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Subject: Re: [PATCH] mtd: ubi: avoid expensive do_div() on 32-bit machines
To: Arnd Bergmann <arnd@arndb.de>, Arnd Bergmann <arnd@kernel.org>, Richard
 Weinberger <richard@nod.at>, Miquel Raynal <miquel.raynal@bootlin.com>,
	Vignesh Raghavendra <vigneshr@ti.com>, Daniel Golle <daniel@makrotopia.org>
CC: <linux-mtd@lists.infradead.org>, <linux-kernel@vger.kernel.org>
References: <20240313084707.3292300-1-arnd@kernel.org>
 <4f945f4a-ac29-3ef7-9e15-123962f2a0e9@huawei.com>
 <e0684202-6926-4bd1-86f6-2bb682524712@app.fastmail.com>
From: Zhihao Cheng <chengzhihao1@huawei.com>
Message-ID: <1a12c8ed-0cb5-5650-24a2-84b021c444c3@huawei.com>
Date: Wed, 13 Mar 2024 20:10:54 +0800
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在 2024/3/13 19:53, Arnd Bergmann 写道:
> On Wed, Mar 13, 2024, at 12:29, Zhihao Cheng wrote:
>> 在 2024/3/13 16:46, Arnd Bergmann 写道:
>>> From: Arnd Bergmann <arnd@arndb.de>
>>>
>>> The use of do_div() in ubi_nvmem_reg_read() makes calling it on
>>> 32-bit machines rather expensive. Since the 'from' variable is
>>> known to be a 32-bit quantity, it is clearly never needed and
>>> can be optimized into a regular division operation.
>>>
>> Do you meet a performance problem on a 32-bit machine? There are too
>> many places invoking do_div, why do you optimize this one?
>> Have you tested the influence on a x86_64 platform after this patch
>> applied? Looks like that do_div is more efficient in x86.
> 
> This one was just introduced. The call site looks like a fast
> path and it caused a build regression that Daniel addressed with
> an suboptimal commit b8a77b9a5f9c ("mtd: ubi: fix NVMEM over
> UBI volumes on 32-bit systems").
> 
> The way it usually goes is that someone adds an open-coded
> 64-bit division that causes a link failure, which prompts
I'm a little confused, what kind of link failure? Could you show an example?
> the original developer to either rewrite the code to avoid
> the long division if possible, or add do_div() after showing
> that it is now performance critical, e.g. only called at
> probe time.
> 
>       Arnd
> .
>