linux-kernel - Re: [PATCH v3 56/65] clk: ingenic: cgu: Switch to determine

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Date:   Wed, 5 Apr 2023 17:19:06 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Andreas Färber <afaerber@...e.de>,
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Subject: Re: [PATCH v3 56/65] clk: ingenic: cgu: Switch to determine_rate

Hi Paul,

On Wed, Apr 05, 2023 at 03:04:05PM +0200, Paul Cercueil wrote:
> Le mardi 04 avril 2023 à 12:11 +0200, Maxime Ripard 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().
> 
> As I said in the v2 (IIRC), clk_set_parent() is used when re-parenting
> from the device tree.

Yep, it's indeed an oversight in the commit log.

> > 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 just to be sure, this patch won't make clk_set_rate() automatically
> switch parents, right?
> 
> Allowing automatic re-parenting sounds like a huge can of worms...

The behaviour is strictly equivalent before that patch and after: the
driver will not reparent on a rate change. It just makes it explicit.

Maxime

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