[PATCH] powerpc/bootwrapper: Add documentation of boot wrapper targets

[John Linn]

Thanks Grant, that's a great start for helping us to better understand
the bootwrapper.

The wrapper is somewhat complex and hard to get your head around, or at
least my head ;).

-- John

-----Original Message-----
From: Grant Likely [mailto:grant.likely at secretlab.ca] 
Sent: Wednesday, June 25, 2008 2:21 PM
To: John Linn; Stephen Neuendorffer; linuxppc-dev at ozlabs.com;
paulus at samba.org
Cc: petermendham at computing.dundee.ac.uk
Subject: [PATCH] powerpc/bootwrapper: Add documentation of boot wrapper
targets

From: Grant Likely <grant.likely at secretlab.ca>

There have been many questions on and off the mailing list about how
exactly the bootwrapper is used for embedded targets.  Add some
documentation and help text to try and clarify the system.

Signed-off-by: Grant Likely <grant.likely at secretlab.ca>
---

 Documentation/powerpc/bootwrapper.txt |   78
+++++++++++++++++++++++++++++++++
 arch/powerpc/Makefile                 |   15 ++++++
 arch/powerpc/boot/wrapper             |    4 ++
 3 files changed, 96 insertions(+), 1 deletions(-)

diff --git a/Documentation/powerpc/bootwrapper.txt
b/Documentation/powerpc/bootwrapper.txt
new file mode 100644
index 0000000..8dcb560
--- /dev/null
+++ b/Documentation/powerpc/bootwrapper.txt
@@ -0,0 +1,78 @@
+The PowerPC boot wrapper
+------------------------
+Copyright (C) Secret Lab Technologies Ltd.
+
+PowerPC image targets compresses and wraps the kernel image (vmlinux)
with
+a boot wrapper to make it usable by the system firmware.  There is no
+standard PowerPC firmware interface, so the boot wrapper is designed to
+be adaptable for each kind of image that needs to be built.
+
+The boot wrapper can be found in the arch/powerpc/boot/ directory.  The
+Makefile in that directory has targets for all the available image
types.
+Currently, the following image targets exist:
+
+   cuImage.%:		Backwards compatible uImage for older version of
+			U-Boot (for versions that don't understand the
device
+			tree).  This image embeds a device tree blob
inside
+			the image.
+   dtbImage.%:		Similar to zImage, except device tree blob is
embedded
+			inside the image.
+   simpleImage.%:	Firmware independent compressed image that does
not
+			depend on any particular firmware interface and
embeds
+			a device tree blob.  This image can be loaded to
any
+			location in RAM and jumped to.  Firmware cannot
pass
+			any configuration data to the kernel with this
image
+			type and the kernel depends on the device tree
to
+			determine its console device.
+   treeImage.%;		Image format for some versions of ppc4xx
firmware (non
+			U-Boot firmware).  This image embeds a device
tree
+			blob inside the image.
+   uImage:		Native image format used by U-Boot.  The uImage
target
+			does not add any boot code.  It just wraps a
compressed
+			vmlinux in the uImage data structure.
+   zImage.%:		Image usable by OpenFirmware.  Image expects
firmware
+			to provide the device tree using OpenFirmware
+			interfaces.  Typically general purpose hardware
uses
+			this image format.
+
+Image types which embed a device tree blob (simpleImage, dtbImage,
treeImage,
+and cuImage) all generate the device tree blob from a file in the
+arch/powerpc/boot/dts/ directory.  The Makefile selects the correct
device
+tree source based on the name of the target.  Therefore, if the kernel
is
+built with 'make treeImage.walnut simpleImage.virtex405-ml403', then
the
+build system will use arch/powerpc/boot/dts/walnut.dts to build
+treeImage.walnut and arch/powerpc/boot/dts/virtex405-ml403.dts to build
+the simpleImage.virtex405-ml403.
+
+Two special targets called 'zImage' and 'zImage.initrd' also exist.
These
+targets build all the default images as selected by the kernel
configuration.
+Default images are selected by the boot wrapper Makefile
+(arch/powerpc/boot/Makefile) by adding targets to the $image-y
variable.  Look
+at the Makefile to see which default image targets are available.
+
+How it is built
+---------------
+arch/powerpc is designed to support multiplatform kernels, which means
+that a single vmlinux image can be booted on many different target
boards.
+It also means that the boot wrapper must be able to wrap for many kinds
of
+images on a single build.  The design decision was made to not use any
+conditional compilation code (#ifdef, etc) in the boot wrapper source
code.
+All of the boot wrapper pieces are buildable at any time regardless of
the
+kernel configuration.  Building all the wrapper bits on every kernel
build
+also ensures that obscure parts of the wrapper are at the very least
compile
+tested in a large variety of environments.
+
+The wrapper is adapted for different image types at link time by
linking in
+just the wrapper bits that are appropriate for the image type.  The
'wrapper'
+script (found in arch/powerpc/boot/wrapper) is called by the Makefile
and
+is responsible for selecting the correct wrapper bits for the image
type.
+The arguments are well documented in the script's comment block, so
they
+are not repeated here.  However, it is worth mentioning that the script
+uses the -p (platform) argument as the main method of deciding which
wrapper
+bits to compile in.  Look for the large 'case "$platform" in' block in
the
+middle of the script.  This is also the place where platform specific
fixups
+can be selected by changing the link order.
+
+In particular, care should be taken when working with cuImages.
cuImage
+wrapper bits are very board specific and care should be taken to make
sure
+the target you are trying to build is supported by the wrapper bits.
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index b7d4c4c..754c7eb 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -169,12 +169,25 @@ bootwrapper_install %.dtb:
 	$(Q)$(MAKE) ARCH=ppc64 $(build)=$(boot) $(patsubst
%,$(boot)/%,$@)
 
