Working with the Ubuntu kernel config

The Ubuntu kernel source tree produces several kernel binaries, one for each valid combination of architecture (amd64, arm64, ppc64el, etc) and flavour (generic, lowlatency, 64k-pages, etc), and each of this combination requires a corresponding .config file.

Once a new patch is applied, or after a rebase on a new upstream release, all .config files need to be refreshed (config options could have been added/removed or renamed, their dependencies could have changed, etcetc) and we must assure that some critical config options are always match a specific value in order to have a sound kernel.

The Ubuntu approach is to store all config information (config name, per-arch/per-flavour value and other useful metadata) into a single text file (debian.master/config/annotations) and generate the required .config file at build time.

To know more about annotations inner working, refer to this document.

Annotations format is self-explanatory:

# ARCH: amd64 arm64 armhf ppc64el riscv64 s390x
# FLAVOUR: amd64-generic arm64-generic arm64-generic-64k armhf-generic armhf-generic-lpae ppc64el-generic riscv64-generic s390x-generic

CONFIG_104_QUAD_8         policy<{'amd64': 'm'}>
CONFIG_60XX_WDT           policy<{'amd64': 'm'}>
CONFIG_64BIT              policy<{'amd64': 'y', 'arm64': 'y', 'ppc64el': 'y', 'riscv64': 'y', 's390x': 'y'}>
CONFIG_6LOWPAN_DEBUGFS    policy<{'amd64': 'n', 'arm64': 'n', 'armhf': 'n', 'ppc64el': 'n', 'riscv64': 'n', 's390x': '-'}>
...

nonetheless, a helper script (debian/scripts/misc/annotations) is provided to ease querying and modifying the configutation.

For example, to find the value of a specific option:

$ ./debian/scripts/misc/annotations --query --config CONFIG_DEBUG_FS
{
        "CONFIG_DEBUG_FS": {
        "policy": {
              "amd64": "y",
              "arm64": "y",
              "armhf": "y",
              "ppc64el": "y",
              "riscv64": "y",
              "s390x": "y"
        },
        "note": "'required debug option'"
        }
}

Or to change the its value:

$ ./debian/scripts/misc/annotations --config CONFIG_DEBUG_FS --arch arm64 --flavour generic --write n
{
        "CONFIG_DEBUG_FS": {
        "policy": {
              "amd64": "y",
              "arm64": "y",
              "arm64-generic": "n",
              "armhf": "y",
              "ppc64el": "y",
              "riscv64": "y",
              "s390x": "y"
        },
        "note": "'required debug option'"
        }
}

And once you are done with your changes, commit them:

$ git add debian.master/config/annotations
$ git commit -s -m "UBUNTU: [Config] arm64/generic: disable DEBUG_FS"

pay particular attention to:

• always add a Signed-off-by line (git commit -s switch)

• config changes should always use a UBUNTU: [Config] $arch/$flavour: … subject format (if arch or flavour is omitted, it means all architectures/flavours are affected).