linux-kernel - Re: [RESEND PATCH v14 04/11] pwm: clps711x: Cast period to u32 before use as divisor

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Date:   Thu, 21 May 2020 13:25:25 -0700
From:   Guru Das Srinagesh <gurus@...eaurora.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     linux-pwm@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
        David Collins <collinsd@...eaurora.org>,
        linux-kernel@...r.kernel.org, Joe Perches <joe@...ches.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RESEND PATCH v14 04/11] pwm: clps711x: Cast period to u32
 before use as divisor

On Thu, May 21, 2020 at 11:19:34AM +0100, Daniel Thompson wrote:
> On Wed, May 20, 2020 at 03:55:57PM -0700, Guru Das Srinagesh 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 the numerator will cause the quotient to be zero, so return 0 in
> > that case to efficiently skip the division.
> > 
> > Signed-off-by: Guru Das Srinagesh <gurus@...eaurora.org>
> > ---
> >  drivers/pwm/pwm-clps711x.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pwm/pwm-clps711x.c b/drivers/pwm/pwm-clps711x.c
> > index 924d39a..da771b1 100644
> > --- a/drivers/pwm/pwm-clps711x.c
> > +++ b/drivers/pwm/pwm-clps711x.c
> > @@ -43,7 +43,10 @@ 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);
> > +	if (pwm->args.period > (v * 0xf))
> > +		return 0;
> 
> This doesn't look right to me.
> 
> DIV_ROUND_CLOSEST() does rounded division and the short circuit doesn't
> implement that.

My initial patch [1] was to simply use DIV64_U64_ROUND_CLOSEST(), but I
got review feedback to add a short-circuit (same thread, [2]). I feel
like I should skip the short-circuiting and type casting and simply just
use DIV64_U64_ROUND_CLOSEST() - what do you think?

[1] https://lore.kernel.org/lkml/587f9ccae68ad7e1ce97fa8da6037292af1a5095.1584473399.git.gurus@codeaurora.org/
[2] https://lore.kernel.org/lkml/CAK8P3a2Hi_AoRC3g7qKth4e_Y1jZrbBDhWUb3YPZm10FWMu-ig@mail.gmail.com/