Re: [PATCH v2 1/2] clk: sunxi-ng: nkm: consider alternative parent rates when finding rate

[Maxime Ripard]

On Mon, Jun 12, 2023 at 10:51:52AM +0200, Frank Oltmanns wrote:
> > @@ -28,12 +68,17 @@ static unsigned long ccu_nkm_find_best(unsigned long parent, unsigned long rate,
> >  			for (_m = nkm->min_m; _m <= nkm->max_m; _m++) {
> 
> According to the manual M/N has to be <= 3. Therefore we need a
> different maximum value for the _m-for-loop:
> 
>         unsigned long max_m = min(3 * _n, nkm->max_m);
>         for (_m = nkm->min_m; _m <= max_m; _m++) {
> 
> I suggest that I add an optional member max_mn_ratio to the structs
> ccu_nkm and _ccu_nkm. Optional meaning: Ignore if 0.

Which workload is affected by this restriction?

> >  				unsigned long tmp_rate;
> >
> > -				tmp_rate = parent * _n * _k / _m;
> > -
> > +				if (parent_hw) {
> > +					tmp_parent = optimal_parent_rate(rate, _n, _k, _m);
> > +					tmp_parent = clk_hw_round_rate(parent_hw, tmp_parent);
> > +				}
> 
> Another constraint is PLL-VIDEO0 rate / M >= 24 MHz. Therefore we also need:
>         if (tmp_parent < 24000000 * _m)
>                 continue;
>
> So, we need another optional member min_m_times_parent or, for
> shortness, maybe min_m_parent. I could use help finding a good name for
> this.

Again, if it's not causing any harm I'd rather keep the code as simple
and maintainable as possible.

Maxime

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