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Date:   Mon, 28 Feb 2022 11:50:43 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     linux-kbuild@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masahiro Yamada <masahiroy@kernel.org>, llvm@lists.linux.dev,
        Jonathan Corbet <corbet@lwn.net>,
        Federico Vaga <federico.vaga@vaga.pv.it>,
        Alex Shi <alexs@kernel.org>, Hu Haowen <src.res@email.cn>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-doc-tw-discuss@lists.sourceforge.net,
        linux-arm-kernel@lists.infradead.org,
        intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        greybus-dev@lists.linaro.org, linux-staging@lists.linux.dev,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH] [v2] Kbuild: move to -std=gnu11
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On Mon, Feb 28, 2022 at 11:27:43AM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.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.
> 
> An earlier attempt to do this when gcc-5 started defaulting to
> -std=gnu11 failed because at the time that caused warnings about
> designated initializers with older compilers. Now that gcc-5.1 is the
> minimum compiler version used for building kernels, that is no longer a
> concern. Similarly, the behavior of 'inline' functions changes between
> gnu89 and gnu11, but this was taken care of by defining 'inline' to
> include __attribute__((gnu_inline)) in order to allow building with
> clang a while ago.
> 
> One minor issue that remains is an added gcc warning for shifts of
> negative integers when building with -Werror, which happens with the
> 'make W=1' option, as well as for three drivers in the kernel that always
> enable -Werror, but it was only observed with the i915 driver so far.
> To be on the safe side, add -Wno-shift-negative-value to any -Wextra
> in a Makefile.
> 
> Nathan Chancellor reported an additional -Wdeclaration-after-statement
> warning that appears in a system header on arm, this still needs a
> workaround.
> 
> The differences between gnu99, gnu11, gnu1x and gnu17 are fairly
> minimal and mainly impact warnings at the -Wpedantic level that the
> kernel never enables. Between these, gnu11 is the newest version
> that is supported by all supported compiler versions, though it is
> only the default on gcc-5, while all other supported versions of
> gcc or clang default to gnu1x/gnu17.
> 
> Link: https://lore.kernel.org/lkml/CAHk-=wiyCH7xeHcmiFJ-YgXUy2Jaj7pnkdKpcovt8fYbVFW3TA@mail.gmail.com/
> Link: https://github.com/ClangBuiltLinux/linux/issues/1603
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: linux-kbuild@vger.kernel.org
> Cc: llvm@lists.linux.dev
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> [v2]
>  - added -std=gnu11 back, rather than just relying on the default
>  - minor changes to changelog text
> ---
>  Documentation/process/programming-language.rst              | 4 ++--
>  .../translations/it_IT/process/programming-language.rst     | 4 ++--
>  .../translations/zh_CN/process/programming-language.rst     | 4 ++--
>  .../translations/zh_TW/process/programming-language.rst     | 4 ++--
>  Makefile                                                    | 6 +++---
>  arch/arm64/kernel/vdso32/Makefile                           | 2 +-
>  drivers/gpu/drm/i915/Makefile                               | 1 +
>  drivers/staging/greybus/tools/Makefile                      | 3 ++-
>  fs/btrfs/Makefile                                           | 1 +
>  scripts/Makefile.extrawarn                                  | 1 +
>  10 files changed, 17 insertions(+), 13 deletions(-)

For the greybus tools section:

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>