linux-kernel - Re: [PATCH v11 04/12] pwm: clps711x: Cast period to u32 before use as divisor

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Date:   Mon, 6 Apr 2020 17:26:31 -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>,
        Alexander Shiyan <shc_work@...l.ru>
Subject: Re: [PATCH v11 04/12] pwm: clps711x: Cast period to u32 before use
 as divisor

On Fri, Mar 20, 2020 at 06:11:42PM +0100, Arnd Bergmann wrote:
> On Fri, Mar 20, 2020 at 2:41 AM Guru Das Srinagesh <gurus@...eaurora.org> wrote:
> >
> > Since the PWM framework is switching struct pwm_args.period's datatype
> > to u64, prepare for this transition by typecasting it to u32.
> >
> > Also, since the dividend is still a 32-bit number, any divisor greater
> > than UINT_MAX will cause the quotient to be zero, so return 0 in that
> > case to efficiently skip the division.
> >
> > Cc: Alexander Shiyan <shc_work@...l.ru>
> > Cc: Arnd Bergmann <arnd@...db.de>
> >
> > Signed-off-by: Guru Das Srinagesh <gurus@...eaurora.org>
> 
> Reviewed-by: Arnd Bergmann <arnd@...db.de>

The stated aim of adding the if condition is to determine when the
division operation may be skipped as the quotient would be zero anyway
[1]. That said, I think the current if condition is incorrect. The
quotient would be zero only when the denominator of the division exceeds
(v * 0xf) and not UINT_MAX. In fact, UINT_MAX has no bearing on whether
the quotient becomes zero or not.

Therefore, the correct if condition should be:

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

What do you think?

[1] https://www.spinics.net/lists/linux-pwm/msg11908.html

Thank you.

Guru Das.