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

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Message-ID: <20231018093556.GB24128@varda-linux.qualcomm.com>
Date:   Wed, 18 Oct 2023 15:05:57 +0530
From:   Varadarajan Narayanan <quic_varada@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
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, Oct 16, 2023 at 11:46:56AM +0300, Dmitry Baryshkov wrote:
> 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 for the suggestion. Have posted v3 with this and addressing
Stephen Boyd's other comments. Please take a look.
(https://lore.kernel.org/linux-arm-msm/cover.1697600121.git.quic_varada@quicinc.com/)

Thanks
Varada