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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Fri, 30 Sep 2022 18:11:28 +0900
Message-ID: <CAK7LNATqfCxwvYMUtoQZkoTk5yqZ_q+HJgcf934ib3NEG91oiw@mail.gmail.com>
Subject: Re: [PATCH v3] kbuild: add debug level and macro defs options
To:     Dmitrii Bundin <dmitrii.bundin.a@gmail.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Eric Dumazet <edumazet@google.com>,
        Isabella Basso <isabbasso@riseup.net>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Fangrui Song <maskray@google.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Nathan Chancellor <nathan@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>
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On Fri, Sep 30, 2022 at 6:06 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Tue, Aug 23, 2022 at 7:42 AM Dmitrii Bundin
> <dmitrii.bundin.a@gmail.com> wrote:
> >
> > On Tue, Aug 23, 2022 at 12:36 AM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > or perhaps that simply needs to be `-g -gsplit-dwarf`?  In which case,
> > > that if/else could just be re-arranged.
> >
> > How about simply assigning DEBUG_CFLAGS   := -g at the very beginning
> > without any conditions? This would provide the default with the
> > possibility of overriding later and -gsplit-dwarf does not necessarily
> > come with -g implicitly.
>
> This was fixed by commit 32ef9e5054ec0321b9336058c58ec749e9c6b0fe,
> which is now in the mainline.
>
>
>
>
> > > Honestly, I really don't think we need to be wrapping every compiler
> > > command line flag under the sun in a kconfig option.
> >
> > This indeed sounds reasonable to me. So the key point here is to not
> > bloat the kconfig with options related to every compiler flag. But I
> > think it still might be useful to provide some option that would
> > include sort of full debug information compilers may produce. With
> > this approach there would be, in fact 3 different levels of debug
> > information supported by Kconfig: reduced, default and full. The full
> > level would increase everything like -g3, and -fdebug-macro for Clang,
> > and probably others.
>
>
> I think that would be much saner than this patch.
>
>
>
> CONFIG_DEBUG_INFO_LEVEL is a direct way to specify the debug level.
>
> CONFIG_DEBUG_MACRO_DEFINITIONS is feature-driven.
>
> Do not mix two different ways.
>
>
>
>
>
>
> CONFIG_DEBUG_INFO_LEVEL is here just because Andrew Morton suggested that.
>
>
> The debug level is compiler-specific. There is no guarantee
> that there is a common range.
>
>
> The debug level range of GCC is 0-3.
> Clang accepts 3, but -g3 has no effect.
> The debug level range of Rustc is 0-2.
>
> See how badly scripts/Makefile.debug looks in linux-next.
>
>
>
>
>
> How should Rustc behave for CONFIG_DEBUG_INFO_LEVEL=3 ?
>
> -Cdebuginfo=3 is a compile error.
>
>   RUSTC L rust/core.o
> error: debug info level needs to be between 0-2 (instead was `3`)
>
>
>
> You cannot directly specify the debug level number given that
> we support multiple compilers with different policy for
> debug level options.
>
>
>
>
>
>
> > > Or add -g1 to CONFIG_DEBUG_INFO_REDUCED.
> >
> > I ran some tests and there was indeed some decrease in size. That
> > combination probably might be useful.
> >
> > Any thoughts?
> >
> > Regards
> > Dmitrii
>
>
>
>
>
> --
> Best Regards
> Masahiro Yamada


I proposed to do a ground-work like the following first.
https://patchwork.kernel.org/project/linux-kbuild/patch/20220930085351.2648034-1-masahiroy@kernel.org/


On top of that, it is easier to add CONFIG_DEBUG_INFO_FULL or whatever.


And, -g1 for CONFIG_DEBUG_INFO_REDUCED if you think it is worthwhile.


-- 
Best Regards
Masahiro Yamada