linux-kernel - Re: [v5 2/2] pwm: Add Aspeed ast2600 PWM support

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Message-ID: <20210517060615.3hyifoebyrddsrta@pengutronix.de>
Date:   Mon, 17 May 2021 08:06:15 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Billy Tsai <billy_tsai@...eedtech.com>
Cc:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "andrew@...id.au" <andrew@...id.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "joel@....id.au" <joel@....id.au>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        BMC-SW <BMC-SW@...eedtech.com>,
        "lee.jones@...aro.org" <lee.jones@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [v5 2/2] pwm: Add Aspeed ast2600 PWM support

Hello Billy,

On Mon, May 17, 2021 at 02:53:44AM +0000, Billy Tsai wrote:
> On 2021/5/15, 11:57 PM,Uwe Kleine-Königwrote:
> 
> 	>	> +	div_h = DIV_ROUND_DOWN_ULL(div_h,
> 	>	> +				   (FIELD_MAX(PWM_ASPEED_CTRL_CLK_DIV_L) + 1));
> 	>	> +	div_h = DIV_ROUND_DOWN_ULL(div_h, NSEC_PER_SEC);
> 
> 	> As a division is an expensive operation you can better first multiply
> 	> NSEC_PER_SEC and FIELD_MAX(PWM_ASPEED_CTRL_CLK_DIV_L) + 1 and divide by
> 	> the result.
> 
> When I multiply NSEC_PER_SEC and FIELD_MAX(PWM_ASPEED_CTRL_CLK_DIV_L) + 1 the result will overflow
> for 32-bits and the divisor type of do_div is 32-bits so I need to do div twice to avoid the issue.
> Can you give me some suggests?

Hmm, you're right. There doesn't seem to be a div64_64, I thought there
was one. Anyhow, while looking at the various divide functions I saw
that dividing by a constant shouldn't be that expensive, so I think the
sane way is to keep the two divisions and add a comment describing the
problem.

Best regards
Uwe

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

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