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References: <20220301145233.3689119-1-arnd@kernel.org> <Yh5PhDDLcVe4kSqk@dev-arch.archlinux-ax161>
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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Fri, 4 Mar 2022 14:03:50 +0900
Message-ID: <CAK7LNAQYdUEPiAsLp749u9NiVXkvZzasHjCjKR+HA81qUDB+yw@mail.gmail.com>
Subject: Re: [PATCH 1/3] [v3] Kbuild: move to -std=gnu11
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Arnd Bergmann <arnd@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Marco Elver <elver@google.com>,
        Jani Nikula <jani.nikula@intel.com>,
        David Sterba <dsterba@suse.com>, Alex Shi <alexs@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        llvm@lists.linux.dev,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Mar 2, 2022 at 1:53 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Tue, Mar 01, 2022 at 03:52:31PM +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 standa=
rds
> > 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 using gnu_inline behavior and gnu11 using standard c99+
> > behavior, 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 -Wextra, which happens with the
> > 'make W=3D1' option, as well as for three drivers in the kernel that al=
ways
> > enable -Wextra, 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-=3DwiyCH7xeHcmiFJ-YgXUy2Jaj7pnk=
dKpcovt8fYbVFW3TA@mail.gmail.com/
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1603
> > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Acked-by: Marco Elver <elver@google.com>
> > Acked-by: Jani Nikula <jani.nikula@intel.com>
> > Acked-by: David Sterba <dsterba@suse.com>
> > Reviewed-by: Alex Shi <alexs@kernel.org>
> > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> > Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
> > Cc: Masahiro Yamada <masahiroy@kernel.org>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: linux-kbuild@vger.kernel.org
> > Cc: llvm@lists.linux.dev
> > Cc: linux-doc@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
>
> One comment below.
>
> > ---
> > [v3]
> >  - split out USERCFLAGS to a separate patch
> >  - add -Wdeclaration-after-statement patch from Mark Rutland
> >  - leave out C17 reference
> >  - more rewording the descrption
> >
> > [v2]
> >  - added -std=3Dgnu11 back, rather than just relying on the default
> >  - minor changes to changelog text
> > ---
> >  Documentation/process/programming-language.rst              | 6 +++---
> >  .../translations/it_IT/process/programming-language.rst     | 4 ++--
> >  .../translations/zh_CN/process/programming-language.rst     | 3 +--
> >  .../translations/zh_TW/process/programming-language.rst     | 3 +--
> >  Makefile                                                    | 4 ++--
> >  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, 15 insertions(+), 13 deletions(-)
> >
> > diff --git a/Documentation/process/programming-language.rst b/Documenta=
tion/process/programming-language.rst
> > index ec474a70a02f..5fc9160ca1fa 100644
> > --- a/Documentation/process/programming-language.rst
> > +++ b/Documentation/process/programming-language.rst
> > @@ -5,9 +5,9 @@ Programming Language
> >
> >  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 IS=
O C90
> > -(including some C99 features). ``clang`` [clang]_ is also supported, s=
ee
> > -docs on :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
> > +under ``-std=3Dgnu11`` [gcc-c-dialect-options]_: the GNU dialect of IS=
O C11.
> > +``clang`` [clang]_ is also supported, see docs on
> > +:ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
> >
> >  This dialect contains many extensions to the language [gnu-extensions]=
_,
> >  and many of them are used within the kernel as a matter of course.
> > diff --git a/Documentation/translations/it_IT/process/programming-langu=
age.rst b/Documentation/translations/it_IT/process/programming-language.rst
> > index 41db2598ce11..c1a9b481a6f9 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
> >
> >  Il kernel =C3=A8 scritto nel linguaggio di programmazione C [it-c-lang=
uage]_.
