netdev - Re: [PATCH net-next v2 1/2] Makefile.extrawarn: Add symbol for W=1 warnings for today

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Date:   Mon, 12 Oct 2020 10:11:53 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH net-next v2 1/2] Makefile.extrawarn: Add symbol for W=1
 warnings for today

On Sun, Oct 11, 2020 at 3:04 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
> On Tue, Oct 6, 2020 at 6:08 AM Andrew Lunn <andrew@...n.ch> wrote:
>
>
> I am not a big fan of KBUILD_CFLAGS_W1_<timestamp>
> since it is ugly.
>
> I'd like to start with adding individual flags
> like drivers/gpu/drm/i915/Makefile, and see
> how difficult it would be to maintain it.

I don't really like that either, to be honest, mostly because that is
somewhat incompatible with my plan to move all the warning flags
out the command line and into _Pragma() lines in header files.

> One drawback of your approach is that
> you cannot set KBUILD_CFLAGS_W1_20200930
> until you eliminate all the warnings in the
> sub-directory in interest.
> (i.e. all or nothing approach)
>
> At best, you can only work out from 'old -> new' order
> because KBUILD_CFLAGS_W1_20200326 is a suer-set of
> KBUILD_CFLAGS_W1_20190907, which is a suer-set of
> KBUILD_CFLAGS_W1_20190617 ...
>
>
>
> If you add flags individually, you can start with
> low-hanging fruits, or ones with higher priority
> as Arnd mentions about -Wmissing-{declaration,prototypes}.
>
>
> For example, you might be able to set
> 'subdir-ccflags-y += -Wmissing-declarations'
> to drivers/net/Makefile, while
> 'subdir-ccflags-y += -Wunused-but-set-variable'
> stays in drivers/net/ethernet/Makefile.

I agree with the goal though. In particular it may be helpful to pick
a set of warning flags to always be enabled without also setting
-Wextra, which has different meanings depending on compiler
version, or between gcc and clang.

I wonder how different they really are, and we can probably find
out from https://github.com/Barro/compiler-warnings, but I have
not checked myself.

      Arnd