linux-kernel - Re: [PATCH v9 04/11] pwm: clps711x: Use 64-bit division macro

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Date:   Wed, 18 Mar 2020 10:00:10 -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 <uwe@...ine-koenig.org>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alexander Shiyan <shc_work@...l.ru>
Subject: Re: [PATCH v9 04/11] pwm: clps711x: Use 64-bit division macro

On Wed, Mar 18, 2020 at 10:49:34AM +0100, Arnd Bergmann wrote:
> On Wed, Mar 18, 2020 at 12:30 AM Guru Das Srinagesh
> <gurus@...eaurora.org> wrote:
> >
> > On Tue, Mar 17, 2020 at 11:22:06PM +0100, Arnd Bergmann wrote:
> > > > diff --git a/drivers/pwm/pwm-clps711x.c b/drivers/pwm/pwm-clps711x.c
> > > > index 924d39a..ba9500a 100644
> > > > --- a/drivers/pwm/pwm-clps711x.c
> > > > +++ b/drivers/pwm/pwm-clps711x.c
> > > > @@ -43,7 +43,7 @@ static void clps711x_pwm_update_val(struct clps711x_chip *priv, u32 n, u32 v)
> > > >  static unsigned int clps711x_get_duty(struct pwm_device *pwm, unsigned int v)
> > > >  {
> > > >         /* Duty cycle 0..15 max */
> > > > -       return DIV_ROUND_CLOSEST(v * 0xf, pwm->args.period);
> > > > +       return DIV64_U64_ROUND_CLOSEST(v * 0xf, pwm->args.period);
> > > >  }
> > >
> > > Is it actually going to exceed U32_MAX? If not, a type cast may be
> > > more appropriate here than the expensive 64-bit division.
> >
> > With the final change in this patch series, the framework will support
> > periods that exceed U32_MAX. My concern is that using a typecast would
> > mean that in those cases, this driver will not support > U32_MAX values.
> > Using DIV64_U64_ROUND_CLOSEST makes the driver future proof and able to
> > handle > U32_MAX values correctly. What do you think?
> 
> Ah, so if the period can actually be larger than U32_MAX, you need to
> handle that case. However, I see that the divident in this code (v * 0xf)
> is still a 32-bit number, so a correct and efficient implementation could be
> 
>    if (pwm->args.period > (UINT_MAX / 0xf))

Shouldn't the if condition be the following? Or am I missing
something here?

     if (pwm->args.period > (UINT_MAX / (v * 0xf)))
     					^^^^^^^^^

>           return 0;
>    return DIV_ROUND_CLOSEST(v * 0xf, (u32)pwm->args.period);

Thank you.

Guru Das.