Re: make olddefconfig surprises

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Jan 23, 2026 at 02:54:27PM -0800, Song Liu wrote:
> I think in many cases, we use selftests/xx/config as the base .config
> and run olddefconfig on it. This is how I got confused in the first
> place.

Okay, that makes much more sense in light of everything! I would expect
that to work but only if the dependencies of all the required
configurations are also included in the files.

> I guess the right way to achieve the this goal is:
>     make defconfig
>     ./scripts/kconfig/merge_config.sh .config selftests/xxx/config
> 
> Actually, from my simple tests, this is the same as
>     make defconfig
>     cat  selftests/xxx/config >> .config
>     make olddefconfig

Right, I believe those two are generally equivalent. merge_config.sh
becomes more useful as you have more fragments to merge (one recent use
case for performance improvements to merge_config.sh has over 100
fragments to merge) and helps avoid some "redefined" warnings, IIRC.

> It appears merge_config.sh and olddefconfig cannot figure out
> dependencies, so we need to do it manually. For example, if I
> want CONFIG_LIVEPATCH=y, I also need to include all the
> dependencies in selftests/xxx/config:
> 
>     CONFIG_FUNCTION_TRACER=y
>     CONFIG_DYNAMIC_FTRACE=y
>     CONFIG_DYNAMIC_FTRACE_WITH_REGS=y
>     CONFIG_LIVEPATCH=y

Right, that is exactly the issue here: if certain configurations are
expected to be enabled after these files are used as standalone bases or
merged into other files, these files need to have all of their
dependencies included as well.

> I haven't figured out a way to include all the dependencies
> automatically. Did I miss something?

I do not think something like this exists (maybe a SAT solver? :P), at
least not as part of the kernel tree. This is basically the same thing
as not being able to turn on a configuration in menuconfig until you
have gone through and enabled all of its dependencies. I personally use
menuconfig when trying to create a configuration fragment for minimized
reproducers on top of defconfigs because it is easy to see the final
diff when everything is switched:

  $ make defconfig
  $ make menuconfig
  $ git diff --no-index .config.old .config

You could start with defconfig or a more minimal configuration like
allnoconfig depending on how the selftests config is expected to be
used (as a base or merged into other configs).

Cheers,
Nathan