> >  Pi=C3=B9 precisamente, il kernel viene compilato con ``gcc`` [it-gcc]_=
 usando
> > -l'opzione ``-std=3Dgnu89`` [it-gcc-c-dialect-options]_: il dialetto GN=
U
> > -dello standard ISO C90 (con l'aggiunta di alcune funzionalit=C3=A0 da =
C99).
> > +l'opzione ``-std=3Dgnu11`` [it-gcc-c-dialect-options]_: il dialetto GN=
U
> > +dello standard ISO C11.
> >  Linux supporta anche ``clang`` [it-clang]_, leggete la documentazione
> >  :ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
> >
> > diff --git a/Documentation/translations/zh_CN/process/programming-langu=
age.rst b/Documentation/translations/zh_CN/process/programming-language.rst
> > index 2a47a1d2ec20..fabdc338dbfb 100644
> > --- a/Documentation/translations/zh_CN/process/programming-language.rst
> > +++ b/Documentation/translations/zh_CN/process/programming-language.rst
> > @@ -9,8 +9,7 @@
> >  =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> >
> >  =E5=86=85=E6=A0=B8=E6=98=AF=E7=94=A8C=E8=AF=AD=E8=A8=80 :ref:`c-langua=
ge <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-diale=
ct-options>` =E4=B8=8B=E7=BC=96=E8=AF=91=E7=9A=84=EF=BC=9AISO C90=E7=9A=84 =
GNU =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-diale=
ct-options>` =E4=B8=8B=E7=BC=96=E8=AF=91=E7=9A=84=EF=BC=9AISO C11=E7=9A=84 =
GNU =E6=96=B9=E8=A8=80
> >
> >  =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
> >
> > diff --git a/Documentation/translations/zh_TW/process/programming-langu=
age.rst b/Documentation/translations/zh_TW/process/programming-language.rst
> > index 54e3699eadf8..144bdaf81a41 100644
> > --- a/Documentation/translations/zh_TW/process/programming-language.rst
> > +++ b/Documentation/translations/zh_TW/process/programming-language.rst
> > @@ -12,8 +12,7 @@
> >  =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> >
> >  =E5=85=A7=E6=A0=B8=E6=98=AF=E7=94=A8C=E8=AA=9E=E8=A8=80 :ref:`c-langua=
ge <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-diale=
ct-options>` =E4=B8=8B=E7=B7=A8=E8=AD=AF=E7=9A=84=EF=BC=9AISO C90=E7=9A=84 =
GNU =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-diale=
ct-options>` =E4=B8=8B=E7=B7=A8=E8=AD=AF=E7=9A=84=EF=BC=9AISO C11=E7=9A=84 =
GNU =E6=96=B9=E8=A8=80
> >
> >  =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
> >
> > diff --git a/Makefile b/Makefile
> > index 289ce2be8032..94fa9a849a7a 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -515,7 +515,7 @@ KBUILD_CFLAGS   :=3D -Wall -Wundef -Werror=3Dstrict=
-prototypes -Wno-trigraphs \
> >                  -fno-strict-aliasing -fno-common -fshort-wchar -fno-PI=
E \
> >                  -Werror=3Dimplicit-function-declaration -Werror=3Dimpl=
icit-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_I=
MPLICIT_FALLTHROUGH)
> >
> >  ifdef CONFIG_CC_IS_CLANG
> >  KBUILD_CPPFLAGS +=3D -Qunused-arguments
> > -# The kernel builds with '-std=3Dgnu89' so use of GNU extensions is ac=
ceptable.
> > +# The kernel builds with '-std=3Dgnu11' so use of GNU extensions is ac=
ceptable.
> >  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 white=
listed names.
> > diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso=
32/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 -W=
no-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/Makef=
ile
> > 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-variab=
le)
> >  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/g=
reybus/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
>
> This Makefile uses its own -std=3D option because it is a userland tool.
> Seems like this hunk could just be dropped or the -std=3D argument could
> be bumped here too?
>

Right, this is a userland tool.

I do not mind either way.

BTW, this makefile is somewhat corrupted
because ../greybus_protocols.h does not exist.
(but it is working, relying on the built-in rules)


If Arnd does not mind sending v4, maybe we
can split this into two paches:

1/4 : add -Wno-shift-negative-value where -Wextra is used
2/4 : Kbuild: move to -std=3Dgnu11


1/4 adds -Wno-shift-negative-value
whether it is the kernel space or user-land.


--
Best Regards
Masahiro Yamada