linux-kernel - Re: [PATCH] kbuild: mkcompile_h: consider timestamp if KBUILD_BUILD

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Date:   Thu, 17 Jun 2021 10:43:19 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Matthias Maennich <maennich@...gle.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH] kbuild: mkcompile_h: consider timestamp if
 KBUILD_BUILD_TIMESTAMP is set

On Thu, Jun 17, 2021 at 10:05 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Sat, Jun 12, 2021 at 11:18 PM Matthias Maennich <maennich@...gle.com> wrote:
> >
> > To avoid unnecessary recompilations, mkcompile_h does not regenerate
> > compile.h if just the timestamp changed.
> > Though, if KBUILD_BUILD_TIMESTAMP is set, an explicit timestamp for the
> > build was requested, in which case we should not ignore it.
> >
> > If a user follows the documentation for reproducible builds [1] and
> > defines KBUILD_BUILD_TIMESTAMP as the git commit timestamp, a clean
> > build will have the correct timestamp. A subsequent cherry-pick (or
> > amend) changes the commit timestamp and if an incremental build is done
> > with a different KBUILD_BUILD_TIMESTAMP now, that new value is not taken
> > into consideration. But it should for reproducibility.
> >
> > Hence, whenever KBUILD_BUILD_TIMESTAMP is explicitly set, do not ignore
> > UTS_VERSION when making a decision about whether the regenerated version
> > of compile.h should be moved into place.
> >
> > [1] https://www.kernel.org/doc/html/latest/kbuild/reproducible-builds.html
> >
> > Cc: Masahiro Yamada <masahiroy@...nel.org>
> > Cc: linux-kbuild@...r.kernel.org
> > Signed-off-by: Matthias Maennich <maennich@...gle.com>
> > ---
>
>
> Applied to linux-kbuild. Thanks.
>

This may not be a big deal, but when KBUILD_BUILD_TIMESTAMP is unset,
the timestamp is not updated.  It still has a user-specified string.




-- 
Best Regards
Masahiro Yamada