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 input, Kconfig already tracks
that state internally, but it does not provide a 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. Print the diagnostic to stderr directly, not through the
normal message callback, so it remains visible when conf is run with -s,
such as from make -s.

Keep the diagnostic out of the conf_message() formatting buffer so long
warning lists are not truncated, 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 olddefconfig,
savedefconfig, and the silent-conf path.

Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
---
Changes since v2:
https://lore.kernel.org/all/20260521022824.38591-1-pengpeng@iscas.ac.cn/
- print the changed-input diagnostic to stderr directly so it remains
  visible when conf is run with -s, as reported by Nathan
- move the selftest expectation from stdout to stderr
- add explicit silent-conf coverage for the KCONFIG_WARN_CHANGED_INPUT
  warning path
- keep Nicolas's v2 Reviewed-by/Tested-by out of this revision because
  the warning output path changed

 Documentation/kbuild/kconfig.rst              |   5 +
 scripts/kconfig/confdata.c                    | 106 +++++++++++++++++-
 scripts/kconfig/tests/conftest.py             |   8 +-
 .../kconfig/tests/warn_changed_input/Kconfig  |  40 +++++++
 .../tests/warn_changed_input/__init__.py      |  33 ++++++
 .../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_stderr  |   4 +
 9 files changed, 200 insertions(+), 6 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_stderr

diff --git a/Documentation/kbuild/kconfig.rst b/Documentation/kbuild/kconfig.rst
index fc4e845bc249..9b2625c768f0 100644
--- a/Documentation/kbuild/kconfig.rst
+++ b/Documentation/kbuild/kconfig.rst
@@ -59,6 +59,11 @@ Environment variables for ``*config``:
     This environment variable makes Kconfig warn about all unrecognized
     symbols in the config input.
 
+``KCONFIG_WARN_CHANGED_INPUT``
+    If set to a non-blank value, Kconfig prints optional warnings for
+    user-provided values that change after Kconfig resolves dependencies
+    or applies other constraints such as ranges.
+
 ``KCONFIG_WERROR``
     If set, Kconfig treats warnings as errors.
 
diff --git a/scripts/kconfig/confdata.c b/scripts/kconfig/confdata.c
index ac95661a1c9d..34be06d4b563 100644
--- a/scripts/kconfig/confdata.c
+++ b/scripts/kconfig/confdata.c
@@ -206,6 +206,78 @@ static void conf_message(const char *fmt, ...)
 	va_end(ap);
 }
 
+static void conf_changed_input_warning(const char *s)
+{
+	fputs(s, stderr);
+}
+
+static bool conf_warn_changed_input_enabled(void)
+{
+	const char *env = getenv("KCONFIG_WARN_CHANGED_INPUT");
+
+	return env && *env;
+}
+
+static const char *sym_get_user_value_string(struct symbol *sym)
+{
+	switch (sym->type) {
+	case S_BOOLEAN:
+	case S_TRISTATE:
+		switch (sym->def[S_DEF_USER].tri) {
+		case yes:
+			return "y";
+		case mod:
+			return "m";
+		default:
+			return "n";
+		}
+	default:
+		return sym->def[S_DEF_USER].val ?: "";
+	}
+}
+
+static bool sym_user_value_changed(struct symbol *sym)
+{
+	if (!sym_has_value(sym) || sym->type == S_UNKNOWN)
+		return false;
+
+	switch (sym->type) {
+	case S_BOOLEAN:
+	case S_TRISTATE:
+		return sym->def[S_DEF_USER].tri != sym_get_tristate_value(sym);
+	default:
+		return strcmp(sym_get_user_value_string(sym),
+			      sym_get_string_value(sym));
+	}
+}
+
+static void conf_clear_written_flags(void)
+{
+	struct symbol *sym;
+
+	for_all_symbols(sym)
+		sym->flags &= ~SYMBOL_WRITTEN;
+}
+
+static void conf_append_changed_input_warning(struct gstr *gs,
+					      struct symbol *sym,
+					      bool *changed_input_found)
+{
+	if (!sym_user_value_changed(sym))
+		return;
+
+	if (!*changed_input_found) {
+		str_printf(gs,
+			   "warning: user-provided values changed by Kconfig:\n");
+		*changed_input_found = true;
+	}
+
+	str_printf(gs, "  %s%s: %s -> %s\n",
+		   CONFIG_, sym->name,
+		   sym_get_user_value_string(sym),
+		   sym_get_string_value(sym));
+}
+
 const char *conf_get_configname(void)
 {
 	char *name = getenv("KCONFIG_CONFIG");
@@ -759,11 +831,15 @@ int conf_write_defconfig(const char *filename)
 {
 	struct symbol *sym;
 	struct menu *menu;
+	struct gstr gs;
 	FILE *out;
+	bool warn_changed_input = conf_warn_changed_input_enabled();
+	bool changed_input_found = false;
 
 	out = fopen(filename, "w");
 	if (!out)
 		return 1;
+	gs = str_new();
 
 	sym_clear_all_valid();
 
