Software development notes
by Andrew
Hostboot is one part of the firmware stack that initialises an OpenPOWER system - it is the first piece of software to execute on the “main” cores of the CPU. Its job is to initialise the CPU’s configuration and internal buses, stitch together the sockets into a coherent SMP system, train and initialise main memory, and finally hand off control of the CPU to the “PAYLOAD” firmware.
Hacking on hostboot is similarly non-trivial, so this post exists to document what I’ve come to as a workflow.
Hostboot is most easily built by using the buildroot-based op-build system. However, if you are hacking hostboot, generally you will want to build your own hostboot tree, not the current stable release or master as op-build’s buildroot configuration allows. op-build also offers a “Custom” option, though you have to supply a commit ID, and that is irritating as it’s forever changing.
Also irritating is that building hostboot from scratch takes enough time to go for a reasonable walk, even on a beefy machine. This wouldn’t be so bad if the build system didn’t also mostly defeat ccache, which means we’re left with trying to build changes from the same working tree to avoid giving up in despair.
This last point eliminates taking advantage of overriding buildroot’s
<pkg>_VERSION,
<pkg>_SITE
and
<pkg>_SITE_METHOD
variables on the commandline, as similar to the Custom configuration option
they create ID-tagged directories in output/build (e.g.
output/build/hostboot-8a8b2e4719156ae124e7bd198b089529b4040081). Further, even
if we were to disregard the lengthy build times, using
<pkg>_SITE_METHOD=git with a local path assigned to <pkg>_SITE
breaks the build as hostboot makes use of symlinks with relative paths and
these get broken by buildroot’s archiving of git
repositories
The correct way to go about this is to use buildroot’s
<pkg>_OVERRIDE_SRCDIR
variable, which is demonstrated below.
As a final tangent, I like to keep my laptop as the canonical source of the
work that I’m doing, but it’s also not beefy enough to build hostboot in a
reasonable amount of time. To this end I’ve set up a git remote configured with
git config receive.denyCurrentBranch warn on a bigger machine, and then I
force push to that from my laptop. On the remote machine I do a git reset
--hard to pick up the new changes, and then issue the following to build them:
$ op-build HOSTBOOT_OVERRIDE_SRCDIR=/path/to/hostboot/ hostboot-rebuild machine-xml-rebuild openpower-pnor-rebuild
The resulting PNOR image can then be flashed to the target machine for testing.
Hostboot series:
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