linux-kernel - Re: [PATCH] mtd: spi-nor: replace unnecessary div64_u64() with div

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Message-Id: <D0WNIVK1DBL5.3KQOD1K2O128P@kernel.org>
Date: Mon, 29 Apr 2024 15:47:13 +0200
From: "Michael Walle" <mwalle@...nel.org>
To: "Pratyush Yadav" <pratyush@...nel.org>
Cc: "Tudor Ambarus" <tudor.ambarus@...aro.org>, "Miquel Raynal"
 <miquel.raynal@...tlin.com>, "Richard Weinberger" <richard@....at>,
 "Vignesh Raghavendra" <vigneshr@...com>, "Dan Carpenter"
 <dan.carpenter@...aro.org>, <linux-mtd@...ts.infradead.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mtd: spi-nor: replace unnecessary div64_u64() with
 div_u64()

Hi,

On Mon Apr 29, 2024 at 3:27 PM CEST, Pratyush Yadav wrote:
> On Mon, Apr 29 2024, Michael Walle wrote:
>
> > Both occurences of div64_u64() just have a u8 or u32 divisor. Use
> > div_u64() instead.
>
> Does this improve performance or is this only for correctness?

See function doc for div_u64():

 * This is the most common 64bit divide and should be used if possible,
 * as many 32bit archs can optimize this variant better than a full 64bit
 * divide.

> Patch LGTM otherwise.
>
> Reviewed-by: Pratyush Yadav <pratyush@...nel.org>
>
> BTW, I also noticed that there is a do_div() call in spi_nor_write()
> that also uses a u64 dividend and u32 divisor. I was wondering why it
> uses do_div() and not div_u64() (I am not sure what the difference
> between the two is) but I suppose it doesn't matter much since your
> spring cleaning series will delete that code anyway.

do_div() is a macro and is modifying the dividend in place, whereas
div_u64() will return it. do_div() is using u32 for the divisor
anyway.

-michael

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