@@ -772,10 +848,14 @@ int conf_write_defconfig(const char *filename)
 
 		sym = menu->sym;
 
-		if (!sym || sym_is_choice(sym))
+		if (!sym || sym_is_choice(sym) || sym->flags & SYMBOL_WRITTEN)
 			continue;
 
 		sym_calc_value(sym);
+		if (warn_changed_input)
+			conf_append_changed_input_warning(&gs, sym,
+							  &changed_input_found);
+		sym->flags |= SYMBOL_WRITTEN;
 		if (!(sym->flags & SYMBOL_WRITE))
 			continue;
 		sym->flags &= ~SYMBOL_WRITE;
@@ -798,6 +878,13 @@ int conf_write_defconfig(const char *filename)
 		print_symbol_for_dotconfig(out, sym);
 	}
 	fclose(out);
+
+	conf_clear_written_flags();
+
+	if (changed_input_found)
+		conf_changed_input_warning(str_get(&gs));
+
+	str_free(&gs);
 	return 0;
 }
 
@@ -809,7 +896,10 @@ int conf_write(const char *name)
 	const char *str;
 	char tmpname[PATH_MAX + 1], oldname[PATH_MAX + 1];
 	char *env;
+	struct gstr gs;
 	bool need_newline = false;
+	bool warn_changed_input = conf_warn_changed_input_enabled();
+	bool changed_input_found = false;
 
 	if (!name)
 		name = conf_get_configname();
@@ -838,6 +928,7 @@ int conf_write(const char *name)
 	}
 	if (!out)
 		return 1;
+	gs = str_new();
 
 	conf_write_heading(out, &comment_style_pound);
 
@@ -859,13 +950,16 @@ int conf_write(const char *name)
 		} else if (!sym_is_choice(sym) &&
 			   !(sym->flags & SYMBOL_WRITTEN)) {
 			sym_calc_value(sym);
+			if (warn_changed_input)
+				conf_append_changed_input_warning(&gs, sym,
+								  &changed_input_found);
+			sym->flags |= SYMBOL_WRITTEN;
 			if (!(sym->flags & SYMBOL_WRITE))
 				goto next;
 			if (need_newline) {
 				fprintf(out, "\n");
 				need_newline = false;
 			}
-			sym->flags |= SYMBOL_WRITTEN;
 			print_symbol_for_dotconfig(out, sym);
 		}
 
@@ -892,8 +986,12 @@ int conf_write(const char *name)
 	}
 	fclose(out);
 
-	for_all_symbols(sym)
-		sym->flags &= ~SYMBOL_WRITTEN;
+	conf_clear_written_flags();
+
+	if (changed_input_found)
+		conf_changed_input_warning(str_get(&gs));
+
+	str_free(&gs);
 
 	if (*tmpname) {
 		if (is_same(name, tmpname)) {
diff --git a/scripts/kconfig/tests/conftest.py b/scripts/kconfig/tests/conftest.py
index 2a2a7e2da060..87860b1bfd9f 100644
--- a/scripts/kconfig/tests/conftest.py
+++ b/scripts/kconfig/tests/conftest.py
@@ -37,7 +37,8 @@ class Conf:
 
     # runners
     def _run_conf(self, mode, dot_config=None, out_file='.config',
-                  interactive=False, in_keys=None, extra_env={}):
+                  interactive=False, in_keys=None, extra_env={},
+                  silent=False):
         """Run text-based Kconfig executable and save the result.
 
         mode: input mode option (--oldaskconfig, --defconfig=<file> etc.)
@@ -48,7 +49,10 @@ class Conf:
         extra_env: additional environments
         returncode: exit status of the Kconfig executable
         """
-        command = [CONF_PATH, mode, 'Kconfig']
+        command = [CONF_PATH]
+        if silent:
+            command.append('-s')
+        command += [mode, 'Kconfig']
 
