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

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Message-ID: <20200331202058.GB25781@codeaurora.org>
Date:   Tue, 31 Mar 2020 13:20:58 -0700
From:   Guru Das Srinagesh <gurus@...eaurora.org>
To:     Arnd Bergmann <arnd@...db.de>
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 Tue, Mar 31, 2020 at 05:24:52PM +0200, Arnd Bergmann wrote:
> 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.

Makes sense, I agree. Would this be an acceptable fix?

Instead of multiplying c and state->period first and then dividing by
10^9, first divide state->period by 10^9 and then multiply the quotient
of that division with c and assign it to period_cycles. Like so:

	c = clk_get_rate(imx->clk_per);
	c *= div_u64(state->period, 1000000000);
	period_cycles = c;

This should take care of overflow not happening because state->period is
converted from nanoseconds to seconds early on and so becomes a small
number.

Thank you.

Guru Das.