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

[Maxime Ripard]

On Tue, Jun 13, 2023 at 12:17:06PM +0200, Frank Oltmanns wrote:
> Hi Maxime,
> 
> I'll now only respond to one aspect of your mail, because it's the
> foundation for the whole behaviour.
> 
> On 2023-06-13 at 11:10:08 +0200, Maxime Ripard <maxime@xxxxxxxxxx> wrote:
> [...]
> >> >> ccu_nkm_find_best is called in the following two situations:
> >> >>  a. from ccu_nkm_set_rate when setting the rate
> >> >>  b. from ccu_nkm_round_rate when determining the rate
> >> >>
> >> >> In situation a. we never want ccu_nkm_find_best to try different parent
> >> >> rates because setting the parent rate is a done deal (at least that's my
> >> >> understanding).
> >> >>
> >> >> In situation b. we only want ccu_nkm_find_best to try different parent
> >> >> rates when, as you mentioned, the CLK_SET_RATE_PARENT flag is set.
> >> >
> >> > It doesn't really matter though. The output of that function must be
> >> > stable and must return the same set of factors and parent rate for a
> >> > given target rate.
> >> >
> >>
> >> I'm not sure if we're talking about the same thing here. Of course the
> >> set of factors and parent rate for a given target rate will be different
> >> depending on the fact if we can or cannot adjust the parent rate,
> >> agreed?
> >
> > Yes, but here you also have a different behaviour in clk_round_rate()
> > and in clk_set_rate(), which isn't ok.
> >
> > Basically, clk_set_rate() + clk_get_rate() must be equal to
> > clk_round_rate().
> >
> > If you change if you look for parents depending on whether you're being
> > called in clk_round_rate() and clk_set_rate(), then you're breaking that
> > expectation.
> >
> >> Let me compare my implementation to ccu_mp.
> >>
> >> ccu_mp_round_rate either calls the function ccu_mp_find_best or
> >> ccu_mp_find_best_with_parent_adj, depending on CLK_SET_RATE_PARENT.
> >
> > Yes, and it's fine: the flag is per-clock, and the output is the same
> > depending on whether we're being called by clk_round_rate() and
> > clk_set_rate().
> >
> 
> The output is really not the same.
> 
> ccu_mp_set_rate() always calls ccu_mp_find_best(). It never (!) considers
> changing the parent, independent of any flags.
> 
> ccu_mp_round_rate() is calling ccu_mp_find_best() OR
> ccu_mp_find_best_with_parent_adj() depending on the flag.
> 
> If I understand you correctly, you consider that a bug.

No, sorry, you're right.

clk_set_rate will call round_rate first, which will (possibly) pick up a
new parent, and by the time set_rate is called our parent will have been
changed already so we will just call find_best again considering only
that parent.

The set of factors and dividers should remain the same there, but I
don't think that's a concern.

That leaves us with the rounding stuff, and the overall function
arguments. I like the structure of ccu_mp better, is there a reason to
deviate from it?

Maxime

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