Re: [PATCH] Kbuild: remove -std=gnu89 from compiler arguments

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On 2/27/22 3:52 PM, Arnd Bergmann wrote:
From: Arnd Bergmann <arnd@xxxxxxxx>

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 gnu89, 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.

Nathan Chancellor reported an additional -Wdeclaration-after-statement
warning that appears in a system header on arm, this still needs a

Since the differences between gnu99, gnu11 and gnu17 are fairly minimal
and mainly impact warnings at the -Wpedantic level that the kernel
never enables, the easiest way is to just leave out the -std=gnu89
argument entirely, and rely on the compiler default language setting,
which is gnu11 for gcc-5, and gnu1x/gnu17 for all other supported
versions of gcc or clang.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Cc: linux-kbuild@xxxxxxxxxxxxxxx
Cc: llvm@xxxxxxxxxxxxxxx
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
I put the suggestion into patch form, based on what we discussed
in the thread.  I only gave it minimal testing, but it would
be good to have it in linux-next if we want to do this in the
merge window.

Did you determine what needed the new compiler flag based on
compilation results?

Glancing at the Greybus code, I don't believe there's any
reason it needs to shift a negative value.  Such warnings
could be fixed by making certain variables unsigned, for

I have no objection, I'll just make a note of it.


  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                                                   | 7 +++----
  arch/arm64/kernel/vdso32/Makefile                          | 3 +--
  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(+), 15 deletions(-)

. . .

diff --git a/drivers/staging/greybus/tools/Makefile b/drivers/staging/greybus/tools/Makefile
index ad0ae8053b79..a3bbd73171f2 100644
--- a/drivers/staging/greybus/tools/Makefile
+++ b/drivers/staging/greybus/tools/Makefile
@@ -12,7 +12,8 @@ CFLAGS	+= -std=gnu99 -Wall -Wextra -g \
  	    -Wredundant-decls \
  	    -Wcast-align \
  	    -Wsign-compare \
-	    -Wno-missing-field-initializers
+	    -Wno-missing-field-initializers \
+	    -Wno-shift-negative-value

. . .
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