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

[Arnaud Lacombe]

Hi,

2011/9/9 Michal Marek <mmarek at suse.cz>:
> On 20.8.2011 02:37, 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.
>
> I like this. The only issue I can think of right now, is that if you add
> a large directory to vpath-y, then it would be easy to accidentally
> reuse more files from that directory than intended. But that could be
> easily prevented by isolating those reusable source files.
>
I do not think it is that dangerous. We enforce unique access to VPATH
and we still prioritize $(src) over any other specified path.

That said, what would you want to pull the patch into -next, beside
kernel.org being up ?

Thanks,
 - Arnaud