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From:   Jani Nikula <jani.nikula@linux.intel.com>
To:     Arnd Bergmann <arnd@kernel.org>, linux-kbuild@vger.kernel.org
Cc:     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
In-Reply-To: <20220228103142.3301082-1-arnd@kernel.org>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20220228103142.3301082-1-arnd@kernel.org>
Date:   Mon, 28 Feb 2022 14:36:52 +0200
Message-ID: <87v8wz5frv.fsf@intel.com>
MIME-Version: 1.0
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Precedence: bulk

On Mon, 28 Feb 2022, Arnd Bergmann <arnd@kernel.org> 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=3Dgnu11 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=3D1' option, as well as for three drivers in the kernel that alwa=
ys
> 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.

Do you mean always enable -Wall and/or -Wextra instead of -Werror?

We do use -Werror for our CI and development too, but it's hidden behind
a config option that depends on COMPILE_TEST=3Dn to avoid any problems
with allmodconfig/allyesconfig.

For the future, makes me wonder if we couldn't have a way for drivers to
locally enable -Wall/-Wextra in a way that incorporates the exceptions
from kbuild instead of having to duplicate them.

Anyway, for the i915 changes,

Acked-by: Jani Nikula <jani.nikula@intel.com>

>
> 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-=3DwiyCH7xeHcmiFJ-YgXUy2Jaj7pnkdK=
pcovt8fYbVFW3TA@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=3Dgnu11 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(-)
>
> diff --git a/Documentation/process/programming-language.rst b/Documentati=
on/process/programming-language.rst
> index ec474a70a02f..894f2a6eb9db 100644
> --- a/Documentation/process/programming-language.rst
> +++ b/Documentation/process/programming-language.rst
> @@ -5,8 +5,8 @@ Programming Language
>=20=20
>  The kernel is written in the C programming language [c-language]_.
>  More precisely, the kernel is typically compiled with ``gcc`` [gcc]_
> -under ``-std=3Dgnu89`` [gcc-c-dialect-options]_: the GNU dialect of ISO =
C90
> -(including some C99 features). ``clang`` [clang]_ is also supported, see
> +under ``-std=3Dgnu11`` [gcc-c-dialect-options]_: the GNU dialect of ISO =
C11
> +(including some C17 features). ``clang`` [clang]_ is also supported, see
>  docs on :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
>=20=20
>  This dialect contains many extensions to the language [gnu-extensions]_,
> diff --git a/Documentation/translations/it_IT/process/programming-languag=
e.rst b/Documentation/translations/it_IT/process/programming-language.rst
> index 41db2598ce11..aa21097737ae 100644
> --- a/Documentation/translations/it_IT/process/programming-language.rst
> +++ b/Documentation/translations/it_IT/process/programming-language.rst
> @@ -10,8 +10,8 @@ Linguaggio di programmazione
>=20=20
>  Il kernel =C3=A8 scritto nel linguaggio di programmazione C [it-c-langua=
ge]_.
>  Pi=C3=B9 precisamente, il kernel viene compilato con ``gcc`` [it-gcc]_ u=
sando
> -l'opzione ``-std=3Dgnu89`` [it-gcc-c-dialect-options]_: il dialetto GNU
> -dello standard ISO C90 (con l'aggiunta di alcune funzionalit=C3=A0 da C9=
9).
> +l'opzione ``-std=3Dgnu11`` [it-gcc-c-dialect-options]_: il dialetto GNU
> +dello standard ISO C11 (con l'aggiunta di alcune funzionalit=C3=A0 da C1=
7).
>  Linux supporta anche ``clang`` [it-clang]_, leggete la documentazione
>  :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
>=20=20
> diff --git a/Documentation/translations/zh_CN/process/programming-languag=
