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

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Message-ID: <20200521101934.j5ivjky4e6byveut@holly.lan>
Date:   Thu, 21 May 2020 11:19:34 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Guru Das Srinagesh <gurus@...eaurora.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 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.


Daniel.

> +
> +	return DIV_ROUND_CLOSEST(v * 0xf, (u32)pwm->args.period);
>  }
>  
>  static int clps711x_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
> -- 
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>