Date:   Mon, 7 Nov 2022 09:54:17 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Aidan MacDonald <aidanmacdonald.0x0@...il.com>
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Subject: Re: [PATCH v2 56/65] clk: ingenic: cgu: Switch to determine_rate

Hi,

On Fri, Nov 04, 2022 at 05:35:29PM +0000, Aidan MacDonald wrote:
> 
> 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.

I'm not really sure what you mean there, sorry. Isn't it what
assigned-clock-parents/clk_set_parent() at probe, plus a determine_rate
implementation that would affect best_parent_hw would already provide?

> 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.

Like I said, the default implementation is already working to what you
suggested if I understood properly. However, this has never been tested
for any of the drivers in that series so I don't want to introduce (and
debug ;)) regressions in all those drivers that were not setting any
constraint but never actually tested their reparenting code.

So that series is strictly equivalent to what you had before, it's just
explicit now.

If you find that some other decision make sense for your driver in
particular cases, feel free to change it. I barely know most of these
platforms, so I won't be able to make that decision (and test it)
unfortunately.

Maxime

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