[PATCH v10 1/3] pwm: driver for qualcomm ipq6018 pwm block

[Uwe Kleine-König]

Hello Baruch,

On Tue, Jan 25, 2022 at 06:22:45PM +0200, Baruch Siach wrote:
> On Tue, Jan 25 2022, Uwe Kleine-König wrote:
> > On Tue, Jan 25, 2022 at 03:03:08PM +0200, Baruch Siach wrote:
> >> On Wed, Jan 19 2022, Uwe Kleine-König wrote:
> >> > The task here is to calculate the biggest pwm_div for a given pre_div
> >> > such that
> >> >
> >> >
> >> > 	(pre_div + 1) * (pwm_div + 1) * NSEC_PER_SEC
> >> > 	-------------------------------------------- <= period_ns
> >> > 	                   rate
> >> >
> >> > right?
> >> >
> >> > This is equivalent to:
> >> >
> >> > 	                  period_ns * rate
> >> > 	pre_div <= ---------------------------- - 1
> >> > 	           (pre_div + 1) * NSEC_PER_SEC
> >> >
> >> > As pre_div is integer, rounding down should be fine?!
> >> 
> >> I can't follow. With round down (as in v8) the result is always:
> >> 
> >>   NSEC_PER_SEC * (pre_div + 1) * (pwm_div + 1) <= period_rate
> >
> > Yes, that's the condition that a valid configuration should fulfill
> > because then the configured period is never bigger than the requested
> > period.
> >  
> >> As a result, 'diff' calculation below will always produce diff <= 0. When
> >> there is no diff == 0 result (bingo) we get IPQ_PWM_MAX_DIV in both best_
> >> values at the end of the loop.
> >
> > Looking again, your check is wrong. I think you need:
> >
> > 	diff = period_rate - NSEC_PER_SEC * (pre_div + 1) * (pwm_div + 1)
> >
> > . Given the calculations for pre_div and pwm_div this should never be
> > negative and you should pick values that minimize diff.
> 
> So, if I understand correctly, you suggest to leave round up as in v10,
> and invert the diff calculation. Is that correct?

If you agree that this results in the intended setting and keeps dmesg
clean even with PWM_DEBUG enabled: yes.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |
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