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Date: Fri, 26 Jan 2024 08:30:39 +0100
From: Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= <u.kleine-koenig@pengutronix.de>
To: Wenhua Lin <Wenhua.Lin@unisoc.com>
Cc: Rob Herring <robh+dt@kernel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>, Conor Dooley <conor+dt@kernel.org>, 
	Orson Zhai <orsonzhai@gmail.com>, Baolin Wang <baolin.wang@linux.alibaba.com>, 
	Chunyan Zhang <zhang.lyra@gmail.com>, linux-pwm@vger.kernel.org, devicetree@vger.kernel.org, 
	linux-kernel@vger.kernel.org, wenhua lin <wenhua.lin1994@gmail.com>, 
	Xiongpeng Wu <xiongpeng.wu@unisoc.com>, zhaochen su <zhaochen.su29@gmail.com>, 
	Zhaochen Su <Zhaochen.Su@unisoc.com>, Xiaolong Wang <Xiaolong.Wang@unisoc.com>
Subject: Re: [PATCH V2 3/6] pwm: sprd: Optimize the calculation method of duty
Message-ID: <mdrr6ko2zqsf2osdkecjac6ollnuvlv4irkkiyn4ihkq5fioxx@nm6677kkih4h>
References: <20240125025533.10315-1-Wenhua.Lin@unisoc.com>
 <20240125025533.10315-4-Wenhua.Lin@unisoc.com>
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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 =3D spc->mod[pwm->hwpwm];
> -	duty =3D duty_ns * mod / period_ns;
> +	tmp =3D (u64)duty_ns * mod;
> +	duty =3D 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

--=20
Pengutronix e.K.                           | Uwe Kleine-K=F6nig            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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