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

[Olliver Schinagl]

Hey Jonathan,


On 23-09-16 16:02, Jonathan Liu wrote:
> On 26 August 2016 at 03:50,  <oliver@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
> >   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));
> > +
> > +       spin_lock(&sun4i_pwm->ctrl_lock);
> > +       val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
> >          val &= ~BIT_CH(PWM_CLK_GATING, pwm->hwpwm);
> >          sun4i_pwm_writel(sun4i_pwm, val, PWM_CTRL_REG);
> >          spin_unlock(&sun4i_pwm->ctrl_lock);
> I had some issues where setting the brightness for pwm-backlight to 0
> was not turning off the backlight sometimes.
> This patch fixes the issue for me. Thanks.
You are welcome, but there's some work still being done in this regard, 
or rather, i have to find time to grab a scope and properly check the 
output. So expect the proper fix to be coming to a kernel near you soon :)

Meanwhile, it seems to still confirm that this is needed. Thanks on that.

Olliver
> Tested-by: Jonathan Liu <net147@xxxxxxxxx>