wait for the ready is just as expensive as
the ndelay, or the ndelay is less preferred, then I gladly make the
change; but I think we need the ndelay for the else where we do not
have the ready flag (A10 or A13 iirc?)
Olliver
>
> Maxime
>
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next prev parent reply other threads:[~2016-09-09 9:01 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-25 17:50 [PATCHv2 0/2] pwm: sunxi: give the pwm IP block more time Olliver Schinagl
2016-08-25 17:50 ` [PATCH 1/2] pwm: sunxi: allow the pwm to finish its pulse before disable Olliver Schinagl
2016-08-26 22:19 ` Maxime Ripard
2016-09-06 7:12 ` Olliver Schinagl
2016-09-06 19:51 ` Maxime Ripard
2016-09-09 9:01 ` Olliver Schinagl [this message]
2016-09-24 20:25 ` Maxime Ripard
2016-09-26 8:46 ` Olliver Schinagl
2016-09-27 20:16 ` Maxime Ripard
[not found] ` <afcb938d-d2df-4740-6c85-cdf2766f671c@schinagl.nl>
2016-12-12 12:24 ` Maxime Ripard
2017-01-03 15:59 ` Olliver Schinagl
2017-01-03 16:55 ` Alexandre Belloni
2017-01-04 6:36 ` Thierry Reding
2016-09-23 14:02 ` [1/2] " Jonathan Liu
2016-09-23 14:03 ` Olliver Schinagl
2017-05-05 1:54 ` Jonathan Liu
2016-08-25 17:50 ` [PATCH 2/2] pwm: sunxi: Yield some time to the pwm-block to become ready Olliver Schinagl
2016-08-26 22:25 ` Maxime Ripard
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