4.9. Kconfig Diffs

For packages using Kconfig as their configuration system, PTXdist can generate their config file from another reference config file and a Kconfig diff on the fly. This mechanism can be used to build configuration variants of a kernel or barebox package. For example, a “debug” kernel package can refer to the config file of the “production” kernel as its reference config file, and its Kconfig diff then only contains the Kconfig symbols that were changed in the “debug” variant:

release config symbol	debug kconfig diff	resulting debug config symbol
CONFIG_LOCALVERSION=”release”	CONFIG_LOCALVERSION=”debug”	CONFIG_LOCALVERSION=”debug”
# CONFIG_ARCH_MULTI_V6 is not set		# CONFIG_ARCH_MULTI_V6 is not set
CONFIG_ARCH_MULTI_V7=y		CONFIG_ARCH_MULTI_V7=y
# CONFIG_CGROUP_DEBUG is not set	CONFIG_CGROUP_DEBUG=y	CONFIG_CGROUP_DEBUG=y
# CONFIG_DEBUG_PREEMPT is not set	CONFIG_DEBUG_PREEMPT=y	CONFIG_DEBUG_PREEMPT=y
CONFIG_WATCHDOG=y	# CONFIG_WATCHDOG is not set	# CONFIG_WATCHDOG is not set
CONFIG_WATCHDOG_CORE=y	# CONFIG_WATCHDOG_CORE is undefined
CONFIG_BCM2835_WDT=y	# CONFIG_BCM2835_WDT is undefined

Kconfig does not write symbols to the config file that have unfulfilled dependencies (e.g., in the above example CONFIG_WATCHDOG_CORE and CONFIG_BCM2835_WDT depend on CONFIG_WATCHDOG). Those symbols that are not present in the resulting debug config file are represented as … is undefined in the diff.

The first line of the Kconfig diff file contains the MD5 sum of the reference config file. PTXdist uses this checksum to detect when the reference config has changed, in which case the diff needs to be regenerated from the package’s config file, and to make sure that the diff is applicable to the reference config.

Using Kconfig diffs

The main part is setting the <PKG>_REF_CONFIG variable of a package to the reference config file, for example:

# rules/kernel.make:
KERNEL_CONFIG                := $(call ptx/in-platformconfigdir, kernelconfig-release)

# rules/kernel-debug.make:
KERNEL_DEBUG_CONFIG          := $(call ptx/in-platformconfigdir, kernelconfig-debug)
KERNEL_DEBUG_REF_CONFIG      := $(call ptx/in-platformconfigdir, kernelconfig-release)

PTXdist will now automatically generate kernelconfig-debug from kernelconfig-release and kernelconfig-debug.diff whenever doing and oldconfig or menuconfig on the kernel-debug package. If any symbols were changed by that operation, both the kernelconfig-debug.diff and the kernelconfig-debug are updated afterwards.

Kconfig diffs and layers

PTXdist uses the same diff mechanism whenever the config file of a Kconfig-style package is changed in an inherited layer (see Layers in PTXdist). In that case, the diff is calculated between the package’s config file in the base layer and the package’s (adapted) config file in the current layer.

Note

When using both <PKG>_REF_CONFIG and inter-layer Kconfig diffs, the inter-layer diff takes precedence, and the reference config file in the inherited layer is ignored. In the following example, arrows represent the config diff relation: