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From:   Sedat Dilek <sedat.dilek@gmail.com>
Date:   Thu, 29 Aug 2019 23:01:39 +0200
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Subject: Re: [PATCH v2] kbuild: refactor scripts/Makefile.extrawarn
To:     Masahiro Yamada <yamada.masahiro@socionext.com>
Cc:     linux-kbuild@vger.kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Clang-Built-Linux ML <clang-built-linux@googlegroups.com>,
        linux-kernel@vger.kernel.org
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On Thu, Aug 29, 2019 at 8:13 PM Masahiro Yamada
<yamada.masahiro@socionext.com> wrote:
>
> Instead of the warning-[123] magic, let's accumulate compiler options
> to KBUILD_CFLAGS directly as the top Makefile does. I think this makes
> easier to understand what is going on in this file.
>
> This commit slightly changes the behavior, I think all of which are OK.
>
> [1] Currently, cc-option calls are needlessly evaluated. For example,
>       warning-3 += $(call cc-option, -Wpacked-bitfield-compat)
>     needs evaluating only when W=3, but it is actually evaluated for
>     W=1, W=2 as well. With this commit, only relevant cc-option calls
>     will be evaluated. This is a slight optimization.
>
> [2] Currently, unsupported level like W=4 is checked by:
>       $(error W=$(KBUILD_ENABLE_EXTRA_GCC_CHECKS) is unknown)
>     This will no longer be checked, but I do not think it is a big
>     deal.
>
> [3] Currently, 4 Clang warnings (Winitializer-overrides, Wformat,
>     Wsign-compare, Wformat-zero-length) are shown by any of W=1, W=2,
>     and W=3. With this commit, they will be warned only by W=1. I
>     think this is a more correct behavior since each warning belongs
>     to only one group.
>
> For understanding this commit correctly:
>
> We have 3 warning groups, W=1, W=2, and W=3. You may think W=3 has a
> higher level than W=1, but they are actually independent. If you like,
> you can combine them like W=13. To enable all the warnings, you can
> pass W=123. This is shown by 'make help', but it is often missed
> unfortunately. Since we support W= combination, there should not exist
> intersection among the three groups. If we enable Winitializer-overrides
> for W=1, we do not need to for W=2 or W=3. This is why I believe the
> change [3] makes sense.
>
> The documentation says -Winitializer-overrides is enabled by default.
> (https://clang.llvm.org/docs/DiagnosticsReference.html#winitializer-overrides)
> We negate it by passing -Wno-initializer-overrides for the normal
> build, but we do not do that for W=1. This means, W=1 effectively
> enables -Winitializer-overrides by the clang's default. The same for
> the other three. I wonder if this logic needs detailed commenting,
> but I do not want to be bothered any more. I added comments.
>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
>
> Changes in v2:
>   - Added comments and more commit log
>
>  scripts/Makefile.extrawarn | 105 +++++++++++++++++++------------------
>  1 file changed, 54 insertions(+), 51 deletions(-)
>

Thanks for the v2.

I am impressed by the very informative commit-log.

But... I still miss relevant infos in the "kbuild-docs" aka in
<Documentation/kbuild/kbuild.rst file>

I also was not aware I can combine W=... settings like W=123 (W=132
and W=321 does the same I guess).
In my little world W=3 should include W=1 and W=2.
Such informations I would like to have in kbuild-docs.

BTW, I mixed up kbuild-system with kconfig-system as I normally get in
touch with the 2nd.

Renaming the kbuild-variable is up to you.
I am OK when you want to wait for Arnd's rework of extrawarn compiler options.
I did change it in one of my patches treewide.

- Sedat -