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

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Message-ID: <20200422123809.GO25745@shell.armlinux.org.uk>
Date:   Wed, 22 Apr 2020 13:38:09 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4] ARM: decompressor: simplify libfdt builds

On Wed, Apr 22, 2020 at 09:30:58PM +0900, Masahiro Yamada wrote:
> 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.

I've no problem with replacing your patch...

-- 
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