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

[Guru Das Srinagesh]

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@xxxxxxxxxxxxxx> 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@xxxxxxx>
> > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> >
> > Signed-off-by: Guru Das Srinagesh <gurus@xxxxxxxxxxxxxx>
> 
> Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>

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.