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

[Olliver Schinagl <oliver@schinagl.nl>]

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On za, 2016-09-24 at 22:25 +0200, Maxime Ripard wrote:
> Hi Oliver,
> 
> Sorry for the slow answer.
> 
> On Fri, Sep 09, 2016 at 11:01:08AM +0200, Olliver Schinagl wrote:
> > 
> > > 
> > > > 
> > > > > 
> > > > > > 
> > > > > > *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;
> 
> For the spin_lock part, I was just comparing it to a
> spin_lock_irqsave, which is pretty expensive since it masks all the
> interrupts in the system, introducing latencies.
so spin_lock is very expensive and we should avoid if we can?
> 
> > 
> > but I think we need the ndelay for the else where we do not
> > have the ready flag (A10 or A13 iirc?)
> 
> Hmmmm, good point. But that would also apply to your second patch
> then, wouldn't it?
yeah, you would have an if/else for the case of !hasready.

this is what i've been dabbling in the train last week, but haven't
thought it through yet, let alone tested it:


+       if (!(sun4i_pwm->data->has_rdy))
+               ndelay(pwm_get_period(pwm));
+       else
+               do {
+                       spin_lock(&sun4i_pwm->ctrl_lock);
+                       val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
+                       spin_unlock(&sun4i_pwm->ctrl_lock);
+               } while (!(val & PWM_RDY(pwm->hwpwm)))

Here I assumed the spin_lock is cheap to make, expensive to hold for
long, e.g. reducing the length the spin-lock is active for. the
alternative was to remove the spin_lock here, and remove unlock-lock
before-after this block where you basically get a very long lasting
spin_lock, the alternative.
 
        spin_lock(&sun4i_pwm->ctrl_lock);
        val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
+       if (sun4i_pwm->data->has_rdy && (!(val & PWM_RDY(pwm->hwpwm))))
+               dev_warn(chip->dev, "never became ready\n");

this may be useful for debugging i thought.

        val &= ~BIT_CH(PWM_CLK_GATING, pwm->hwpwm);
        sun4i_pwm_writel(sun4i_pwm, val, PWM_CTRL_REG);
        spin_unlock(&sun4i_pwm->ctrl_lock);

Olliver
> 
> Maxime
> 

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