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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Tue, 27 Aug 2019 14:14:59 +0900
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Subject: Re: [PATCH] riscv: add arch/riscv/Kbuild
To:     Christoph Hellwig <hch@infradead.org>
Cc:     linux-riscv@lists.infradead.org,
        Palmer Dabbelt <palmer@sifive.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Paul Walmsley <paul.walmsley@sifive.com>
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On Mon, Aug 26, 2019 at 8:35 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Wed, Aug 21, 2019 at 06:26:58PM +0900, Masahiro Yamada wrote:
> > Use the standard obj-y form to specify the sub-directories under
> > arch/riscv/. No functional change intended.
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
>
> Do you have a document what the grand scheme here is?

There is a small documentation about "Makefile" vs "Kbuild"
in Documentation/kbuild/modules.rst section 3.2

It is talking about external modules, but the benefit applies
to arch/$(SRCARCH)/Kbuild as well.

arch/$(SRCARCH)/Makefile is included by the top Makefile
to specify arch-specific compiler flags, etc.

On the other hand, arch/$(SRCARCH)/Kbuild, if exists, is included
when Kbuild actually descends into arch/$(SRCARCH)/.

This allows you to hierarchize the sub-directories to visit
instead of specifying everything in flat in arch/$(SRCARCH)/Makefile.

Major architectures are already doing this.

See
arch/x86/Kbuild
arch/sparc/Kbuild
arch/powerpc/Kbuild
etc.

(and arm64 also adopted this recently)


The trick is "Kbuild" has precedence over "Makefile".

If you are interested in the actual code,
see line 41 of scripts/Makefile.build


>  Less of the magic
> in arch/$(ARCH)/Makefile sounds like a good idea, but unless we have
> a very specific split between the kbuild makefile and various override
> I fear just splitting things up into two files doesn't really help much.

Why not?


-- 
Best Regards
Masahiro Yamada