Re: [PATCH] kconfig: warn on dead default

[Nathan Chancellor <nathan@kernel.org>]

Hi Julian,

On Sat, Jun 06, 2026 at 03:00:08PM +0100, Julian Braha wrote:
> The dead default check was originally introduced with kconfirm:
> https://lore.kernel.org/all/6ec4df6d-1445-48ca-8f54-1d1a83c4716d@gmail.com/
> 
> While I'm still working on that tool, it's not yet ready for inclusion
> into the tree. I am currently waiting for common distro packagers to
> package the parsing library before submitting the next RFC iteration.
> 
> However, the dead default check is more impactful than the other checks:
> all 4 dead defaults that were detected should not have been dead and could
> cause misconfiguration bugs. But fortunately, these were just for kunit
> tests. The 3 patches to fix them have all since been merged:
> commit aef656a0e6c0 ("powerpc: fix dead default for GUEST_STATE_BUFFER_TEST")
> commit 30cc5e2ad826 ("s390/Kconfig: Cleanup defaults for selftests")
> commit df75430515c3 ("drm: fix dead default for DRM_TTM_KUNIT_TEST")
> 
> We can actually check for dead defaults while evaluating Kconfig, which
> should be even more effective at preventing future instances than keeping
> it in a static checker.
> 
> Note that this patch will only trigger a warning when the default values
> are different, in other words, pure duplicate defaults won't cause a
> warning, as they are simply redundant.
> 
> Signed-off-by: Julian Braha <julianbraha@gmail.com>

Nicolas pushed this into -next, where it fires several times for me. I
have not looked at these closely yet, I just wanted to get the report
out so that more eyes could be on it.

ARCH=powerpc allmodconfig:

  arch/powerpc/platforms/Kconfig.cputype:269:warning: default for 'TUNE_CPU' has the same condition as the earlier default at arch/powerpc/platforms/Kconfig.cputype:268
  arch/powerpc/platforms/Kconfig.cputype:270:warning: default for 'TUNE_CPU' has the same condition as the earlier default at arch/powerpc/platforms/Kconfig.cputype:268
  lib/Kconfig.debug:1083:warning: default for 'PANIC_TIMEOUT' is unreachable: earlier default at arch/powerpc/Kconfig:350 is unconditional
  arch/Kconfig:1194:warning: default for 'PGTABLE_LEVELS' is unreachable: earlier default at arch/powerpc/Kconfig:482 is unconditional

ARCH=s390 allmodconfig:

  arch/Kconfig:1194:warning: default for 'PGTABLE_LEVELS' is unreachable: earlier default at arch/s390/Kconfig:295 is unconditional
  lib/Kconfig:372:warning: default for 'HAS_IOMEM' is unreachable: earlier default at arch/s390/Kconfig:760 is unconditional

When building with clang-23, which supports '-fms-anonymous-struct':

  init/Kconfig:978:warning: default for 'CC_MS_EXTENSIONS' is unreachable: earlier default at init/Kconfig:977 is unconditional

I think the init/Kconfig and arch/powerpc/platforms/Kconfig.cputype ones
are intentional due to how cc-option works but I know those might be
hard to silence because they are already evaluated by this stage if I
understand correctly? Not sure about the other ones (apologies if they
are already known or have fixes).

-- 
Cheers,
Nathan