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

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Message-Id: <b08aaae4-8c69-47eb-9658-5f3f5c8e4056@app.fastmail.com>
Date: Wed, 13 Mar 2024 13:21:26 +0100
From: "Arnd Bergmann" <arnd@...nel.org>
To: "Zhihao Cheng" <chengzhihao1@...wei.com>, "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

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.

     Arnd