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

[Pengpeng Hou]

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.

Document the new environment variable and add tests for both olddefconfig
and savedefconfig.

Signed-off-by: Pengpeng Hou <pengpeng@xxxxxxxxxxx>
---
 Documentation/kbuild/kconfig.rst         |  5 ++
 scripts/kconfig/confdata.c               | 94 +++++++++++++++++++++++++++++++-
 .../tests/warn_changed_input/Kconfig     | 27 +++++++++
 .../tests/warn_changed_input/__init__.py | 27 +++++++++
 .../tests/warn_changed_input/config      |  2 +
 .../warn_changed_input/expected_config   |  6 ++
 .../expected_defconfig                   |  1 +
 .../warn_changed_input/expected_stdout   |  3 +
 8 files changed, 162 insertions(+), 3 deletions(-)

diff --git a/Documentation/kbuild/kconfig.rst b/Documentation/kbuild/kconfig.rst
index d213c4f599a4..e35dd1d5f9d3 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 9599a0408862..bcbac5a204e9 100644
--- a/scripts/kconfig/confdata.c
+++ b/scripts/kconfig/confdata.c
@@ -206,6 +206,72 @@ static void conf_message(const char *fmt, ...)
 	va_end(ap);
 }
 
+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 *found)
+{
+	if (!sym_user_value_changed(sym))
+		return;
+
+	if (!*found) {
+		str_printf(gs,
+			   "warning: user-provided values changed by Kconfig:\n");
+		*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");
@@ -759,7 +825,10 @@ int conf_write_defconfig(const char *filename)
 {
 	struct symbol *sym;
 	struct menu *menu;
+	struct gstr gs = str_new();
 	FILE *out;
+	bool warn_changed_input = conf_warn_changed_input_enabled();
+	bool found = false;
 
 	out = fopen(filename, "w");
 	if (!out)
@@ -772,10 +841,13 @@ 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, &found);
+		sym->flags |= SYMBOL_WRITTEN;
 		if (!(sym->flags & SYMBOL_WRITE))
 			continue;
 		sym->flags &= ~SYMBOL_WRITE;
@@ -798,6 +870,13 @@ int conf_write_defconfig(const char *filename)
 		print_symbol_for_dotconfig(out, sym);
 	}
 	fclose(out);
+
+	conf_clear_written_flags();
+
+	if (found)
+		conf_message("%s", str_get(&gs));
+
+	str_free(&gs);
 	return 0;
 }
 
@@ -809,7 +888,10 @@ int conf_write(const char *name)
 	const char *str;
 	char tmpname[PATH_MAX + 1], oldname[PATH_MAX + 1];
 	char *env;
+	struct gstr gs = str_new();
 	bool need_newline = false;
+	bool warn_changed_input = conf_warn_changed_input_enabled();
+	bool found = false;
 
 	if (!name)
 		name = conf_get_configname();
@@ -859,6 +941,8 @@ 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, &found);
 			if (!(sym->flags & SYMBOL_WRITE))
 				goto next;
 			if (need_newline) {
@@ -892,8 +976,12 @@ int conf_write(const char *name)
 	}
 	fclose(out);
 
-	for_all_symbols(sym)
-		sym->flags &= ~SYMBOL_WRITTEN;
+	conf_clear_written_flags();
+
+	if (found)
+		conf_message("%s", str_get(&gs));
+
+	str_free(&gs);
 
 	if (*tmpname) {
 		if (is_same(name, tmpname)) {
diff --git a/scripts/kconfig/tests/warn_changed_input/Kconfig b/scripts/kconfig/tests/warn_changed_input/Kconfig
new file mode 100644
index 000000000000..4b749eccbe28
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/Kconfig
@@ -0,0 +1,27 @@
+# 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.
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..5a2b68fb1033
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/__init__.py
@@ -0,0 +1,27 @@
+# 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.stdout
+
+    assert conf._run_conf('--olddefconfig', dot_config='config',
+                          extra_env={
+                              'KCONFIG_WARN_CHANGED_INPUT': '1',
+                          }) == 0
+    assert conf.stdout_contains('expected_stdout')
+    assert conf.config_matches('expected_config')
+
+    assert conf._run_conf('--savedefconfig=defconfig', dot_config='config',
+                          out_file='defconfig',
+                          extra_env={
+                              'KCONFIG_WARN_CHANGED_INPUT': '1',
+                          }) == 0
+    assert conf.stdout_contains('expected_stdout')
+    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..91156997e58d
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/config
@@ -0,0 +1,2 @@
+CONFIG_A=y
+CONFIG_NUM=30
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_stdout b/scripts/kconfig/tests/warn_changed_input/expected_stdout
new file mode 100644
index 000000000000..83ca08b7187f
--- /dev/null
+++ b/scripts/kconfig/tests/warn_changed_input/expected_stdout
@@ -0,0 +1,3 @@
+warning: user-provided values changed by Kconfig:
+  CONFIG_A: y -> n
+  CONFIG_NUM: 30 -> 20
-- 
2.50.1 (Apple Git-155)