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Date:   Mon, 12 Jun 2023 14:31:21 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Frank Oltmanns <frank@oltmanns.dev>
Cc:     Andre Przywara <andre.przywara@arm.com>,
        Chen-Yu Tsai <wens@csie.org>,
        Jernej Skrabec <jernej.skrabec@gmail.com>,
        Michael Turquette <mturquette@baylibre.com>,
        Roman Beranek <me@crly.cz>,
        Samuel Holland <samuel@sholland.org>,
        Stephen Boyd <sboyd@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-clk@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-sunxi@lists.linux.dev
Subject: Re: [PATCH v2 1/2] clk: sunxi-ng: nkm: consider alternative parent
 rates when finding rate
Message-ID: <sfni3vehkhotsqrrirklhzrxzkcxzkq6nbtqokeab5if3jgm53@frh7z3iowsfe>
References: <20230611090143.132257-1-frank@oltmanns.dev>
 <20230611090143.132257-2-frank@oltmanns.dev>
 <87edmh12s7.fsf@oltmanns.dev>
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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 lon=
g parent, unsigned long rate,
> >  			for (_m =3D nkm->min_m; _m <=3D nkm->max_m; _m++) {
>=20
> According to the manual M/N has to be <=3D 3. Therefore we need a
> different maximum value for the _m-for-loop:
>=20
>         unsigned long max_m =3D min(3 * _n, nkm->max_m);
>         for (_m =3D nkm->min_m; _m <=3D max_m; _m++) {
>=20
> 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 =3D parent * _n * _k / _m;
> > -
> > +				if (parent_hw) {
> > +					tmp_parent =3D optimal_parent_rate(rate, _n, _k, _m);
> > +					tmp_parent =3D clk_hw_round_rate(parent_hw, tmp_parent);
> > +				}
>=20
> Another constraint is PLL-VIDEO0 rate / M >=3D 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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