linux-kernel - Re: [PATCH V2 3/6] pwm: sprd: Optimize the calculation method of duty

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Date: Fri, 26 Jan 2024 08:30:39 +0100
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Wenhua Lin <Wenhua.Lin@...soc.com>
Cc: Rob Herring <robh+dt@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	Orson Zhai <orsonzhai@...il.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Chunyan Zhang <zhang.lyra@...il.com>, linux-pwm@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, wenhua lin <wenhua.lin1994@...il.com>, 
	Xiongpeng Wu <xiongpeng.wu@...soc.com>, zhaochen su <zhaochen.su29@...il.com>, 
	Zhaochen Su <Zhaochen.Su@...soc.com>, Xiaolong Wang <Xiaolong.Wang@...soc.com>
Subject: Re: [PATCH V2 3/6] pwm: sprd: Optimize the calculation method of duty

Hello,

On Thu, Jan 25, 2024 at 10:55:30AM +0800, Wenhua Lin wrote:
> diff --git a/drivers/pwm/pwm-sprd.c b/drivers/pwm/pwm-sprd.c
> index cc54aa77c7e6..8de3f9e154ce 100644
> --- a/drivers/pwm/pwm-sprd.c
> +++ b/drivers/pwm/pwm-sprd.c
> @@ -156,7 +156,8 @@ static int sprd_pwm_config(struct sprd_pwm_chip *spc, struct pwm_device *pwm,
>  	 * given settings (MOD and input clock).
>  	 */
>  	mod = spc->mod[pwm->hwpwm];
> -	duty = duty_ns * mod / period_ns;
> +	tmp = (u64)duty_ns * mod;
> +	duty = DIV_ROUND_CLOSEST_ULL(tmp, period_ns);

Please stick to rounding down in .apply() (and so sprd_pwm_config()).
Given that duty_ns is an u64 in .apply(), you're loosing precision
anyhow. Look at how the microchip-core driver uses mul_u64_u64_div_u64()
for how to do that properly.

You tested your patch with CONFIG_PWM_DEBUG enabled, right?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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