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

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Date:   Thu, 15 Jun 2023 18:04:53 +0200 (GMT+02:00)
From:   Frank Oltmanns <frank@...manns.dev>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Andre Przywara <andre.przywara@....com>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Michael Turquette <mturquette@...libre.com>,
        Roman Beranek <me@...y.cz>,
        Samuel Holland <samuel@...lland.org>,
        Stephen Boyd <sboyd@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-sunxi@...ts.linux.dev
Subject: Re: [PATCH 1/2] clk: sunxi-ng: nkm: consider alternative parent
 rates when finding rate


15.06.2023 16:47:33 Maxime Ripard <maxime@...no.tech>:

> 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@...no.tech> wrote:
>> [...]
>>>>>> …
>>>>>
>>>>> 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.

Ok, no worries. That was my understanding, so your previous statement shattered my worldview. ;) That's why I may have seemed a bit alarmed.

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

Ack. The output is stable when called with the same rate.

> 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?

I'm still pondering the rounding stuff. I'm just not sure why you are so relaxed about the fact that when calling round_rate with 449064000 we get 449035712, but when we call get round_rate with 449035712 we get 449018181, and when we call round_rate with 449018181, we get 449018180.

But ultimately, you have the final word, of course. But I need some time to be sure, that this does not become a problem in some cases.

Regarding the arguments, I can adopt ccu_mp's style. The only reason was to avoid code duplication, no other reason.

Thanks,
  Frank

> > Maxime