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

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Message-ID: <20200319205117.GA17255@codeaurora.org>
Date:   Thu, 19 Mar 2020 13:51:17 -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 08:38:29PM +0100, Arnd Bergmann wrote:
> On Wed, Mar 18, 2020 at 6:00 PM Guru Das Srinagesh <gurus@...eaurora.org> wrote:
> >
> > 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)))
> >                                         ^^^^^^^^^
> 
> That would require performing a division, which is an external function
> call on ARMv4. My version just checks for an upper bound and completely
> avoids the division. You could also just check for ">UINT_MAX" if you
> find that clearer.

Thanks, have checked for UINT_MAX in v10 of my patchset that I just
uploaded. Could you please review it?

Thank you.

Guru Das.