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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Sat, 31 Aug 2019 22:04:53 +0900
Message-ID: <CAK7LNAR2JuZkdJGxO=f2hUxmQca5d7430NC-2hiqZwkJphJ9sA@mail.gmail.com>
Subject: Re: [PATCH] riscv: add arch/riscv/Kbuild
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Albert Ou <aou@eecs.berkeley.edu>,
        Palmer Dabbelt <palmer@sifive.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        linux-riscv@lists.infradead.org
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On Sat, Aug 31, 2019 at 12:53 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Tue, Aug 27, 2019 at 02:14:59PM +0900, Masahiro Yamada wrote:
> > There is a small documentation about "Makefile" vs "Kbuild"
> > in Documentation/kbuild/modules.rst section 3.2
>
> I know that part.
>
> >
> > It is talking about external modules, but the benefit applies
> > to arch/$(SRCARCH)/Kbuild as well.
>
> I fail to fully understand this part, though.

Kbuild support two file names, "Makefile" and "Kbuild"
for describing obj-y, obj-m, etc.

If both exist in the same directory, "Kbuild" takes precedence.

This is useful where we want to use "Makefile"
for other purposes.

For external modules, we use "Makefile" as a start point,
so we often want to  describe obj-m in "Kbuild" instead of "Makefile".


Similarly, arch/$(SRCARCH)/Makefile is very special
in that it is included from the top-level Makefile,
and specify arch-specific compiler flags etc.

We can use arch/$(SRCARCH)/Kbuild
to specify obj-y, obj-m.
The top-level Makefile does not need to know
the directory structure under arch/$(SRCARCH)/.

This is logical separation.


>
> > 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.
>
> Yes, but what is the plan in the long run?  arch/$(ARCH)/Makefile
> is still a weird hodge-podge of overriding global variables and misc
> Makefile targets, it now just has a tiny little work.

There are lots of arch-specific build settings.
We definitely need a place to describe them,
hence arch/$(SRCARCH)/Makefile.


>  Is there any
> actual benefit from using Kbuild for the build process?

For example, arch/x86/Kbuild is quite useful
since there are many directories in arch/x86.

riscv does not have many directories for now since it is a new architecture,
but it will be useful when it will have more directories.


>  Can we
> eventually move the setting of variables in the Makefile into another
> special purpose file?  Is the support for actually compiling kernel
> source files from the arch/$(ARCH) Makefile eventually going away?

No, I do not think so.

>
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> linux-riscv@lists.infradead.org
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-- 
Best Regards
Masahiro Yamada