Date:   Wed, 9 Nov 2022 12:00:45 +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

On Mon, Nov 07, 2022 at 08:57:22PM +0000, Aidan MacDonald wrote:
> 
> Maxime Ripard <maxime@...no.tech> writes:
> 
> > 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?
> 
> Assigning the parent clock in the DT works once, at boot, but going off
> what you wrote in the commit message, if the clock driver has a
> .determine_rate() implementation that *can* reparent clocks then it
> probably *will* reparent them, and the DT assignment will be lost.

Yes, indeed, but assigned-clock-parents never provided any sort of
guarantee on whether or not the clock was allowed to reparent or not.
It's just a one-off thing, right before probe, and a clk_set_parent()
call at probe will override that just fine.

Just like assigned-clock-rates isn't permanent.

> What I'm suggesting is a runtime constraint that the clock subsystem
> would enforce, and actively prevent drivers from changing the parent.
> Either explicitly with clk_set_parent() or due to .determine_rate().
> 
> That way you could write a .determine_rate() implementation that *can*
> select a better parent, but if the DT applies a constraint to fix the
> clock to a particular parent, the clock subsystem will force that parent
> to be used so you can be sure the clock is never reparented by accident.

Yeah, that sounds like a good idea, and CLK_SET_RATE_NO_REPARENT isn't
too far off from this, it's just ignored by clk_set_parent() for now. I
guess we could rename CLK_SET_RATE_NO_REPARENT to CLK_NO_REPARENT, make
clk_set_parent handle it, and set that flag whenever
assigned-clock-parents is set on a clock.

It's out of scope for this series though, and I certainly don't want to
deal with all the regressions it might create :)

Maxime

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