Re: [PATCH] kbuild: mkcompile_h: consider timestamp if KBUILD_BUILD_TIMESTAMP is set

[Matthias Maennich]

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@xxxxxxxxxx> wrote:
> > On Sat, Jun 12, 2021 at 11:18 PM Matthias Maennich <maennich@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > Cc: linux-kbuild@xxxxxxxxxxxxxxx
> > > Signed-off-by: Matthias Maennich <maennich@xxxxxxxxxx>
> > > ---
> >
> >
> > 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