e.rst b/Documentation/translations/zh_CN/process/programming-language.rst
> index 2a47a1d2ec20..58d2b3bd2d85 100644
> --- a/Documentation/translations/zh_CN/process/programming-language.rst
> +++ b/Documentation/translations/zh_CN/process/programming-language.rst
> @@ -9,8 +9,8 @@
>  =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>=20=20
>  =E5=86=85=E6=A0=B8=E6=98=AF=E7=94=A8C=E8=AF=AD=E8=A8=80 :ref:`c-language=
 <cn_c-language>` =E7=BC=96=E5=86=99=E7=9A=84=E3=80=82=E6=9B=B4=E5=87=86=E7=
=A1=AE=E5=9C=B0=E8=AF=B4=EF=BC=8C=E5=86=85=E6=A0=B8=E9=80=9A=E5=B8=B8=E6=98=
=AF=E7=94=A8 :ref:`gcc <cn_gcc>`
> -=E5=9C=A8 ``-std=3Dgnu89`` :ref:`gcc-c-dialect-options <cn_gcc-c-dialect=
-options>` =E4=B8=8B=E7=BC=96=E8=AF=91=E7=9A=84=EF=BC=9AISO C90=E7=9A=84 GN=
U =E6=96=B9=E8=A8=80=EF=BC=88
> -=E5=8C=85=E6=8B=AC=E4=B8=80=E4=BA=9BC99=E7=89=B9=E6=80=A7=EF=BC=89
> +=E5=9C=A8 ``-std=3Dgnu11`` :ref:`gcc-c-dialect-options <cn_gcc-c-dialect=
-options>` =E4=B8=8B=E7=BC=96=E8=AF=91=E7=9A=84=EF=BC=9AISO C11=E7=9A=84 GN=
U =E6=96=B9=E8=A8=80=EF=BC=88
> +=E5=8C=85=E6=8B=AC=E4=B8=80=E4=BA=9BC17=E7=89=B9=E6=80=A7=EF=BC=89
>=20=20
>  =E8=BF=99=E7=A7=8D=E6=96=B9=E8=A8=80=E5=8C=85=E5=90=AB=E5=AF=B9=E8=AF=AD=
=E8=A8=80 :ref:`gnu-extensions <cn_gnu-extensions>` =E7=9A=84=E8=AE=B8=E5=
=A4=9A=E6=89=A9=E5=B1=95=EF=BC=8C=E5=BD=93=E7=84=B6=EF=BC=8C=E5=AE=83=E4=BB=
=AC=E8=AE=B8=E5=A4=9A=E9=83=BD=E5=9C=A8=E5=86=85=E6=A0=B8=E4=B8=AD=E4=BD=BF=
=E7=94=A8=E3=80=82
>=20=20
> diff --git a/Documentation/translations/zh_TW/process/programming-languag=
e.rst b/Documentation/translations/zh_TW/process/programming-language.rst
> index 54e3699eadf8..235de05f7e2c 100644
> --- a/Documentation/translations/zh_TW/process/programming-language.rst
> +++ b/Documentation/translations/zh_TW/process/programming-language.rst
> @@ -12,8 +12,8 @@
>  =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>=20=20
>  =E5=85=A7=E6=A0=B8=E6=98=AF=E7=94=A8C=E8=AA=9E=E8=A8=80 :ref:`c-language=
 <tw_c-language>` =E7=B7=A8=E5=AF=AB=E7=9A=84=E3=80=82=E6=9B=B4=E6=BA=96=E7=
=A2=BA=E5=9C=B0=E8=AA=AA=EF=BC=8C=E5=85=A7=E6=A0=B8=E9=80=9A=E5=B8=B8=E6=98=
=AF=E7=94=A8 :ref:`gcc <tw_gcc>`
> -=E5=9C=A8 ``-std=3Dgnu89`` :ref:`gcc-c-dialect-options <tw_gcc-c-dialect=
-options>` =E4=B8=8B=E7=B7=A8=E8=AD=AF=E7=9A=84=EF=BC=9AISO C90=E7=9A=84 GN=
U =E6=96=B9=E8=A8=80=EF=BC=88
> -=E5=8C=85=E6=8B=AC=E4=B8=80=E4=BA=9BC99=E7=89=B9=E6=80=A7=EF=BC=89
> +=E5=9C=A8 ``-std=3Dgnu11`` :ref:`gcc-c-dialect-options <tw_gcc-c-dialect=
-options>` =E4=B8=8B=E7=B7=A8=E8=AD=AF=E7=9A=84=EF=BC=9AISO C11=E7=9A=84 GN=
U =E6=96=B9=E8=A8=80=EF=BC=88
> +=E5=8C=85=E6=8B=AC=E4=B8=80=E4=BA=9BC17=E7=89=B9=E6=80=A7=EF=BC=89
>=20=20
>  =E9=80=99=E7=A8=AE=E6=96=B9=E8=A8=80=E5=8C=85=E5=90=AB=E5=B0=8D=E8=AA=9E=
=E8=A8=80 :ref:`gnu-extensions <tw_gnu-extensions>` =E7=9A=84=E8=A8=B1=E5=
=A4=9A=E6=93=B4=E5=B1=95=EF=BC=8C=E7=95=B6=E7=84=B6=EF=BC=8C=E5=AE=83=E5=80=
=91=E8=A8=B1=E5=A4=9A=E9=83=BD=E5=9C=A8=E5=85=A7=E6=A0=B8=E4=B8=AD=E4=BD=BF=
=E7=94=A8=E3=80=82
>=20=20
> diff --git a/Makefile b/Makefile
> index 289ce2be8032..66496eaeb9ec 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -432,7 +432,7 @@ HOSTCXX	=3D g++
>  endif
>=20=20
>  export KBUILD_USERCFLAGS :=3D -Wall -Wmissing-prototypes -Wstrict-protot=
ypes \
> -			      -O2 -fomit-frame-pointer -std=3Dgnu89
> +			      -O2 -fomit-frame-pointer -std=3Dgnu11
>  export KBUILD_USERLDFLAGS :=3D
>=20=20
>  KBUILD_HOSTCFLAGS   :=3D $(KBUILD_USERCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTC=
FLAGS)
> @@ -515,7 +515,7 @@ KBUILD_CFLAGS   :=3D -Wall -Wundef -Werror=3Dstrict-p=
rototypes -Wno-trigraphs \
