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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 26 Jan 2022 15:28:20 +0100
Message-ID: <CAMuHMdXRoVt_zRBNvugJjYhJnyYbABeCWv9fFRM0r_=s7FYvJQ@mail.gmail.com>
Subject: Re: [PATCH v3 1/2] docs: add a document about regression handling
To:     Thorsten Leemhuis <linux@leemhuis.info>
Cc:     "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        workflows@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Randy Dunlap <rdunlap@infradead.org>,
        regressions@lists.linux.dev,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>
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Hi Thorsten,

On Tue, Jan 25, 2022 at 5:45 PM Thorsten Leemhuis <linux@leemhuis.info> wrote:
> +How to create a configuration similar to the one of an older kernel?
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Start a known-good kernel and configure the newer Linux version with ``make
> +olddefconfig``. This makes the kernel's build scripts pick up the configuration
> +file (the `.config` file) from the running kernel as base for the new one you
> +are about to compile; afterwards they set all new configuration options to their
> +default value, which should disable new features that might cause regressions.

Doing so may actually cause mutations to appear in your .config
when going back and forth (i.e. when bisecting), interfering with
the bisection process.

To avoid that, I usually start bisecting with
"cp .config <src>/arch/<arch>/configs/bisect_defconfig", and use
"make bisect_defconfig" in every bisection step. That way all steps
are reproducible, and unaffected by config mutations.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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