Hi Uwe,
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?
Thanks,
baruch
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