linux-kernel - Re: [PATCH 12/37] clk: renesas: rzg2l: reduce the critical area

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Date:   Fri, 15 Sep 2023 09:05:48 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     claudiu beznea <claudiu.beznea@...on.dev>
Cc:     mturquette@...libre.com, sboyd@...nel.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
        ulf.hansson@...aro.org, linus.walleij@...aro.org,
        gregkh@...uxfoundation.org, jirislaby@...nel.org,
        magnus.damm@...il.com, catalin.marinas@....com, will@...nel.org,
        prabhakar.mahadev-lad.rj@...renesas.com,
        biju.das.jz@...renesas.com, quic_bjorande@...cinc.com,
        arnd@...db.de, konrad.dybcio@...aro.org, neil.armstrong@...aro.org,
        nfraprado@...labora.com, rafal@...ecki.pl,
        wsa+renesas@...g-engineering.com,
        linux-renesas-soc@...r.kernel.org, linux-clk@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mmc@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
Subject: Re: [PATCH 12/37] clk: renesas: rzg2l: reduce the critical area

Hi Claudiu,

On Fri, Sep 15, 2023 at 7:51 AM claudiu beznea <claudiu.beznea@...on.dev> wrote:
> On 14.09.2023 16:12, Geert Uytterhoeven wrote:
> > On Tue, Sep 12, 2023 at 6:52 AM Claudiu <claudiu.beznea@...on.dev> wrote:
> >> From: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
> >>
> >> spinlock in rzg2l_mod_clock_endisable() is intended to protect the accesses
> >> to hardware register. There is no need to protect the instructions that set
> >> temporary variable which will be then written to register. Thus limit the
> >> spinlock only to the hardware register access.
> >>
> >> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
> >
> > Thanks for your patch!
> >
> >> --- a/drivers/clk/renesas/rzg2l-cpg.c
> >> +++ b/drivers/clk/renesas/rzg2l-cpg.c
> >> @@ -912,13 +912,13 @@ static int rzg2l_mod_clock_endisable(struct clk_hw *hw, bool enable)
> >>
> >>         dev_dbg(dev, "CLK_ON %u/%pC %s\n", CLK_ON_R(reg), hw->clk,
> >>                 enable ? "ON" : "OFF");
> >> -       spin_lock_irqsave(&priv->rmw_lock, flags);
> >>
> >>         value = bitmask << 16;
> >>         if (enable)
> >>                 value |= bitmask;
> >> -       writel(value, priv->base + CLK_ON_R(reg));
> >>
> >> +       spin_lock_irqsave(&priv->rmw_lock, flags);
> >> +       writel(value, priv->base + CLK_ON_R(reg));
> >>         spin_unlock_irqrestore(&priv->rmw_lock, flags);
> >
> > After this, it becomes obvious there is nothing to protect at all,
> > so the locking can just be removed from this function?
>
> I tend to be paranoid when writing to hardware resources thus I kept it.
> Would you prefer to remove it at all?

Yes please. I guess this was copied from R-Car and friends, where
there is a RMW operation on an MSTPCR register.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds