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

[Maxime Ripard <maxime.ripard@free-electrons.com>]

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On Thu, Dec 08, 2016 at 02:23:39PM +0100, Olliver Schinagl wrote:
> Hey Maxime,
> 
> first off, also sorry for the slow delay :) (pun not intended)
> 
> On 27-08-16 00:19, 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?
> 
> I started to implement our earlier discussed suggestions, but I do not think
> they will work. The read bit is not to let the user know it is ready with
> all of its commands, but only if the period registers are ready. I think it
> is some write lock while it copies the data into its internal control loop.
> From the manual:
> PWM0 period register ready.
> 0: PWM0 period register is ready to write,
> 1: PWM0 period register is busy.
> 
> 
> So no, I don't think i can use the ready bit here at all. The only thing we
> can do here, but I doubt it's worth it, is to read the period register,
> caluclate a time from it, and then ndelay(pwm_get_period(pwm) - ran_time)
> 
> The only 'win' then is that we could are potentially not waiting the full
> pwm period, but only a fraction of it. Since we are disabling the hardware
> (for power reasons) anyway, I don't think this is any significant win,
> except for extreme situations. E.g. we have a pwm period of 10 seconds, we
> disable it after 9.9 second, and now we have to wait for 10 seconds before
> the pwm_disable is finally done. So this could in that case be reduced to
> then only wait for 0.2 seconds since it is 'done' sooner.
> 
> However that optimization is also not 'free'. We have to read the period
> register and calculate back the time. I suggest to do that when reworking
> this driver to work with atomic mode, and merge this patch 'as is' to
> atleast fix te bug where simply not finish properly.

That whole discussion made me realise something that is really
bad. AFAIK, pwm_get_period returns a 32 bits register, which means a
theorical period of 4s. Busy looping during 4 seconds is already very
bad, as you basically kill one CPU during that time, but doing so in a
(potentially) atomic context is even worse.

NACK.

Maxime

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

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