linux-kernel - Re: [PATCH v2 1/8] clk: qcom: clk-alpha-pll: introduce stromer plus ops

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Message-ID: <CAA8EJpoQwDaUa+-WyM6FBzQJo9gn1k2rYLmKSFBLUH00epGJ0Q@mail.gmail.com>
Date:   Mon, 16 Oct 2023 11:46:56 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Varadarajan Narayanan <quic_varada@...cinc.com>
Cc:     Stephen Boyd <sboyd@...nel.org>, agross@...nel.org,
        andersson@...nel.org, conor+dt@...nel.org,
        devicetree@...r.kernel.org, ilia.lin@...nel.org,
        konrad.dybcio@...aro.org, krzysztof.kozlowski+dt@...aro.org,
        linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        mturquette@...libre.com, quic_kathirav@...cinc.com,
        rafael@...nel.org, robh+dt@...nel.org, viresh.kumar@...aro.org
Subject: Re: [PATCH v2 1/8] clk: qcom: clk-alpha-pll: introduce stromer plus ops

On Mon, 16 Oct 2023 at 10:03, Varadarajan Narayanan
<quic_varada@...cinc.com> wrote:
>
> On Thu, Oct 12, 2023 at 01:55:36PM -0700, Stephen Boyd wrote:
> > Quoting Varadarajan Narayanan (2023-10-12 02:26:17)
> > > Stromer plus APSS PLL does not support dynamic frequency scaling.
> > > To switch between frequencies, we have to shut down the PLL,
> > > configure the L and ALPHA values and turn on again. So introduce the
> > > separate set of ops for Stromer Plus PLL.
> >
> > Does this assume the PLL is always on?
>
> Yes once the PLL is configured by apss-ipq-pll driver, it is always on.
>
> > > Signed-off-by: Kathiravan T <quic_kathirav@...cinc.com>
> > > Signed-off-by: Varadarajan Narayanan <quic_varada@...cinc.com>
> > > ---
> > > v2:     Use clk_alpha_pll_stromer_determine_rate, instead of adding new
> > >         clk_alpha_pll_stromer_plus_determine_rate as the alpha pll width
> > >         is same for both
> > >
> > >         Fix review comments
> > >                 udelay(50) -> usleep_range(50, 60)
> > >                 Remove SoC-specific from print message
> > > ---
> > >  drivers/clk/qcom/clk-alpha-pll.c | 57 ++++++++++++++++++++++++++++++++++++++++
> > >  drivers/clk/qcom/clk-alpha-pll.h |  1 +
> > >  2 files changed, 58 insertions(+)
> > >
> > > diff --git a/drivers/clk/qcom/clk-alpha-pll.c b/drivers/clk/qcom/clk-alpha-pll.c
> > > index 4edbf77..5221b6c 100644
> > > --- a/drivers/clk/qcom/clk-alpha-pll.c
> > > +++ b/drivers/clk/qcom/clk-alpha-pll.c
> > > @@ -2508,3 +2508,60 @@ const struct clk_ops clk_alpha_pll_stromer_ops = {
> > >         .set_rate = clk_alpha_pll_stromer_set_rate,
> > >  };
> > >  EXPORT_SYMBOL_GPL(clk_alpha_pll_stromer_ops);
> > > +
> > > +static int clk_alpha_pll_stromer_plus_set_rate(struct clk_hw *hw,
> > > +                                              unsigned long rate,
> > > +                                              unsigned long prate)
> > > +{
> > > +       struct clk_alpha_pll *pll = to_clk_alpha_pll(hw);
> > > +       u32 l, alpha_width = pll_alpha_width(pll);
> > > +       int ret;
> > > +       u64 a;
> > > +
> > > +       rate = alpha_pll_round_rate(rate, prate, &l, &a, alpha_width);
> > > +
> > > +       regmap_write(pll->clkr.regmap, PLL_MODE(pll), 0);
> >
> > There's a theoretical problem here if I understand correctly. A call to
> > clk_enable() can happen while clk_set_rate() is in progress or vice
> > versa. Probably we need some sort of spinlock for this PLL that
> > synchronizes any enable/disable with the rate change so that when we
> > restore the enable bit the clk isn't enabled when it was supposed to be
> > off.
>
> Since the PLL is always on, should we worry about enable/disable?
> If you feel it is better to synchronize with a spin lock, will
> add and post a new revision. Please let me know.

Probably another option might be to change stromer PLL ops to use
prepare/unprepare instead of enable/disable. This way the
clk_prepare_lock() in clk_set_rate() will take care of locking.

>
> Thanks
> Varada



-- 
With best wishes
Dmitry