Received-SPF: pass (google.com: domain of linux-kernel+bounces-101548-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Feedback-ID: i36794607:Fastmail
User-Agent: Cyrus-JMAP/3.11.0-alpha0-251-g8332da0bf6-fm-20240305.001-g8332da0b
Precedence: bulk
MIME-Version: 1.0
Message-Id: <21cd0f32-c94b-4b2b-8fa6-ebe12756b0c4@app.fastmail.com>
In-Reply-To: <610231c3-c3ee-e543-1a8a-8e1098ee6a7c@huawei.com>
References: <20240313084707.3292300-1-arnd@kernel.org>
 <4f945f4a-ac29-3ef7-9e15-123962f2a0e9@huawei.com>
 <e0684202-6926-4bd1-86f6-2bb682524712@app.fastmail.com>
 <1a12c8ed-0cb5-5650-24a2-84b021c444c3@huawei.com>
 <b08aaae4-8c69-47eb-9658-5f3f5c8e4056@app.fastmail.com>
 <610231c3-c3ee-e543-1a8a-8e1098ee6a7c@huawei.com>
Date: Wed, 13 Mar 2024 14:39:01 +0100
From: "Arnd Bergmann" <arnd@kernel.org>
To: "Zhihao Cheng" <chengzhihao1@huawei.com>, "Arnd Bergmann" <arnd@arndb.de>,
 "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
Subject: Re: [PATCH] mtd: ubi: avoid expensive do_div() on 32-bit machines
Content-Type: text/plain;charset=utf-8
Content-Transfer-Encoding: quoted-printable

On Wed, Mar 13, 2024, at 14:29, Zhihao Cheng wrote:
> =E5=9C=A8 2024/3/13 20:21, Arnd Bergmann =E5=86=99=E9=81=93:
>> On Wed, Mar 13, 2024, at 13:10, Zhihao Cheng wrote:
>>> =E5=9C=A8 2024/3/13 19:53, Arnd Bergmann =E5=86=99=E9=81=93:
>>>> 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?
>>=20
>> The open-coded 64-bit division without using do_div() shows up as
>>=20
>> 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 `__udi=
vdi3'
>> 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.
>>=20
>
> Thanks for explaination, which means that do_div is used for 64-bit=20
> division to solve the link failure caused by missed libgcc. Since=20
> parameter 'from' is u32, there is no need to invoke do_div on a 32-bit=20
> platform, you just want to stop the wasting behavior on a 32-bit=20
> platform. Do I understand right?

Yes, correct.

      Arnd