linux-kernel - Re: [PATCH 08/37] clk: renesas: rzg2l: trust value returned by hardware

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Message-ID: <CAMuHMdXjhCth125PHKxrWJENFUhrvXfFJPLhL2ZRY6++azzczA@mail.gmail.com>
Date:   Thu, 14 Sep 2023 14:18:44 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Sergey Shtylyov <s.shtylyov@....ru>
Cc:     Claudiu <claudiu.beznea@...on.dev>, 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 08/37] clk: renesas: rzg2l: trust value returned by hardware

On Tue, Sep 12, 2023 at 6:43 PM Sergey Shtylyov <s.shtylyov@....ru> wrote:
> On 9/12/23 7:51 AM, Claudiu wrote:
>
> > From: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
> >
> > Initial value of CPG_PL2SDHI_DSEL bits 0..1 or 4..6 is 01b. Hardware user's
> > manual (r01uh0914ej0130-rzg2l-rzg2lc.pdf) specifies that setting 0 is
> > prohibited. The rzg2l_cpg_sd_clk_mux_get_parent() should just read
> > CPG_PL2SDHI_DSEL, trust the value and return the proper clock parent index
> > based on the read value. Do this.
> >
> > Fixes: eaff33646f4cb ("clk: renesas: rzg2l: Add SDHI clk mux support")
> > Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>

> > --- a/drivers/clk/renesas/rzg2l-cpg.c
> > +++ b/drivers/clk/renesas/rzg2l-cpg.c
> > @@ -239,14 +239,8 @@ static u8 rzg2l_cpg_sd_clk_mux_get_parent(struct clk_hw *hw)
> >
> >       val >>= GET_SHIFT(hwdata->conf);
> >       val &= GENMASK(GET_WIDTH(hwdata->conf) - 1, 0);
> > -     if (val) {
> > -             val--;
> > -     } else {
> > -             /* Prohibited clk source, change it to 533 MHz(reset value) */
> > -             rzg2l_cpg_sd_clk_mux_set_parent(hw, 0);
> > -     }
> >
> > -     return val;
> > +     return val ? --val : val;
>
>         return val ? val - 1 : 0;

Definitely, mixing multiple users of the same variable and pre-decrement
is ill-defined.

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