linux-kernel - Re: [PATCH] Makefile.extrawarn: Move -Wunaligned-access to W=2

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Date:   Wed, 2 Feb 2022 09:12:06 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev, "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH] Makefile.extrawarn: Move -Wunaligned-access to W=2

On Wed, Feb 2, 2022 at 12:22 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> -Wunaligned-access is a new warning in clang that is default enabled for
> arm and arm64 under certain circumstances within the clang frontend (see
> LLVM commit below). Under an ARCH=arm allmodconfig, there are
> 1284 total/70 unique instances of this warning (most of the instances
> are in header files), which is quite noisy.
>
> To keep the build green through CONFIG_WERROR, only allow this warning
> with W=2, which seems appropriate according to its description:
> "warnings which occur quite often but may still be relevant".
>
> This intentionally does not use the -Wno-... + -W... pattern that the
> rest of the Makefile does because this warning is not enabled for
> anything other than certain arm and arm64 configurations within clang so
> it should only be "not disabled", rather than explicitly enabled, so
> that other architectures are not disturbed by the warning.
>
> Cc: stable@...r.kernel.org
> Link: https://github.com/llvm/llvm-project/commit/35737df4dcd28534bd3090157c224c19b501278a
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

The warning seems important enough to be considered for W=1 on 32-bit arm,
otherwise the chances of anyone actually fixing drivers for it is
relatively slim.
Can you point post the (sufficiently trimmed) output that you get with
the warning
enabled in an allmodconfig build?

I'm not sure why this is enabled by default for arm64, which does not have
the problem with fixup handlers.

       Arnd