Hi Uwe,
Jones <lee.jones@xxxxxxxxxx>; Philipp
> Zabel <p.zabel@xxxxxxxxxxxxxx>
> Subject: Re: [PATCH v7 2/2] pwm: Add support for RZ/G2L GPT
>
> 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.
Thanks.
Tested on target and results are ok now.
+ for (i =0 ; i < 256; i++) {
+ pr_info("####### %s ####%u/%u\n",__func__,i,(fls(i) + 1) / 2);
+ }
[ 34.740902] ####### rzg2l_gpt_calculate_prescale ####0/0
[ 34.759151] ####### rzg2l_gpt_calculate_prescale ####3/1
[ 34.825619] ####### rzg2l_gpt_calculate_prescale ####15/2
[ 35.094655] ####### rzg2l_gpt_calculate_prescale ####63/3
[ 36.184126] ####### rzg2l_gpt_calculate_prescale ####255/4
Cheers,
Biju
