linux-kernel - Re: [PATCH v11 06/12] pwm: imx27: Use 64-bit division macro and function

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Date:   Tue, 31 Mar 2020 17:24:52 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Guru Das Srinagesh <gurus@...eaurora.org>
Cc:     Linux PWM List <linux-pwm@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>
Subject: Re: [PATCH v11 06/12] pwm: imx27: Use 64-bit division macro and function

On Mon, Mar 30, 2020 at 10:44 PM Guru Das Srinagesh
<gurus@...eaurora.org> wrote:
>
> On Fri, Mar 20, 2020 at 06:09:39PM +0100, Arnd Bergmann wrote:
> > On Fri, Mar 20, 2020 at 2:42 AM Guru Das Srinagesh <gurus@...eaurora.org> wrote:
> >
> > > @@ -240,8 +240,7 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > >
> > >         period_cycles /= prescale;
> > >         c = (unsigned long long)period_cycles * state->duty_cycle;
> > > -       do_div(c, state->period);
> > > -       duty_cycles = c;
> > > +       duty_cycles = div64_u64(c, state->period);
> > >
> >
> > This change looks fine, but I wonder if the code directly above it
> >
> >         c = clk_get_rate(imx->clk_per);
> >         c *= state->period;
> >         do_div(c, 1000000000);
> >         period_cycles = c;
> >
> > might run into an overflow when both the clock rate and the period
> > are large numbers.
>
> Hmm. Seems to me like addressing this would be outside the scope of this
> patch series.

I think it should be part of the same series, addressing bugs that
were introduced
by the change to 64-bit period. If it's not getting fixed along with
the other regressions,
I fear nobody is going to go back and fix it later.

      Arnd