linux-kernel - Re: [PATCH] pwm: sun4i: Fix incorrect calculation of duty

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Date:   Wed, 16 Oct 2019 09:40:28 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     megous@...ous.com
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        "open list:PWM SUBSYSTEM" <linux-pwm@...r.kernel.org>,
        "moderated list:ARM/Allwinner sunXi SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pwm: sun4i: Fix incorrect calculation of
 duty_cycle/period

On Mon, Oct 14, 2019 at 03:53:03PM +0200, megous@...ous.com wrote:
> From: Ondrej Jirman <megous@...ous.com>
> 
> Since 5.4-rc1, pwm_apply_state calls ->get_state after ->apply
> if available, and this revealed an issue with integer precision
> when calculating duty_cycle and period for the currently set
> state in ->get_state callback.
> 
> This issue manifested in broken backlight on several Allwinner
> based devices.
> 
> Previously this worked, because ->apply updated the passed state
> directly.
> 
> Fixes: deb9c462f4e53 ("pwm: sun4i: Don't update the state for the caller of pwm_apply_state")
> Signed-off-by: Ondrej Jirman <megous@...ous.com>
> ---
>  drivers/pwm/pwm-sun4i.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Applied, thanks.

Thierry

> diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> index 6f5840a1a82d..05273725a9ff 100644
> --- a/drivers/pwm/pwm-sun4i.c
> +++ b/drivers/pwm/pwm-sun4i.c
> @@ -137,10 +137,10 @@ static void sun4i_pwm_get_state(struct pwm_chip *chip,
>  
>  	val = sun4i_pwm_readl(sun4i_pwm, PWM_CH_PRD(pwm->hwpwm));
>  
> -	tmp = prescaler * NSEC_PER_SEC * PWM_REG_DTY(val);
> +	tmp = (u64)prescaler * NSEC_PER_SEC * PWM_REG_DTY(val);
>  	state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, clk_rate);
>  
> -	tmp = prescaler * NSEC_PER_SEC * PWM_REG_PRD(val);
> +	tmp = (u64)prescaler * NSEC_PER_SEC * PWM_REG_PRD(val);
>  	state->period = DIV_ROUND_CLOSEST_ULL(tmp, clk_rate);
>  }
>  
> -- 
> 2.23.0
> 

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