linux-kernel - Re: [PATCH v3 1/5] clk: Respect CLK_OPS_PARENT

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Message-ID: <aPaPSeClhiq2WYJN@redhat.com>
Date: Mon, 20 Oct 2025 15:36:41 -0400
From: Brian Masney <bmasney@...hat.com>
To: Nicolas Frattaroli <nicolas.frattaroli@...labora.com>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...nel.org>,
	Dong Aisheng <aisheng.dong@....com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Yassine Oudjana <y.oudjana@...tonmail.com>,
	Laura Nao <laura.nao@...labora.com>,
	Nícolas F. R. A. Prado <nfraprado@...labora.com>,
	Chia-I Wu <olvaffe@...il.com>, Chen-Yu Tsai <wenst@...omium.org>,
	kernel@...labora.com, linux-clk@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v3 1/5] clk: Respect CLK_OPS_PARENT_ENABLE during recalc

Hi Nicolas,

On Fri, Oct 17, 2025 at 02:21:55PM +0200, Nicolas Frattaroli wrote:
> On Thursday, 16 October 2025 22:52:30 Central European Summer Time Brian Masney wrote:
> > On Fri, Oct 10, 2025 at 10:47:09PM +0200, Nicolas Frattaroli wrote:
> > > When CLK_OPS_PARENT_ENABLE was introduced, it guarded various clock
> > > operations, such as setting the rate or switching parents. However,
> > > another operation that can and often does touch actual hardware state is
> > > recalc_rate, which may also be affected by such a dependency.
> > > 
> > > Add parent enables/disables where the recalc_rate op is called directly.
> > > 
> > > Fixes: fc8726a2c021 ("clk: core: support clocks which requires parents enable (part 2)")
> > > Fixes: a4b3518d146f ("clk: core: support clocks which requires parents enable (part 1)")
> > > Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
> > > Reviewed-by: Chen-Yu Tsai <wenst@...omium.org>
> > > Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@...labora.com>
> > > ---
> > >  drivers/clk/clk.c | 13 +++++++++++++
> > >  1 file changed, 13 insertions(+)
> > > 
> > > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > > index 85d2f2481acf360f0618a4a382fb51250e9c2fc4..1b0f9d567f48e003497afc98df0c0d2ad244eb90 100644
> > > --- a/drivers/clk/clk.c
> > > +++ b/drivers/clk/clk.c
> > > @@ -1921,7 +1921,14 @@ static unsigned long clk_recalc(struct clk_core *core,
> > >  	unsigned long rate = parent_rate;
> > >  
> > >  	if (core->ops->recalc_rate && !clk_pm_runtime_get(core)) {
> > > +		if (core->flags & CLK_OPS_PARENT_ENABLE)
> > > +			clk_core_prepare_enable(core->parent);
> > > +
> > >  		rate = core->ops->recalc_rate(core->hw, parent_rate);
> > > +
> > > +		if (core->flags & CLK_OPS_PARENT_ENABLE)
> > > +			clk_core_disable_unprepare(core->parent);
> > > +
> > >  		clk_pm_runtime_put(core);
> > >  	}
> > >  	return rate;
> > 
> > clk_change_rate() has the following code:
> > 
> > 
> >         if (core->flags & CLK_OPS_PARENT_ENABLE)
> >                 clk_core_prepare_enable(parent);
> > 
> > 	...
> > 
> >         core->rate = clk_recalc(core, best_parent_rate);
> > 
> > 	...
> > 
> >         if (core->flags & CLK_OPS_PARENT_ENABLE)
> >                 clk_core_disable_unprepare(parent);
> > 
> > clk_change_rate() ultimately is called by various clk_set_rate
> > functions. Will that be a problem for the double calls to
> > clk_core_prepare_enable()?
> 
> I don't see how multiple prepares are a problem as long as they're
> balanced.
> 
> > 
> > Fanning this out to the edge further is going to make the code even
> > more complicated. What do you think about moving this to
> > clk_core_enable_lock()? I know the set_parent operation has a special
> > case that would need to be worked around.
> 
> __clk_core_init also needs special code in that case, as it calls the
> bare recalc_rate op with no clk_core_enable_lock beforehand. It's also
> wrong, in that recalc_rate does not necessitate the clock being on as
> far as I'm aware. (if it did, this wouldn't be a problem in the first
> place, as enabling it would enable the parent as well). Changing the
> semantics of clk_recalc, and therefore clk_get_rate, to also enable
> the clock, would be a major change in how the common clock framework
> functions.
> 
> In my case, the __clk_core_init callback was the one that crashed,
> so it really needs to happen there, and I really don't want to
> refactor every location where `CLK_OPS_PARENT_ENABLE` is used for
> a bugfix just to avoid potentially checking the same flag twice.
> 
> Having `CLK_OPS_PARENT_ENABLE` cleaned up such that every clk op
> that has potential register access is never directly called by the
> clk core except for one place, an accessor function that does both
> pmdomain and `CLK_OPS_PARENT_ENABLE` checks, would be nice, e.g.
> by keeping the clk_recalc change and then having __clk_core_init
> call clk_recalc instead of the recalc op directly. But then the
> __clk_core_init logic needs further refactoring as well.
> 
> I'm not sure I want to do that in this series, because it's quite
> a bit different from just adding the missing check and parent
> toggling, and has the chance of me introducing subtle logic bugs
> in what is supposed to be a bugfix.

I agree and that makes sense. Thanks for the explanation. What you have
is a good compromise.

Reviewed-by: Brian Masney <bmasney@...hat.com>