linux-kernel - Re: [PATCH 3/8] clk: sunxi-ng: mp: Fix dual-divider clock rate readback

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Message-ID: <CAGb2v65utA6PsoAS-wLPskYy9GGXfESvtiwOBC4qSdi1XvLeKQ@mail.gmail.com>
Date: Fri, 5 Sep 2025 23:17:19 +0800
From: Chen-Yu Tsai <wens@...nel.org>
To: Andre Przywara <andre.przywara@....com>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
	Conor Dooley <conor+dt@...nel.org>, Stephen Boyd <sboyd@...nel.org>, 
	Jernej Skrabec <jernej@...nel.org>, Samuel Holland <samuel@...lland.org>, linux-sunxi@...ts.linux.dev, 
	linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] clk: sunxi-ng: mp: Fix dual-divider clock rate readback

On Fri, Sep 5, 2025 at 11:12 PM Andre Przywara <andre.przywara@....com> wrote:
>
> On Sun, 31 Aug 2025 01:08:56 +0800
> Chen-Yu Tsai <wens@...nel.org> wrote:
>
> > From: Chen-Yu Tsai <wens@...e.org>
> >
> > When dual-divider clock support was introduced, the P divider offset was
> > left out of the .recalc_rate readback function. This causes the clock
> > rate to become bogus or even zero (possibly due to the P divider being
> > 1, leading to a divide-by-zero).
>
> Ah, a nice catch, thanks for that! Just curious, how did you find this?
> The MMC clocks use the dual divider type as well, but I didn't observe
> them being wrong?

I was looking at clk_summary in debugfs and kept seeing some of the audio
clocks returning 0 and dumped the register to get the actual value. I then
did a printk trace of the .determine_rate callback and saw that one of the
dividers was always 1. I guessed that the .recalc_rate callback was to blame.

> Regardless:
>
> > Fix this by incorporating the P divider offset into the calculation.
> >
> > Fixes: 45717804b75e ("clk: sunxi-ng: mp: introduce dual-divider clock")
> > Signed-off-by: Chen-Yu Tsai <wens@...e.org>
>
> Reviewed-by: Andre Przywara <andre.przywara@....com>

Thanks!


> Thanks,
> Andre
>
> > ---
> >  drivers/clk/sunxi-ng/ccu_mp.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/clk/sunxi-ng/ccu_mp.c b/drivers/clk/sunxi-ng/ccu_mp.c
> > index 354c981943b6..4221b1888b38 100644
> > --- a/drivers/clk/sunxi-ng/ccu_mp.c
> > +++ b/drivers/clk/sunxi-ng/ccu_mp.c
> > @@ -185,7 +185,7 @@ static unsigned long ccu_mp_recalc_rate(struct clk_hw *hw,
> >       p &= (1 << cmp->p.width) - 1;
> >
> >       if (cmp->common.features & CCU_FEATURE_DUAL_DIV)
> > -             rate = (parent_rate / p) / m;
> > +             rate = (parent_rate / (p + cmp->p.offset)) / m;
> >       else
> >               rate = (parent_rate >> p) / m;
> >
>