linux-kernel - Re: [PATCH v4] ARM: decompressor: simplify libfdt builds

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Message-ID: <CAK7LNAQ+3Jgf9sNUju3gTJvvRxUn6bM1r1A6j9HCTNmeOWXFzQ@mail.gmail.com>
Date:   Wed, 22 Apr 2020 21:30:58 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Russell King <linux@...linux.org.uk>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] ARM: decompressor: simplify libfdt builds

On Wed, Apr 22, 2020 at 4:44 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> Hi Yamada-san, Kees, Russell,
>
> -CC RMK's patch system
> +CC lakml
>
> On Sun, Apr 19, 2020 at 9:26 PM Masahiro Yamada <masahiroy@...nel.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 $(obj)/, fdt_rw.c fdt_ro.c fdt_wip.c fdt.c))
> >   endif
> >
> > This will need to stay for a while because "git bisect" crossing this
> > commit, otherwise, would result in a build error.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
>
> This is now commit 365a6327cd643eed ("ARM: 8968/1: decompressor:
> simplify libfdt builds") in arm/for-next.
>
> In light of reworking "[PATCH v5] ARM: boot: Obtain start of physical
> memory from DTB"[1] on top of this, which would conditionally add
> another source file to libfdt_objs, I have a few questions.
>
> > --- a/arch/arm/boot/compressed/Makefile
> > +++ b/arch/arm/boot/compressed/Makefile
> > @@ -76,29 +76,31 @@ compress-$(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-----------------------------




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.



-- 
Best Regards
Masahiro Yamada