Re: [PATCH v4] ARM: decompressor: simplify libfdt builds

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Yamada-san,

On Wed, Apr 22, 2020 at 2:32 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
> On Wed, Apr 22, 2020 at 4:44 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > On Sun, Apr 19, 2020 at 9:26 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
> > > Copying source files during the build time may not end up with
> > > as clean code as expected.
> > >
> > > lib/fdt*.c simply wrap scripts/dtc/libfdt/fdt*.c, and it works
> > > nicely. Let's follow this approach for the arm decompressor, too.
> > >
> > > Add four wrappers, arch/arm/boot/compressed/fdt*.c and remove
> > > the Makefile messes. Another nice thing is we no longer need to
> > > maintain the own libfdt_env.h because the decompressor can include
> > > <linux/libfdt_env.h>.
> > >
> > > There is a subtle problem when generated files are turned into
> > > check-in files.
> > >
> > > When you are doing a rebuild of an existing object tree with O=
> > > option, there exists stale "shipped" copies that the old Makefile
> > > implementation created. The build system ends up with compiling the
> > > stale generated files because Make searches for prerequisites in the
> > > current directory, i.e. $(objtree) first, and then the directory
> > > listed in VPATH, i.e. $(srctree).
> > >
> > > To mend this issue, I added the following code:
> > >
> > >   ifdef building_out_of_srctree
> > >   $(shell rm -f $(addprefix $(obj)/, fdt_rw.c fdt_ro.c fdt_wip.c fdt.c))
> > >   endif
> > >
> > > This will need to stay for a while because "git bisect" crossing this
> > > commit, otherwise, would result in a build error.
> > >
> > > Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> >
> > This is now commit 365a6327cd643eed ("ARM: 8968/1: decompressor:
> > simplify libfdt builds") in arm/for-next.
> >
> > In light of reworking "[PATCH v5] ARM: boot: Obtain start of physical
> > memory from DTB"[1] on top of this, which would conditionally add
> > another source file to libfdt_objs, I have a few questions.
> >
> > > --- a/arch/arm/boot/compressed/Makefile
> > > +++ b/arch/arm/boot/compressed/Makefile
> > > @@ -76,29 +76,31 @@ compress-$(CONFIG_KERNEL_LZMA) = lzma
> > >  compress-$(CONFIG_KERNEL_XZ)   = xzkern
> > >  compress-$(CONFIG_KERNEL_LZ4)  = lz4
> > >
> > > -# Borrowed libfdt files for the ATAG compatibility mode
> > > -
> > > -libfdt         := fdt_rw.c fdt_ro.c fdt_wip.c fdt.c
> > > -libfdt_hdrs    := fdt.h libfdt.h libfdt_internal.h
> > > -
> > > -libfdt_objs    := $(addsuffix .o, $(basename $(libfdt)))
> > > +ifeq ($(CONFIG_ARM_ATAG_DTB_COMPAT),y)
> > > +libfdt_objs = fdt_rw.o fdt_ro.o fdt_wip.o fdt.o atags_to_fdt.o
> > >
> >
> > I guess the code below can be moved out of the ifeq block, as it doesn't
> > really do anything if CONFIG_ARM_ATAG_DTB_COMPAT=n, and $(libfdt_objs)
> > becomes empty?
> > If not, I think I'll have to add a new Kconfig symbol ARM_BOOT_LIBFDT,
> > to be selected by ARM_ATAG_DTB_COMPAT and USE_OF.
>
>
>
> Right. We can narrow the ifeq block.
> I did not know your on-going work.
>
>
> If I had known your work adding one more file here,
> I would have written this part as follows:
>
>
> ------------------------------>8----------------------------------
> libfdt_objs := fdt_rw.o fdt_ro.o fdt_wip.o fdt.o
>
> ifeq ($(CONFIG_ARM_ATAG_DTB_COMPAT),y)
> OBJS += $(libfdt_objs) atags_to_fdt.o
> endif
>
> # -fstack-protector-strong triggers protection checks in this code,
> # but it is being used too early to link to meaningful stack_chk logic.
> nossp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
> $(foreach o, $(libfdt_objs) atags_to_fdt.o, \
> $(eval CFLAGS_$(o) := -I $(srctree)/scripts/dtc/libfdt $(nossp-flags-y)))
>
> # These were previously generated C files. When you are building the kernel
> # with O=, make sure to remove the stale files in the output tree. Otherwise,
> # the build system wrongly compiles the stale ones.
> ifdef building_out_of_srctree
> $(shell rm -f $(addprefix $(obj)/, fdt_rw.c fdt_ro.c fdt_wip.c fdt.c))
> endif
> -------------------------------------->8-----------------------------

Thanks, looks good to me!

> So, how shall we move forward?
>
> Leave the necessary Makefile change to Geert?
>
> If Geert and Russell want to replace my patch,
> I can send v5 with the code above.

I can fix it myself when rebasing my patch, unless you get to a v5 before me.
I just wanted to find a good approach, to avoid delaying my patch.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds