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

[Nicolas Pitre]

On Wed, 7 Sep 2011, Arnaud Lacombe wrote:

> Hi,
> 
> On Wed, Sep 7, 2011 at 3:07 PM, Nicolas Pitre <nico at fluxnic.net> wrote:
> > 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.
> >
> What was the exact change you made which triggered this ?

In its simplest expression (not caring about the now undefined lib1funcs 
variable):

diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index 0c74a6fab9..b34ed80977 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -70,6 +70,10 @@ ifeq ($(CONFIG_ARCH_SHMOBILE),y)
 OBJS		+= head-shmobile.o
 endif
 
+# For __aeabi_uidivmod
+OBJS		+= lib1funcs.o
+vpath-y		+= $(srctree)/arch/arm/lib
+
 #
 # We now have a PIC decompressor implementation.  Decompressors running
 # from RAM should not define ZTEXTADDR.  Decompressors running directly
@@ -120,12 +124,6 @@ LDFLAGS_vmlinux += -X
 # Next argument is a linker script
 LDFLAGS_vmlinux += -T
 
-# For __aeabi_uidivmod
-lib1funcs = $(obj)/lib1funcs.o
-
-$(obj)/lib1funcs.S: $(srctree)/arch/$(SRCARCH)/lib/lib1funcs.S FORCE
-	$(call cmd,shipped)
-
 # We need to prevent any GOTOFF relocs being used with references
 # to symbols in the .bss section since we cannot relocate them
 # independently from the rest at run time.  This can be achieved by


Nicolas