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

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Message-ID: <CAMuHMdXqJX_35WJDx_H4y39+sED5MeYeQVnqWXmStUTsfX1C2A@mail.gmail.com>
Date:   Fri, 10 May 2019 09:24:46 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        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,

On Fri, May 10, 2019 at 9:03 AM Sam Ravnborg <sam@...nborg.org> wrote:
> On Fri, May 10, 2019 at 08:46:35AM +0200, Geert Uytterhoeven wrote:
> > 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".

> > 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 no longer updated if .config has not changed.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.
>
> > > My workflow involves always running my own script "linux-oldconfig",
> > instead of "make oldconfig", so I immediately see what has changed:
> >
> >     $ cat $(type -p linux-oldconfig)
> >     #!/bin/bash
> >     make ${0#*/linux-} && colordiff -u .config{.old,}
> So scripts relying on the old (broken) behaviour will no longer work.
> The new behaviour is better as it is usefaul in many typical situations.
>
> Hacking, hack. What did I change in the config?

I agree both semantics have their merits.
Sometimes I also wanted to see the last real change...

No worries, updating my script, so it works with both semantics:

    $ cat $(type -p linux-oldconfig)
    #!/bin/bash
    cp -a .config .config.orig
    make ${0#*/linux-} && colordiff -u .config{.orig,}

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds