Message-Id: <Z8W2LR.DTD355V5EAE02@crapouillou.net>
Date:   Wed, 09 Nov 2022 11:36:35 +0000
From:   Paul Cercueil <paul@...pouillou.net>
To:     Maxime Ripard <maxime@...no.tech>
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Subject: Re: [PATCH v2 56/65] clk: ingenic: cgu: Switch to determine_rate

Hi Maxime,

Le mer. 9 nov. 2022 à 11:53:01 +0100, Maxime Ripard 
<maxime@...no.tech> a écrit :
> Hi Paul,
> 
> On Sat, Nov 05, 2022 at 10:33:54AM +0000, Paul Cercueil wrote:
>>  Hi Maxime,
>> 
>>  Le ven. 4 nov. 2022 à 15:59:46 +0100, Maxime Ripard 
>> <maxime@...no.tech> a
>>  écrit :
>>  > 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.
>> 
>>  But that would introduce policy into the driver...
> 
> I'm not sure why you're bringing policies into that discussion. 
> There's
> plenty of policy in the driver already, and the current code doesn't 
> do
> something that the old wasn't doing (implicitly).

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

> And there's plenty of policies in drivers in general. Whether you 
> 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 
just following what the hardware allows you to do. The absence of 
policy means that the driver allows you to configure the hardware in 
any way you might want to.

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

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

>>  The fact that I don't want the MMC parented to the PLLs, doesn't 
>> mean
>>  that it's an invalid configuration per se.
> 
> 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 
the patch description should probably be updated. We have .set_parent 
callbacks to configure clocks from DT, there's nothing more to it.

Cheers,
-Paul

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