linux-kernel - Re: [PATCH] mtd: ubi: avoid expensive do

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Message-ID: <610231c3-c3ee-e543-1a8a-8e1098ee6a7c@huawei.com>
Date: Wed, 13 Mar 2024 21:29:38 +0800
From: Zhihao Cheng <chengzhihao1@...wei.com>
To: Arnd Bergmann <arnd@...nel.org>, Arnd Bergmann <arnd@...db.de>, Richard
 Weinberger <richard@....at>, Miquel Raynal <miquel.raynal@...tlin.com>,
	Vignesh Raghavendra <vigneshr@...com>, Daniel Golle <daniel@...rotopia.org>
CC: <linux-mtd@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mtd: ubi: avoid expensive do_div() on 32-bit machines

在 2024/3/13 20:21, Arnd Bergmann 写道:
> On Wed, Mar 13, 2024, at 13:10, Zhihao Cheng wrote:
>> 在 2024/3/13 19:53, Arnd Bergmann 写道:
>>> On Wed, Mar 13, 2024, at 12:29, Zhihao Cheng wrote:
>>>
>>> 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 open-coded 64-bit division without using do_div() shows up as
> 
> x86_64-linux-ld: drivers/mtd/ubi/nvmem.o: in function `ubi_nvmem_reg_read':
> nvmem.c:(.text+0x10a): undefined reference to `__umoddi3'
> x86_64-linux-ld: nvmem.c:(.text+0x11f): undefined reference to `__udivdi3'
> x86_64-linux-ld: drivers/mtd/ubi/nvmem.o: in function `ubi_nvmem_reg_read.cold':
> nvmem.c:(.text.unlikely+0x2d): undefined reference to `__umoddi3'
>  > The idea is that gcc expects __umoddi3 to be provided by libgcc,
> but Linux intentionally leaves it out in order to catch accidental
> 64-bit divisions.
> 

Thanks for explaination, which means that do_div is used for 64-bit 
division to solve the link failure caused by missed libgcc. Since 
parameter 'from' is u32, there is no need to invoke do_div on a 32-bit 
platform, you just want to stop the wasting behavior on a 32-bit 
platform. Do I understand right?