>  		   -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE \
>  		   -Werror=3Dimplicit-function-declaration -Werror=3Dimplicit-int \
>  		   -Werror=3Dreturn-type -Wno-format-security \
> -		   -std=3Dgnu89
> +		   -std=3Dgnu11
>  KBUILD_CPPFLAGS :=3D -D__KERNEL__
>  KBUILD_AFLAGS_KERNEL :=3D
>  KBUILD_CFLAGS_KERNEL :=3D
> @@ -782,7 +782,7 @@ KBUILD_CFLAGS +=3D $(KBUILD_CFLAGS-y) $(CONFIG_CC_IMP=
LICIT_FALLTHROUGH)
>=20=20
>  ifdef CONFIG_CC_IS_CLANG
>  KBUILD_CPPFLAGS +=3D -Qunused-arguments
> -# The kernel builds with '-std=3Dgnu89' so use of GNU extensions is acce=
ptable.
> +# The kernel builds with '-std=3Dgnu11' so use of GNU extensions is acce=
ptable.
>  KBUILD_CFLAGS +=3D -Wno-gnu
>  # CLANG uses a _MergedGlobals as optimization, but this breaks modpost, =
as the
>  # source of a reference will be _MergedGlobals and not on of the whiteli=
sted names.
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32=
/Makefile
> index 6c01b63ff56d..9378ea055bf2 100644
> --- a/arch/arm64/kernel/vdso32/Makefile
> +++ b/arch/arm64/kernel/vdso32/Makefile
> @@ -68,7 +68,7 @@ VDSO_CFLAGS +=3D -Wall -Wundef -Wstrict-prototypes -Wno=
-trigraphs \
>                 -fno-strict-aliasing -fno-common \
>                 -Werror-implicit-function-declaration \
>                 -Wno-format-security \
> -               -std=3Dgnu89
> +               -std=3Dgnu11
>  VDSO_CFLAGS  +=3D -O2
>  # Some useful compiler-dependent flags from top-level Makefile
>  VDSO_CFLAGS +=3D $(call cc32-option,-Wdeclaration-after-statement,)
> diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> index 1b62b9f65196..1618a6e0af4e 100644
> --- a/drivers/gpu/drm/i915/Makefile
> +++ b/drivers/gpu/drm/i915/Makefile
> @@ -17,6 +17,7 @@ subdir-ccflags-y +=3D -Wno-unused-parameter
>  subdir-ccflags-y +=3D -Wno-type-limits
>  subdir-ccflags-y +=3D -Wno-missing-field-initializers
>  subdir-ccflags-y +=3D -Wno-sign-compare
> +subdir-ccflags-y +=3D -Wno-shift-negative-value
>  subdir-ccflags-y +=3D $(call cc-disable-warning, unused-but-set-variable)
>  subdir-ccflags-y +=3D $(call cc-disable-warning, frame-address)
>  subdir-ccflags-$(CONFIG_DRM_I915_WERROR) +=3D -Werror
> diff --git a/drivers/staging/greybus/tools/Makefile b/drivers/staging/gre=
ybus/tools/Makefile
> index ad0ae8053b79..a3bbd73171f2 100644
> --- a/drivers/staging/greybus/tools/Makefile
> +++ b/drivers/staging/greybus/tools/Makefile
> @@ -12,7 +12,8 @@ CFLAGS	+=3D -std=3Dgnu99 -Wall -Wextra -g \
>  	    -Wredundant-decls \
>  	    -Wcast-align \
>  	    -Wsign-compare \
> -	    -Wno-missing-field-initializers
> +	    -Wno-missing-field-initializers \
> +	    -Wno-shift-negative-value
>=20=20
>  CC	:=3D $(CROSS_COMPILE)gcc
>=20=20
> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
> index 4188ba3fd8c3..99f9995670ea 100644
> --- a/fs/btrfs/Makefile
> +++ b/fs/btrfs/Makefile
> @@ -17,6 +17,7 @@ subdir-ccflags-y +=3D $(condflags)
>  subdir-ccflags-y +=3D -Wno-missing-field-initializers
>  subdir-ccflags-y +=3D -Wno-sign-compare
>  subdir-ccflags-y +=3D -Wno-type-limits
> +subdir-ccflags-y +=3D -Wno-shift-negative-value
>=20=20
>  obj-$(CONFIG_BTRFS_FS) :=3D btrfs.o
>=20=20
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 8be892887d71..650d0b8ceec3 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -36,6 +36,7 @@ KBUILD_CFLAGS +=3D $(call cc-option, -Wstringop-truncat=
ion)
>  KBUILD_CFLAGS +=3D -Wno-missing-field-initializers
>  KBUILD_CFLAGS +=3D -Wno-sign-compare
>  KBUILD_CFLAGS +=3D -Wno-type-limits
> +KBUILD_CFLAGS +=3D -Wno-shift-negative-value
>=20=20
>  KBUILD_CPPFLAGS +=3D -DKBUILD_EXTRA_WARN1

--=20
Jani Nikula, Intel Open Source Graphics Center