linux-kernel - Re: [PATCH 2/2] kconfig: do not write .config if the content is the same

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Date:   Fri, 10 May 2019 16:41:23 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Ulf Magnusson <ulfalizer@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] kconfig: do not write .config if the content is the same

Hi Sam, Geert,

On Fri, May 10, 2019 at 4:04 PM Sam Ravnborg <sam@...nborg.org> wrote:
>
> Hi Geert/Masahiro.
>
> On Fri, May 10, 2019 at 08:46:35AM +0200, Geert Uytterhoeven wrote:
> > Hi Yamada-san,
> >
> > On Fri, May 10, 2019 at 8:14 AM Masahiro Yamada
> > <yamada.masahiro@...ionext.com> wrote:
> > > Kconfig updates the .config when it exits even if its content is
> > > exactly the same as before. Since its timestamp becomes newer than
> > > that of other build artifacts, additional processing is invoked,
> > > which is annoying.
> > >
> > > - syncconfig is invoked to update include/config/auto.conf, etc.
> > >
> > > - kernel/config.o is recompiled if CONFIG_IKCONFIG is enabled,
> > >   then vmlinux is relinked as well.
> > >
> > > If the .config is not changed at all, we do not have to even
> > > touch it. Just bail out showing "No change to .config".
> It would be preferable that if nothing changed no output is generated.
> Like we do not tell that we did not build a .o file because the .c file
> had not changed.
> Less noise for a kernel build where nothings happens.
>
> > This causes a semantic change for the meaning of ".config.old", which is
> > no longer updated if .config has not changed.
> > Hence its contents may no longer correspond to the previous config, but to
> > an arbitrary older version.
> This semantic change is good.
> So we now have a .config.old that correspond to the state before
> the last change. Also after several kernel builds.


I agree.

When there is no change in the configuration,
Kconfig will not even attempt to output anything.

Updating only .config.old is strange.

Thanks.



--
Best Regards
Masahiro Yamada