As of commit 2aff5ba6 in the op-build tree, we're able to easily replace the kernel in an OpenPower firmware image.

This commit adds a new partition (called BOOTKERNEL) to the PNOR image, which provides the petitboot bootloader environment. Since it's now in its own partition, we can replace the image with a custom build. Here's a little guide to doing that, using an example of using a separate branch of op-build that provides a little-endian kernel.

You can check if your currently-running firmware has this BOOTKERNEL partition by running pflash -i on the BMC. It should list BOOTKERNEL in the partition table listing:

# pflash -i
Flash info:
-----------
Name          = Micron N25Qx512Ax
Total size    = 64MB 
Erase granule = 4KB 

Partitions:
-----------
ID=00            part 00000000..00001000 (actual=00001000)
ID=01            HBEL 00008000..0002c000 (actual=00024000)
[...]
ID=11            HBRT 00949000..00ca9000 (actual=00360000)
ID=12         PAYLOAD 00ca9000..00da9000 (actual=00100000)
ID=13      BOOTKERNEL 00da9000..01ca9000 (actual=00f00000)
ID=14        ATTR_TMP 01ca9000..01cb1000 (actual=00008000)
ID=15       ATTR_PERM 01cb1000..01cb9000 (actual=00008000)
[...]
#

If your partition table does not contain a BOOTKERNEL partition, you'll need to upgrade to a more recent PNOR image to proceed.

First (if you don't have one already), grab a suitable version of op-build. In this example, we'll use my le branch, which has little-endian support:

git clone --recursive git://github.com/jk-ozlabs/op-build.git
cd op-build
git checkout -b le origin/le
git submodule update

Then, prepare our environment and configure for the relevant platform - in this case, habanero:

. op-build-env
op-build habanero_defconfig

If you'd like to change any of the kernel config (for example, to add or remove drivers), you can do that now, using the 'linux-menuconfig' target. This is only necessary if you wish to make changes. Otherwise, the default kernel config will work.

op-build linux-menuconfig

Next, we build just the userspace and kernel parts of the firmware image, by specifying the linux26-rebuild-with-initramfs build target:

op-build linux26-rebuild-with-initramfs

If you're using a fresh op-build tree, this will take a little while, as it downloads and builds a toolchain, userspace and kernel. Once that's complete, you'll have a built kernel image in the output tree:

 output/build/images/zImage.epapr

Transfer this file to the BMC, and flash using pflash. We specify the -P <PARTITION> argument to write to a single PNOR partition:

pflash -P BOOTKERNEL -e -p /tmp/zImage.epapr

And that's it! The next boot will use your newly-build kernel in the petitboot bootloader environment.

Out-of-tree kernel builds

If you'd like to replace the kernel from op-build with one from your own external source tree, you have two options. Either point op-build at your own tree, or build you own kernel using the initramfs that op-build has produced.

For the former, you can override certain op-build variables to reference a separate source. For example, to use an external git tree:

op-build LINUX_SITE=git://github.com/jk-ozlabs/linux LINUX_VERSION=v3.19

See Customising OpenPower firmware for other examples of using external sources in op-build.

The latter option involves doing a completely out-of-op-build build of a kernel, but referencing the initramfs created by op-build (which is in output/images/rootfs.cpio.xz). From your kernel source directory, add CONFIG_INITRAMFS_SOURCE argument, specifying the relevant initramfs. For example:

make O=obj ARCH=powerpc \
    CONFIG_INITRAMFS_SOURCE=../op-build/output/images/rootfs.cpio.xz