linux-kernel - Re: [PATCH] riscv: add arch/riscv/Kbuild

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Date:   Tue, 27 Aug 2019 14:14:59 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-riscv@...ts.infradead.org,
        Palmer Dabbelt <palmer@...ive.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Paul Walmsley <paul.walmsley@...ive.com>
Subject: Re: [PATCH] riscv: add arch/riscv/Kbuild

On Mon, Aug 26, 2019 at 8:35 PM Christoph Hellwig <hch@...radead.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@...ionext.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