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

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Message-ID: <mafs0mspcb57t.fsf@kernel.org>
Date: Mon, 29 Apr 2024 16:42:30 +0200
From: Pratyush Yadav <pratyush@...nel.org>
To: "Michael Walle" <mwalle@...nel.org>
Cc: "Pratyush Yadav" <pratyush@...nel.org>,  "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()

On Mon, Apr 29 2024, Michael Walle wrote:

> 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.

Thanks. I think it would be good to add this to the commit message:

    Both occurences of div64_u64() just have a u8 or u32 divisor. Use
    div_u64() instead. Many 32 bit architectures can optimize this
    variant better than a full 64 bit divide.

No need to resend, I can do this when applying.

>
>> 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
>

-- 
Regards,
Pratyush Yadav