linux-kernel - Re: [PATCH] kbuild: Disable -Wpointer-to-enum-cast

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Message-ID: <CAK7LNARcTHpd8fzrAhFVB_AR7NoBgenX64de0eS2uN8g0by9PQ@mail.gmail.com>
Date:   Mon, 9 Mar 2020 11:11:05 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] kbuild: Disable -Wpointer-to-enum-cast

Hi Nathan,

On Sun, Mar 8, 2020 at 4:34 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> Clang's -Wpointer-to-int-cast deviates from GCC in that it warns when
> casting to enums. The kernel does this in certain places, such as device
> tree matches to set the version of the device being used, which allows
> the kernel to avoid using a gigantic union.
>
> https://elixir.bootlin.com/linux/v5.5.8/source/drivers/ata/ahci_brcm.c#L428
> https://elixir.bootlin.com/linux/v5.5.8/source/drivers/ata/ahci_brcm.c#L402
> https://elixir.bootlin.com/linux/v5.5.8/source/include/linux/mod_devicetable.h#L264
>
> To avoid a ton of false positive warnings, disable this particular part
> of the warning, which has been split off into a separate diagnostic so
> that the entire warning does not need to be turned off for clang.
>
> Cc: stable@...r.kernel.org
> Link: https://github.com/ClangBuiltLinux/linux/issues/887
> Link: https://github.com/llvm/llvm-project/commit/2a41b31fcdfcb67ab7038fc2ffb606fd50b83a84
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>  Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index 86035d866f2c..90e56d5657c9 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -748,6 +748,10 @@ KBUILD_CFLAGS += -Wno-tautological-compare
>  # source of a reference will be _MergedGlobals and not on of the whitelisted names.
>  # See modpost pattern 2
>  KBUILD_CFLAGS += -mno-global-merge
> +# clang's -Wpointer-to-int-cast warns when casting to enums, which does not match GCC.
> +# Disable that part of the warning because it is very noisy across the kernel and does
> +# not point out any real bugs.
> +KBUILD_CFLAGS += $(call cc-disable-warning, pointer-to-enum-cast)
>  else



I'd rather want to fix all the call-sites (97 drivers?)
instead of having -Wno-pointer-to-enum-cast forever.

If it is tedious to fix them all for now, can we add it
into scripts/Makefile.extrawarn so that this is disabled
by default, but shows up with W=1 builds?

(When we fix most of them, we will be able to
make it a real warning.)


What do you think?

Thanks.




>  # These warnings generated too much noise in a regular build.
> --
> 2.25.1
>
> --
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-- 
Best Regards
Masahiro Yamada