Re: [PATCH v2] kconfig: add optional warnings for changed input values

[Nicolas Schier <nsc@kernel.org>]

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On Thu, May 21, 2026 at 10:28:24AM +0800, Pengpeng Hou wrote:
> When reading .config input, Kconfig stores user-provided values first and
> then resolves the final value after applying dependencies, ranges, and
> other constraints.
> 
> If the final value differs from the user's input, Kconfig already tracks
> that state internally, but it does not provide any focused diagnostic to
> show which explicit inputs were adjusted. This is particularly confusing
> for requested values that get forced down by unmet dependencies or
> clamped by ranges.
> 
> Add an opt-in diagnostic controlled by KCONFIG_WARN_CHANGED_INPUT. Emit
> the warnings from conf_write() and conf_write_defconfig() after value
> resolution and through the existing message callback path so the default
> behavior stays unchanged and interactive frontends remain usable.
> 
> Avoid the conf_message() formatting buffer for this diagnostic so long
> warning lists are not truncated before reaching the callback, and mark
> processed symbols as written before the SYMBOL_WRITE check so duplicate
> menu nodes cannot emit duplicate warnings.
> 
> Document the new environment variable and add tests for both olddefconfig
> and savedefconfig.
> 
> Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
> ---
> Changes since v1: https://lore.kernel.org/all/20260406233001.1-kconfig-warn-changed-input-pengpeng@iscas.ac.cn/
> - rename "found" to "changed_input_found" as suggested by Nicolas
> - avoid the conf_message() 4096-byte formatting buffer so long warning
>   lists are not truncated before the callback sees them
> - mark each processed symbol as SYMBOL_WRITTEN before checking
>   SYMBOL_WRITE to avoid duplicate warnings for duplicate menu nodes
> - add duplicate-definition selftest coverage
> - do not carry the Reviewed-by/Tested-by tags because v2 changes warning
>   emission and duplicate suppression
> 
>  Documentation/kbuild/kconfig.rst              |   5 +
>  scripts/kconfig/confdata.c                    | 107 +++++++++++++++++-
>  .../kconfig/tests/warn_changed_input/Kconfig  |  40 +++++++
>  .../tests/warn_changed_input/__init__.py      |  27 +++++
>  .../kconfig/tests/warn_changed_input/config   |   3 +
>  .../tests/warn_changed_input/expected_config  |   6 +
>  .../warn_changed_input/expected_defconfig     |   1 +
>  .../tests/warn_changed_input/expected_stdout  |   4 +
>  8 files changed, 189 insertions(+), 4 deletions(-)
>  create mode 100644 scripts/kconfig/tests/warn_changed_input/Kconfig
>  create mode 100644 scripts/kconfig/tests/warn_changed_input/__init__.py
>  create mode 100644 scripts/kconfig/tests/warn_changed_input/config
>  create mode 100644 scripts/kconfig/tests/warn_changed_input/expected_config
>  create mode 100644 scripts/kconfig/tests/warn_changed_input/expected_defconfig
>  create mode 100644 scripts/kconfig/tests/warn_changed_input/expected_stdout
> 

Thanks a lot!

Tested-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>

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