 define archhelp
-  @echo '* zImage          - Compressed kernel image
(arch/$(ARCH)/boot/zImage.*)'
+  @echo '* zImage          - Build default images selected by kernel
config'
+  @echo '  zImage.*        - Compressed kernel image
(arch/$(ARCH)/boot/zImage.*)'
+  @echo '  uImage          - U-Book native image format'
+  @echo '  cuImage.<dt>    - Backwards compatible U-Boot image for
older'
+  @echo '                    versions which do not support device
trees'
+  @echo '  dtbImage.<dt>   - zImage with an embedded device tree blob'
+  @echo '  simpleImage.<dt> - Firmware independant image.'
+  @echo '  treeImage.<dt>  - Support for older IBM 4xx firmware (not
U-Boot)'
   @echo '  install         - Install kernel using'
   @echo '                    (your) ~/bin/installkernel or'
   @echo '                    (distribution) /sbin/installkernel or'
   @echo '                    install to $$(INSTALL_PATH) and run lilo'
   @echo '  *_defconfig     - Select default config from
arch/$(ARCH)/configs'
+  @echo ''
+  @echo '  Targets with <dt> embed a device tree blob inside the image'
+  @echo '  These targets support board with firmware that does not'
+  @echo '  support passing a device tree directly.  Replace <dt> with
the'
+  @echo '  name of a dts file from the arch/$(ARCH)/boot/dts/
directory'
+  @echo '  (minus the .dts extension).'
 endef
 
 install:
diff --git a/arch/powerpc/boot/wrapper b/arch/powerpc/boot/wrapper
index d6c96d9..7cd4182 100755
--- a/arch/powerpc/boot/wrapper
+++ b/arch/powerpc/boot/wrapper
@@ -203,6 +203,10 @@ simpleboot-virtex405-*)
     platformo="$object/virtex405-head.o $object/simpleboot.o"
     binary=y
     ;;
+simpleboot-*)
+    platformo="$object/simpleboot.o"
+    binary=y
+    ;;
 esac
 
 vmz="$tmpdir/`basename \"$kernel\"`.$ext"



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