linux-kernel - Re: [greybus-dev] Re: [PATCH] [v2] Kbuild: move to -std=gnu11

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Message-ID: <47c64195-6629-ba2b-4533-b0fe37518da0@roeck-us.net>
Date:   Wed, 18 May 2022 07:07:31 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Jonathan Corbet <corbet@....net>,
        Federico Vaga <federico.vaga@...a.pv.it>,
        Alex Shi <alexs@...nel.org>, Hu Haowen <src.res@...il.cn>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-doc-tw-discuss@...ts.sourceforge.net,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Intel Graphics <intel-gfx@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        greybus-dev@...ts.linaro.org, linux-staging@...ts.linux.dev,
        linux-btrfs <linux-btrfs@...r.kernel.org>
Subject: Re: [greybus-dev] Re: [PATCH] [v2] Kbuild: move to -std=gnu11

On 5/18/22 00:46, Arnd Bergmann wrote:
> On Mon, May 16, 2022 at 3:19 PM Guenter Roeck <linux@...ck-us.net> wrote:
>> On 5/16/22 06:31, Greg KH wrote:
>>> On Mon, May 16, 2022 at 06:10:23AM -0700, Guenter Roeck wrote:
>>>> On Mon, Feb 28, 2022 at 11:27:43AM +0100, Arnd Bergmann wrote:
>>>>> From: Arnd Bergmann <arnd@...db.de>
>>>>>
>>>>> During a patch discussion, Linus brought up the option of changing
>>>>> the C standard version from gnu89 to gnu99, which allows using variable
>>>>> declaration inside of a for() loop. While the C99, C11 and later standards
>>>>> introduce many other features, most of these are already available in
>>>>> gnu89 as GNU extensions as well.
>>>>
>>>> The downside is that backporting affected patches to older kernel branches
>>>> now fails with error messages such as
>>>>
>>>> mm/kfence/core.c: In function ‘kfence_init_pool’:
>>>> mm/kfence/core.c:595:2: error: ‘for’ loop initial declarations are only allowed in C99 or C11 mode
>>>>
>>>> Just something to keep in mind when writing patches.
>>>
>>> I just ran across this very issue on this commit.  It's an easy fixup
>>> for 5.17.y to make this work, so I did that in my tree.  If this gets to
>>> be too much, we might need to reconsider adding c11 to older stable
>>> kernels.
>>>
>>
>> I think I'll do just that for ChromeOS; I don't want to have to deal
>> with the backports, and we are using recent compilers anyway.
> 
> I think it would be better not to have the --std=gnu11 change in the older
> stable kernels by default, as this has introduced build warnings and other
> smaller issues, as well as raising the minimum compiler version.
> 
> The users that are stuck on older kernels for some reason tend to
> overlap with those on older compilers. One example here is Android,
> which used to ship with a gcc-4.9 build as the only non-clang toolchain,
> and was using this for building their kernels. If someone wants to
> pull in stable updates into an older Android, this would fail with
> -std=gnu11. Others may be in the same situation.
> 
> Changing some of the 5.x stable branches to -std=gnu11 is probably
> less of a problem, but I would not know where to draw the line exactly.
> Maybe check with the Android team to see what the newest kernel is
> that they expect to be built with the old gcc-4.9.
> 

I don't think they still build anything with gcc. We (ChromeOS) only
need it for test builds of chromeos-4.4 (sigh), and that will hopefully
be gone in a couple of months.

We already enabled -std=gnu11 in chromeos-5.10 and chromeos-5.15.
We'll see if that is possible with chromeos-5.4 as well.
We won't bother with older kernel branches, but those should not
get many patches from upstream outside stable release merges,
so it is less of a problem.

Guenter