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

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Message-ID: <YMsTWAYSV1ZZFYvH@google.com>
Date:   Thu, 17 Jun 2021 10:18:16 +0100
From:   Matthias Maennich <maennich@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
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 10:43:19AM +0900, Masahiro Yamada wrote:
>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.

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.

I think treating this like any other 'random' prior timestamp is ok.
So, I agree: not a big deal.

Cheers,
Matthias
>
>
>
>
>-- 
>Best Regards
>Masahiro Yamada