Build a B2 Kernel on your crossdev PC

Build a kernel for your B2 in minutes, using your crossdev PC!

Prerequisites

This guide assumes that you have already set up crossdev and the gentoo-b2 overlay on your PC, per these instructions. So, if you haven't yet, please do this first, before proceeding.

Building a B2-Compatible Kernel

First, we need to download some kernel sources, that have been appropriately patched for the B2. You can of course set this up yourself (the necessary patches are here), but for simplicity, in this guide we'll use gentoo-b2-sources package, which will take care of everything for us.

So, let's say you wanted 4.1.3, the version used in the current live USB image. You should issue the following to pull down the sources (note we use --nodeps here; we don't actually want to cross-emerge the perl etc. tools that'll be used to patch the sources, as you already have them in your native architecture (amd64 or whatever) and they'll be called from there):

gentoo_pc ~ # PORTDIR_OVERLAY="/usr/local/portage-crossdev" ARCH="ppc" powerpc-e300c3-linux-gnu-emerge --ask --verbose --nodeps ~sys-kernel/gentoo-b2-sources-4.1.3

This may take some time to complete, depending on the speed of your network connection.

Note also that while this particular kernel version's ebuild (4.1.3) was in the gentoo-b2-sources repository at the time of writing, it may have been replaced by the time you read this. In that case, simply use a more modern version. You can use a tool like eix to show all the currently available ebuild versions for a given package (such as gentoo-b2-sources).

Incidentally, the prefix use of PORTDIR_OVERLAY here is to avoid polluting the /etc/portage/make.conf file (since setting this variable is now deprecated, but is currently required for crossdev).

Now, go into the source directory (crossdev automatically keeps things separate from your normal sysroot, to avoid pollution):

gentoo_pc ~ #  cd /usr/powerpc-e300c3-linux-gnu/usr/src/linux-4.1.3-gentoo

Grab a suitable starter config from somewhere, such as the one used in this project ^-^, save it to .config, and then run the buildkernel-b2 script to build the kernel (see its manpage for options; in particular, add --usb, if you want to build a kernel that is bootable from USB, rather than the B2's hard drive, and --menuconfig if you'd like to change the configuration using the editor, at the start of the build):

gentoo_pc linux-4.1.3-gentoo # cp <your config file> .config
gentoo_pc linux-4.1.3-gentoo # buildkernel-b2

This does a parallel make, so on a modern PC, it shoudn't take long (5-15 mins).

When done, you'll have a deploy_root directory, which will contain:

• a lib directory, with the modules and firmware for the kernel; and
• a boot directory, which will contain either:
  • the uImage, System.map, config, bubba.dtb and 8313E21.dtb files, if compiling for an HDD deployment (the default, where you did not specify the --usb flag to buildkernel-b2, above); or
  • an install subdirectory, containing the install.itb, 8313E21.itb, System.map and config files, if compiling for a USB deployment (where you did specify the --usb flag to buildkernel-b2).

You can now simply copy these files over to the real root (on the B2's hard drive, or the live-USB, as appropriate), and you can try booting with your new kernel!

The next time you invoke buildkernel-b2 in the same directory (perhaps with a slightly altered configuration) it should be significantly faster, since most of the required components will be already compiled.

Next Steps

Now that you have a functioning cross-compilation environment on your PC, why not hook it up to distcc? This will allow your B2 to leverage the power of your PC when performing local builds (for C and C++ compilation and header pre-processing), and makes it possible to use a source-based distribution like Gentoo without huge update times. See these instructions for details on how to proceed.