linux-kernel - Re: [RFC] Kbuild: allow code re-use across different directories

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Message-id: <alpine.LFD.2.00.1109071437400.20358@xanadu.home>
Date:	Wed, 07 Sep 2011 15:07:42 -0400 (EDT)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Arnaud Lacombe <lacombar@...il.com>
Cc:	linux-kbuild@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Kbuild: allow code re-use across different directories

On Fri, 19 Aug 2011, Arnaud Lacombe wrote:

> Hi folks,
> 
> The attached patch modify Kbuild to allow to directly re-use code in multiple
> directory without having to go through a copy. Technically, it changes Kbuild to
> use by default the VPATH feature of GNU make and provides accessors for Makefile
> to change it indirectly.
> 
> Considering:
> 
> arch/foo/lib:
> fancy.c
> 
> We want to be able to build it with -DPANTS=32 in the kernel, but the
> bootloader requires -DPANTS_SIZE=30.
> 
> Currently we would do, either:
> 
> arch/foo/lib/Makefile
> LDFLAGS_fancy.o := -DPANTS=32
> obj-y += fancy.o
> 
> and, either:
> 
> arch/foo/boot/Makefile:
> LDFLAGS_fancy.o := -DPANTS=30
> obj-y += fancy.o
> $(obj)/fancy.c: $(srctree)/arch/foo/lib/fancy.c
> 	$(call cmd,shipped)
> 
> or
> 
> arch/foo/boot/Makefile:
> LDFLAGS_fancy.o := -DPANTS=30
> obj-y += fancy.o
> $(obj)/fancy.o: $(srctree)/arch/foo/lib/fancy.c
> 	$(call cmd,cc_c_o)
> 
> The former implies an extra copy of the source file, the latter expose Kbuild
> internal function.
> 
> With the attached patch, we would do:
> 
> arch/foo/boot/Makefile:
> LDFLAGS_fancy.o := -DPANTS=30
> obj-y += fancy.o
> vpath-y += $(srctree)/arch/foo/lib
> 
> and let GNU make do the job.
> 
> Comments welcome,

It doesn't work.  Whatever I do to arch/arm/boot/compressed/Makefile 
(which admittedly looks a bit hairy and could benefit from a shave) in 
order to remove the $(call cmd,shipped) used with lib1funcs.S, I always 
end up with:

make[2]: *** No rule to make target `arch/arm/boot/compressed/lib1funcs.S', needed by `arch/arm/boot/compressed/lib1funcs.o'.  Stop.

Obviously, restoring the explicit dependency with a $(call cmd,as_o_S) 
does make it work, but this is rather against the point of your patch.

Do you have an example of how arch/arm/boot/compressed/Makefile should 
be changed to benefit from your patch?


Nicolas
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