U-Boot is often used to boot the kernel on ARM boards, but uImage
is not built by "make all", so we are often inclined to do
"make all uImage" to generate DTBs, modules and uImage in a single
command, but we should notice a pitfall behind it.  In fact,
"make all uImage" could generate an invalid uImage if it is run with
the parallel option (-j).

You can reproduce this problem with the following procedure:

[1] First, build "all" and "uImage" separately.
    You will get a valid uImage

  $ git clean -f -x -d
  $ export CROSS_COMPILE=<your-tools-prefix>
  $ make -s -j8 ARCH=arm multi_v7_defconfig
  $ make -s -j8 ARCH=arm all
  $ make -j8 ARCH=arm UIMAGE_LOADADDR=0x80208000 uImage
    CHK     include/config/kernel.release
    CHK     include/generated/uapi/linux/version.h
    CHK     include/generated/utsrelease.h
  make[1]: `include/generated/mach-types.h' is up to date.
    CHK     include/generated/timeconst.h
    CHK     include/generated/bounds.h
    CHK     include/generated/asm-offsets.h
    CALL    scripts/checksyscalls.sh
    CHK     include/generated/compile.h
    Kernel: arch/arm/boot/Image is ready
    Kernel: arch/arm/boot/zImage is ready
    UIMAGE  arch/arm/boot/uImage
  Image Name:   Linux-4.2.0-rc5-00156-gdd2384a7-d
  Created:      Sat Aug  8 23:21:35 2015
  Image Type:   ARM Linux Kernel Image (uncompressed)
  Data Size:    6138648 Bytes = 5994.77 kB = 5.85 MB
  Load Address: 80208000
  Entry Point:  80208000
    Image arch/arm/boot/uImage is ready
  $ ls -l arch/arm/boot/*Image
  -rwxrwxr-x 1 masahiro masahiro 13766656 Aug  8 23:20 arch/arm/boot/Image
  -rw-rw-r-- 1 masahiro masahiro  6138712 Aug  8 23:21 arch/arm/boot/uImage
  -rwxrwxr-x 1 masahiro masahiro  6138648 Aug  8 23:20 arch/arm/boot/zImage

[2] Update some source file(s)

  $ touch init/main.c

[3] Then, re-build "all" and "uImage" simultaneously.
    You will get an invalid uImage at random.

  $ make -j8 ARCH=arm UIMAGE_LOADADDR=0x80208000 all uImage
    CHK     include/config/kernel.release
    CHK     include/generated/uapi/linux/version.h
    CHK     include/generated/utsrelease.h
  make[1]: `include/generated/mach-types.h' is up to date.
    CHK     include/generated/timeconst.h
    CHK     include/generated/bounds.h
    CHK     include/generated/asm-offsets.h
    CALL    scripts/checksyscalls.sh
    CC      init/main.o
    CHK     include/generated/compile.h
    LD      init/built-in.o
    LINK    vmlinux
    LD      vmlinux.o
    MODPOST vmlinux.o
    GEN     .version
    CHK     include/generated/compile.h
    UPD     include/generated/compile.h
    CC      init/version.o
    LD      init/built-in.o
    KSYM    .tmp_kallsyms1.o
    KSYM    .tmp_kallsyms2.o
    LD      vmlinux
    SORTEX  vmlinux
    SYSMAP  System.map
    OBJCOPY arch/arm/boot/Image
    Building modules, stage 2.
    Kernel: arch/arm/boot/Image is ready
    GZIP    arch/arm/boot/compressed/piggy.gzip
    AS      arch/arm/boot/compressed/piggy.gzip.o
    Kernel: arch/arm/boot/Image is ready
    LD      arch/arm/boot/compressed/vmlinux
    GZIP    arch/arm/boot/compressed/piggy.gzip
    OBJCOPY arch/arm/boot/zImage
    Kernel: arch/arm/boot/zImage is ready
    UIMAGE  arch/arm/boot/uImage
  Image Name:   Linux-4.2.0-rc5-00156-gdd2384a7-d
  Created:      Sat Aug  8 23:23:14 2015
  Image Type:   ARM Linux Kernel Image (uncompressed)
  Data Size:    26472 Bytes = 25.85 kB = 0.03 MB
  Load Address: 80208000
  Entry Point:  80208000
    Image arch/arm/boot/uImage is ready
    MODPOST 192 modules
    AS      arch/arm/boot/compressed/piggy.gzip.o
    LD      arch/arm/boot/compressed/vmlinux
    OBJCOPY arch/arm/boot/zImage
    Kernel: arch/arm/boot/zImage is ready
  $ ls -l arch/arm/boot/*Image
  -rwxrwxr-x 1 masahiro masahiro 13766656 Aug  8 23:23 arch/arm/boot/Image
  -rw-rw-r-- 1 masahiro masahiro    26536 Aug  8 23:23 arch/arm/boot/uImage
  -rwxrwxr-x 1 masahiro masahiro  6138648 Aug  8 23:23 arch/arm/boot/zImage

Please notice the uImage is extremely small when this issue is
encountered.  Besides, "Kernel: arch/arm/boot/zImage is ready" is
displayed twice, before and after the uImage log.

The root cause of this is the race condition between zImage and
uImage.  Actually, uImage depends on zImage, but the dependency
between the two is only described in arch/arm/boot/Makefile.
Because arch/arm/boot/Makefile is not included from the top-level
Makefile, it cannot know the dependency between zImage and uImage.

Consequently, when we run make with the parallel option, Kbuild
updates vmlinux first, and then two different threads descends into
the arch/arm/boot/Makefile almost at the same time, one for updating
zImage and the other for uImage.  While one thread is re-generating
zImage, the other also tries to update zImage before creating uImage
on top of that.  zImage is overwritten by the slower thread and then
uImage is created based on the half-written zImage.

This is the reason why "Kernel: arch/arm/boot/zImage is ready" is
displayed twice, and a broken uImage is created.

The same problem could happen on bootpImage.

This commit adds dependencies among Image, zImage, uImage, and
bootpImage to arch/arm/Makefile, which is included from the
top-level Makefile.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>