Date:   Fri, 04 Nov 2022 17:35:29 +0000
From:   Aidan MacDonald <aidanmacdonald.0x0@...il.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Paul Cercueil <paul@...pouillou.net>,
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Subject: Re: [PATCH v2 56/65] clk: ingenic: cgu: Switch to determine_rate


Maxime Ripard <maxime@...no.tech> writes:

> Hi Paul,
>
> On Fri, Nov 04, 2022 at 02:31:20PM +0000, Paul Cercueil wrote:
>> Le ven. 4 nov. 2022 à 14:18:13 +0100, Maxime Ripard <maxime@...no.tech> a
>> écrit :
>> > The Ingenic CGU clocks implements a mux with a set_parent hook, but
>> > doesn't provide a determine_rate implementation.
>> >
>> > This is a bit odd, since set_parent() is there to, as its name implies,
>> > change the parent of a clock. However, the most likely candidate to
>> > trigger that parent change is a call to clk_set_rate(), with
>> > determine_rate() figuring out which parent is the best suited for a
>> > given rate.
>> >
>> > The other trigger would be a call to clk_set_parent(), but it's far less
>> > used, and it doesn't look like there's any obvious user for that clock.
>> >
>> > So, the set_parent hook is effectively unused, possibly because of an
>> > oversight. However, it could also be an explicit decision by the
>> > original author to avoid any reparenting but through an explicit call to
>> > clk_set_parent().
>> >
>> > The driver does implement round_rate() though, which means that we can
>> > change the rate of the clock, but we will never get to change the
>> > parent.
>> >
>> > However, It's hard to tell whether it's been done on purpose or not.
>> >
>> > Since we'll start mandating a determine_rate() implementation, let's
>> > convert the round_rate() implementation to a determine_rate(), which
>> > will also make the current behavior explicit. And if it was an
>> > oversight, the clock behaviour can be adjusted later on.
>>
>> So it's partly on purpose, partly because I didn't know about
>> .determine_rate.
>>
>> There's nothing odd about having a lonely .set_parent callback; in my case
>> the clocks are parented from the device tree.
>>
>> Having the clocks driver trigger a parent change when requesting a rate
>> change sounds very dangerous, IMHO. My MMC controller can be parented to the
>> external 48 MHz oscillator, and if the card requests 50 MHz, it could switch
>> to one of the PLLs. That works as long as the PLLs don't change rate, but if
>> one is configured as driving the CPU clock, it becomes messy.
>> The thing is, the clocks driver has no way to know whether or not it is
>> "safe" to use a designated parent.
>>
>> For that reason, in practice, I never actually want to have a clock
>> re-parented - it's almost always a bad idea vs. sticking to the parent clock
>> configured in the DTS.
>
> Yeah, and this is totally fine. But we need to be explicit about it. The
> determine_rate implementation I did in all the patches is an exact
> equivalent to the round_rate one if there was one. We will never ask to
> change the parent.
>
> Given what you just said, I would suggest to set the
> CLK_SET_RATE_NO_REPARENT flag as well.
>

Ideally there should be a way for drivers and the device tree to
say, "clock X must be driven by clock Y", but the clock framework
would be allowed to re-parent clocks freely as long as it doesn't
violate any DT or driver constraints.

That way allowing reparenting doesn't need to be an all-or-nothing
thing, and it doesn't need to be decided at the clock driver level
with special flags.

Regards,
Aidan

>> > Signed-off-by: Maxime Ripard <maxime@...no.tech>
>> > ---
>> >  drivers/clk/ingenic/cgu.c | 15 ++++++++-------
>> >  1 file changed, 8 insertions(+), 7 deletions(-)
>> >
>> > diff --git a/drivers/clk/ingenic/cgu.c b/drivers/clk/ingenic/cgu.c
>> > index 1f7ba30f5a1b..0c9c8344ad11 100644
>> > --- a/drivers/clk/ingenic/cgu.c
>> > +++ b/drivers/clk/ingenic/cgu.c
>> > @@ -491,22 +491,23 @@ ingenic_clk_calc_div(struct clk_hw *hw,
>> >  	return div;
>> >  }
>> >
>> > -static long
>> > -ingenic_clk_round_rate(struct clk_hw *hw, unsigned long req_rate,
>> > -		       unsigned long *parent_rate)
>> > +static int ingenic_clk_determine_rate(struct clk_hw *hw,
>> > +				      struct clk_rate_request *req)
>> >  {
>> >  	struct ingenic_clk *ingenic_clk = to_ingenic_clk(hw);
>> >  	const struct ingenic_cgu_clk_info *clk_info =
>> > to_clk_info(ingenic_clk);
>> >  	unsigned int div = 1;
>> >
>> >  	if (clk_info->type & CGU_CLK_DIV)
>> > -		div = ingenic_clk_calc_div(hw, clk_info, *parent_rate, req_rate);
>> > +		div = ingenic_clk_calc_div(hw, clk_info, req->best_parent_rate,
>> > +					   req->rate);
>>
>> Sorry but I'm not sure that this works.
>>
>> You replace the "parent_rate" with the "best_parent_rate", and that means
>> you only check the requested rate vs. the parent with the highest frequency,
>> and not vs. the actual parent that will be used.
>
> best_parent_rate is initialized to the current parent rate, not the
> parent with the highest frequency:
> https://elixir.bootlin.com/linux/v6.1-rc3/source/drivers/clk/clk.c#L1471
>
> Maxime

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