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Date:   Wed, 09 Nov 2022 11:36:35 +0000
From:   Paul Cercueil <paul@crapouillou.net>
Subject: Re: [PATCH v2 56/65] clk: ingenic: cgu: Switch to determine_rate
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Stephen Boyd <sboyd@kernel.org>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Chen-Yu Tsai <wens@csie.org>, Daniel Vetter <daniel@ffwll.ch>,
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Hi Maxime,

Le mer. 9 nov. 2022 =E0 11:53:01 +0100, Maxime Ripard=20
<maxime@cerno.tech> a =E9crit :
> Hi Paul,
>=20
> On Sat, Nov 05, 2022 at 10:33:54AM +0000, Paul Cercueil wrote:
>>  Hi Maxime,
>>=20
>>  Le ven. 4 nov. 2022 =E0 15:59:46 +0100, Maxime Ripard=20
>> <maxime@cerno.tech> a
>>  =E9crit :
>>  > Hi Paul,
>>  >
>>  > On Fri, Nov 04, 2022 at 02:31:20PM +0000, Paul Cercueil wrote:
>>  > >  Le ven. 4 nov. 2022 =E0 14:18:13 +0100, Maxime Ripard
>>  > > <maxime@cerno.tech> a
>>  > >  =E9crit :
>>  > >  > The Ingenic CGU clocks implements a mux with a set_parent=20
>> hook,
>>  > > but
>>  > >  > doesn't provide a determine_rate implementation.
>>  > >  >
>>  > >  > This is a bit odd, since set_parent() is there to, as its=20
>> name
>>  > > implies,
>>  > >  > change the parent of a clock. However, the most likely=20
>> candidate
>>  > > to
>>  > >  > trigger that parent change is a call to clk_set_rate(), with
>>  > >  > determine_rate() figuring out which parent is the best=20
>> suited for
>>  > > a
>>  > >  > given rate.
>>  > >  >
>>  > >  > The other trigger would be a call to clk_set_parent(), but=20
>> it's
>>  > > far less
>>  > >  > used, and it doesn't look like there's any obvious user for=20
>> that
>>  > > clock.
>>  > >  >
>>  > >  > So, the set_parent hook is effectively unused, possibly=20
>> because
>>  > > of an
>>  > >  > oversight. However, it could also be an explicit decision by=20
>> the
>>  > >  > original author to avoid any reparenting but through an=20
>> explicit
>>  > > call to
>>  > >  > clk_set_parent().
>>  > >  >
>>  > >  > The driver does implement round_rate() though, which means=20
>> that
>>  > > we can
>>  > >  > change the rate of the clock, but we will never get to=20
>> change the
>>  > >  > parent.
>>  > >  >
>>  > >  > However, It's hard to tell whether it's been done on purpose=20
>> or
>>  > > not.
>>  > >  >
>>  > >  > Since we'll start mandating a determine_rate()=20
>> implementation,
>>  > > let's
>>  > >  > convert the round_rate() implementation to a=20
>> determine_rate(),
>>  > > which
>>  > >  > will also make the current behavior explicit. And if it was=20
>> 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=20
>> callback; in
>>  > > my case
>>  > >  the clocks are parented from the device tree.
>>  > >
>>  > >  Having the clocks driver trigger a parent change when=20
>> 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=20
>> not
>>  > > it is
>>  > >  "safe" to use a designated parent.
>>  > >
>>  > >  For that reason, in practice, I never actually want to have a=20
>> 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=20
>> 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=20
>> ask to
>>  > change the parent.
>>  >
>>  > Given what you just said, I would suggest to set the
>>  > CLK_SET_RATE_NO_REPARENT flag as well.
>>=20
>>  But that would introduce policy into the driver...
>=20
> I'm not sure why you're bringing policies into that discussion.=20
> There's
> plenty of policy in the driver already, and the current code doesn't=20
> do
> something that the old wasn't doing (implicitly).

Yes, I was just talking about the CLK_SET_RATE_NO_REPARENT flag adding=20
policy. The fact that there's plenty of policy in the driver already is=20
not an argument for adding some more.

> And there's plenty of policies in drivers in general. Whether you=20
> limit
> the rate or not, whether you allow reparenting or not, even the
> CLK_SET_RATE_NO_REPARENT flag mentioned above is a policy decision set
> by drivers.

Allowing reparenting and not limiting the rates is not a policy, it's=20
just following what the hardware allows you to do. The absence of=20
policy means that the driver allows you to configure the hardware in=20
any way you might want to.

Limiting rates, forbidding reparenting, that's policy, and it doesn't=20
belong in a driver.

You can argue that choosing not to reparent on rate change is a policy,=20
and it is. That's why we need a way to enforce these policies outside=20
the driver.

>>  The fact that I don't want the MMC parented to the PLLs, doesn't=20
>> mean
>>  that it's an invalid configuration per se.
>=20
> Sure, and that's another policy :)

A policy that is not enforced by the driver.

Going back to the patch itself... I am fine with the change, although=20
the patch description should probably be updated. We have .set_parent=20
callbacks to configure clocks from DT, there's nothing more to it.

Cheers,
-Paul