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

[Alex Elder <elder@linaro.org>]

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-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>
> ---
> 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(-)
> 

. . .

> 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
>   
>   CC	:= $(CROSS_COMPILE)gcc
>   

. . .