On 2/27/22 3:52 PM, 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 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 workaround.
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.
Link: https://lore.kernel.org/lkml/CAHk-=wiyCH7xeHcmiFJ-YgXUy2Jaj7pnkdKpcovt8fYbVF... 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
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 example.
I have no objection, I'll just make a note of it.
-Alex
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(-)
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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-valueCC := $(CROSS_COMPILE)gcc
. . .