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

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Date:   Mon, 16 Oct 2023 12:32:56 +0530
From:   Varadarajan Narayanan <quic_varada@...cinc.com>
To:     Stephen Boyd <sboyd@...nel.org>
CC:     <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 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.

Thanks
Varada