Re: [PATCH v13 04/11] pwm: clps711x: Cast period to u32 before use as divisor

[Geert Uytterhoeven <geert@linux-m68k.org>]

On Wed, Apr 22, 2020 at 5:00 AM Guru Das Srinagesh <gurus@codeaurora.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 the numerator will cause the quotient to be zero, so return 0 in
> that case to efficiently skip the division.
>
> Cc: Alexander Shiyan <shc_work@mail.ru>
> Cc: Arnd Bergmann <arnd@arndb.de>
>
> Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.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..0d368ac 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 ((u32)pwm->args.period > (v * 0xf))

Shouldn't the cast be removed from the expression above?
Else a period larger than or equal to 2^32 may be accepted, but truncated
silently.  And even cause a division by zero error.


> +               return 0;
> +
> +       return DIV_ROUND_CLOSEST(v * 0xf, (u32)pwm->args.period);
>  }
>
>  static int clps711x_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

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