On Fri, Sep 30, 2022 at 06:51:12AM +0000, Biju Das wrote:
> Hi Uwe,
>
> > Subject: Re: [PATCH v7 2/2] pwm: Add support for RZ/G2L GPT
> >
> > Hello Biju,
> >
> > On Thu, Sep 29, 2022 at 05:36:38PM +0000, Biju Das wrote:
> > > > +static u8 rzg2l_calculate_prescale(struct rzg2l_gpt_chip
> > *rzg2l_gpt,
> > > > + u64 period_cycles)
> > > > +{
> > > > + u32 prescaled_period_cycles;
> > > > + u8 prescale;
> > > > +
> > > > + prescaled_period_cycles = period_cycles >> 32;
> > > > +
> > > > + if (prescaled_period_cycles >= 256)
> > > > + prescale = 5;
> > > > + else
> > > > + prescale = (roundup_pow_of_two(prescaled_period_cycles + 1)
> > > > + 1) / 2;
> > >
> > >
> > > This algorithm won't give desired result.
> > >
> > > prescaled_period_cycles Expected result
> > > 0 ->0
> > > 1..3 ->1
> > > 4..15 ->2
> > > 16..63 ->3
> > > 64..255 ->4
> > > 256 > ->5
> >
> > Oh, indeed, it fails for prescaled_period_cycles ∈ { 0, 3, 15, 63, 255
> > }.
> >
> > The correct formula is:
> >
> > if (prescaled_period_cycles >= 256)
> > prescale = 5;
> > else
> > prescale = (roundup_pow_of_two(prescaled_period_cycles) +
> > 1) / 2;
> >
>
> Round_pow_of_two(n) --> n=0 is not acceptable
>
> Round_pow_of_two(58)--> 64 as per the above formula, it becomes 64 + 1 /2 = 32
> Which is very high value.
Oh, I translated my (Python) prototype wrongly to Kernel-C, please make
this:
if (prescaled_period_cycles >= 256)
prescale = 5;
else
prescale = (fls(prescaled_period_cycles) + 1) / 2;
With fls(58) = 6 the result is 3 as intended.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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