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Date:   Wed, 15 Mar 2023 16:51:16 +0100
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To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org
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From:   Bastian Germann <bage@linutronix.de>
Subject: Re: [PATCH 1/1] kbuild: deb-pkg: Allow parallel build
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Am 15.03.23 um 15:18 schrieb Masahiro Yamada:
> "dpkb-buildpackage -j<N>" sets not only DEB_BUILD_OPTIONS
> but also MAKEFLAGS.
> 
> 
> This is clearly explained in "man dpkb-buildpackage".
> 
> 
>   -j, --jobs[=jobs|auto]
>     ...
>    Will add itself to the MAKEFLAGS environment variable, which should
>    cause all subsequent make invocations to inherit the option, thus
>    forcing the parallel setting on the packaging ...

I see. The testing/unstable dpkg-buildpackage version changed the -j behaviour.
-j only sets DEB_BUILD_OPTIONS=parallel= now.

The man page now says:

        --jobs-force[=jobs|auto]
        This option (since dpkg 1.21.10) is equivalent to the --jobs option except that
        it will enable forced parallel mode, by adding the make -j option with the computed
        number of parallel jobs to the MAKEFLAGS environment variable.

> Your statement sounds like
> 
>   'MAKEFLAGS=-j<N> dpkg-buildpackage'
> 
> is the only way to build packages in parallel.

It is in v1.21.10 or later (or using --jobs-force which does the same thing).

> Apparently, dpkg-buildpackage provides a much shorter way
> and invokes internal Make in parallel.
> 
> 
> 
> 
>>
>> So it is not used to run the actual build, just the top level `make -f debian/rules` invocation.
>> You can set --jobs-force=16, which ends up in MAKEFLAGS but the point of the patch is that one can use
>> the usual way of parallelizing.
> 
> 
> What is the "usual" way in this context?

The usual way is dpkg-buildpackage -jN but that does not work with later versions.

> Do you mean
> 'DEB_BUILD_OPTIONS=parallel=16 dpkg-buildpackage -b'
> is the usual way for parallel building?
> 
> If so, I agree. This patch caters to this case.
> 
> 
> But, I think your code should go to debian/rules
> instead of scripts/package/deb-build-option.
> Kbuild's "make clean" works much faster with the parallel option.
> 
> 
> Also, the commit description should explain the
> benefit of this patch.

I will clarify the dpkg-buildpackage behaviour change and will move the stuff to debian/rules
in v2.