         # Override 'srctree' environment to make the test as the top directory
         extra_env['srctree'] = self._test_dir
diff --git a/scripts/kconfig/tests/warn_changed_input/Kconfig b/scripts/kconfig/tests/warn_changed_input/Kconfig
new file mode 100644
index 000000000000..69845e2f3fb3
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/Kconfig
@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: GPL-2.0
+
+config DEP
+	bool "DEP"
+	help
+	  Test dependency symbol for Kconfig warning coverage.
+	  This is used by the warn_changed_input selftest.
+	  It intentionally stays unset in the input fragment.
+	  The test checks how dependent user input is adjusted.
+
+config A
+	bool "A"
+	depends on DEP
+	help
+	  Test bool symbol for changed-input diagnostics.
+	  The input fragment requests this symbol as built-in.
+	  The unmet dependency on DEP forces the final value to n.
+	  The warning should report that downgrade.
+
+config NUM
+	int "NUM"
+	range 10 20
+	help
+	  Test integer symbol for changed-input diagnostics.
+	  The input fragment requests a value outside the allowed range.
+	  Kconfig resolves it to the constrained in-range value.
+	  The warning should report that adjustment.
+
+config DUP
+	bool "DUP"
+	depends on DEP
+	help
+	  Test duplicate-definition handling for changed-input diagnostics.
+	  The input fragment requests this symbol as built-in.
+	  The duplicate definition below must not produce a duplicate warning.
+	  This keeps the warning output stable for repeated menu entries.
+
+config DUP
+	bool
+	depends on DEP
diff --git a/scripts/kconfig/tests/warn_changed_input/__init__.py b/scripts/kconfig/tests/warn_changed_input/__init__.py
new file mode 100644
index 000000000000..4c3bca6af846
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/__init__.py
@@ -0,0 +1,33 @@
+# SPDX-License-Identifier: GPL-2.0
+"""
+Test optional warnings for user-provided values changed by Kconfig.
+
+Warnings should stay disabled by default, and should only appear when
+KCONFIG_WARN_CHANGED_INPUT is enabled.
+"""
+
+
+def test(conf):
+    assert conf.olddefconfig('config') == 0
+    assert 'user-provided values changed by Kconfig' not in conf.stderr
+
+    assert conf._run_conf('--olddefconfig', dot_config='config',
+                          extra_env={
+                              'KCONFIG_WARN_CHANGED_INPUT': '1',
+                          }) == 0
+    assert conf.stderr_contains('expected_stderr')
+    assert conf.config_matches('expected_config')
+
+    assert conf._run_conf('--olddefconfig', dot_config='config',
+                          extra_env={
+                              'KCONFIG_WARN_CHANGED_INPUT': '1',
+                          }, silent=True) == 0
+    assert conf.stderr_contains('expected_stderr')
+
+    assert conf._run_conf('--savedefconfig=defconfig', dot_config='config',
+                          out_file='defconfig',
+                          extra_env={
+                              'KCONFIG_WARN_CHANGED_INPUT': '1',
+                          }) == 0
+    assert conf.stderr_contains('expected_stderr')
+    assert conf.config_matches('expected_defconfig')
diff --git a/scripts/kconfig/tests/warn_changed_input/config b/scripts/kconfig/tests/warn_changed_input/config
new file mode 100644
index 000000000000..dbe93ff26408
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/config
@@ -0,0 +1,3 @@
+CONFIG_A=y
+CONFIG_NUM=30
+CONFIG_DUP=y
diff --git a/scripts/kconfig/tests/warn_changed_input/expected_config b/scripts/kconfig/tests/warn_changed_input/expected_config
new file mode 100644
index 000000000000..fe8bbec66c53
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/expected_config
@@ -0,0 +1,6 @@
+#
+# Automatically generated file; DO NOT EDIT.
+# Main menu
+#
+# CONFIG_DEP is not set
+CONFIG_NUM=20
diff --git a/scripts/kconfig/tests/warn_changed_input/expected_defconfig b/scripts/kconfig/tests/warn_changed_input/expected_defconfig
new file mode 100644
index 000000000000..af9e34851d2a
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/expected_defconfig
@@ -0,0 +1 @@
+CONFIG_NUM=20
diff --git a/scripts/kconfig/tests/warn_changed_input/expected_stderr b/scripts/kconfig/tests/warn_changed_input/expected_stderr
new file mode 100644
index 000000000000..9ec8446b4ac2
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/expected_stderr
@@ -0,0 +1,4 @@
+warning: user-provided values changed by Kconfig:
+  CONFIG_A: y -> n
+  CONFIG_NUM: 30 -> 20
+  CONFIG_DUP: y -> n
-- 
2.50.1 (Apple Git-155)