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

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Date:   Thu, 23 Apr 2020 11:30:19 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Guru Das Srinagesh <gurus@...eaurora.org>
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>,
        David Collins <collinsd@...eaurora.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Shiyan <shc_work@...l.ru>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v13 04/11] pwm: clps711x: Cast period to u32 before use as divisor

On Wed, Apr 22, 2020 at 5:00 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 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@...l.ru>
> Cc: Arnd Bergmann <arnd@...db.de>
>
> 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..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@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds