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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Thu, 18 Jul 2019 11:19:58 +0900
Message-ID: <CAK7LNAR7jkq1fAi_=xgsANCkgP2AAej9Yv7RZB3B_cpD7C_71Q@mail.gmail.com>
Subject: Re: [PATCH] powerpc: remove meaningless KBUILD_ARFLAGS addition
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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On Thu, Jul 18, 2019 at 1:46 AM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Thu, Jul 18, 2019 at 12:19:36AM +0900, Masahiro Yamada wrote:
> > On Wed, Jul 17, 2019 at 11:38 PM Segher Boessenkool
> > <segher@kernel.crashing.org> wrote:
> > >
> > > On Tue, Jul 16, 2019 at 10:15:47PM +1000, Michael Ellerman wrote:
> > > > Segher Boessenkool <segher@kernel.crashing.org> writes:
> > > > And it's definitely calling ar with no flags, eg:
> > > >
> > > >   rm -f init/built-in.a; powerpc-linux-ar rcSTPD init/built-in.a init/main.o init/version.o init/do_mounts.o init/do_mounts_rd.o init/do_mounts_initrd.o init/do_mounts_md.o init/initramfs.o init/init_task.o
> > >
> > > This uses thin archives.  Those will work fine.
> > >
> > > The failing case was empty files IIRC, stuff created from no inputs.
> >
> > Actually, empty files are created everywhere.
>
> >        cmd_ar_builtin = rm -f $@; $(AR) rcSTP$(KBUILD_ARFLAGS) $@
> > $(real-prereqs)
>
> You use thin archives.
>
> Does every config use thin archives always nowadays?

Kbuild always uses thin archives as far as vmlinux is concerned.

But, there are some other call-sites.

masahiro@pug:~/ref/linux$ git grep  '$(AR)' -- :^Documentation :^tools
arch/powerpc/boot/Makefile:    BOOTAR := $(AR)
arch/unicore32/lib/Makefile:    $(Q)$(AR) p $(GNU_LIBC_A) $(notdir $@) > $@
arch/unicore32/lib/Makefile:    $(Q)$(AR) p $(GNU_LIBGCC_A) $(notdir $@) > $@
lib/raid6/test/Makefile:         $(AR) cq $@ $^
scripts/Kbuild.include:ar-option = $(call try-run, $(AR) rc$(1)
"$$TMP",$(1),$(2))
scripts/Makefile.build:      cmd_ar_builtin = rm -f $@; $(AR)
rcSTP$(KBUILD_ARFLAGS) $@ $(real-prereqs)
scripts/Makefile.lib:      cmd_ar = rm -f $@; $(AR)
rcsTP$(KBUILD_ARFLAGS) $@ $(real-prereqs)


Probably, you are interested in arch/powerpc/boot/Makefile.
This does not seem a thin archive.


> > BTW, your commit 8995ac8702737147115e1c75879a1a2d75627b9e
> > dates back to 2008.
> >
> > At that time, thin archive was not used.
>
> Yes, I know.  This isn't about built-in.[oa], it is about *other*
> archives we at least *used to* create.  If we *know* we do not anymore,
> then this workaround can of course be removed (and good riddance).

If it is not about built-in.[oa],
which archive are you talking about?

Can you pin-point the one?

masahiro@pug:~/ref/linux$ git log --oneline  -1
8995ac870273 (HEAD) [POWERPC] Specify GNUTARGET on $(AR) invocations
masahiro@pug:~/ref/linux$ git grep '(AR)'
Documentation/kbuild/makefiles.txt:     per-directory options to $(LD)
and $(AR).
arch/powerpc/Makefile:CROSS32AR := GNUTARGET=elf32-powerpc $(AR)
arch/powerpc/Makefile:override AR       := GNUTARGET=elf$(SZ)-powerpc $(AR)
drivers/md/raid6test/Makefile:   $(AR) cq $@ $^
scripts/Makefile.build:               rm -f $@; $(AR) rcs $@)
scripts/Makefile.build:cmd_link_l_target = rm -f $@; $(AR)
$(EXTRA_ARFLAGS) rcs $@ $(lib-y)
masahiro@pug:~/ref/linux$ git grep '(CROSS32AR)'
arch/powerpc/boot/Makefile:      cmd_bootar = $(CROSS32AR) -cr $@.$$$$
$(filter-out FORCE,$^); mv $@.$$$$ $@


> If ar creates an archive file (a real one, not a thin archive), and it
> has no input files, it uses its default object format as destination
> format, if it isn't told to use something else.  And that doesn't work,
> it needs to use some format compatible with what that archive later is
> linked with.

I compile-tested v4.10, which was before the thin-archive migration,
but I did not see any problem for building ppc32/64.

Whether or not it is a thin archive,
an empty archive is always 8-byte file.

masahiro@pug:~/ref/linux$ cat kernel/livepatch/built-in.o
!<arch>

Is there a room for caring about the under-lying architecture?


-- 
Best Regards
Masahiro Yamada