kconfig: Support conditional deps using "depends on X if Y"

Extend the "depends on" syntax to support conditional dependencies
using "depends on X if Y". While functionally equivalent to "depends
on X || (Y == n)", "depends on X if Y" is much more readable and
makes the kconfig language uniform in supporting the "if <expr>"
suffix.
This also improves readability for "optional" dependencies, which
are the subset of conditional dependencies where X is Y.
Previously such optional dependencies had to be expressed as
the counterintuitive "depends on X || !X", now this can be
represented as "depends on X if X".

The change is implemented by converting the "X if Y" syntax into the
"X || (Y == n)" syntax during "depends on" token processing.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
[Graham Roff: Rewrote commit message, updated patch, added tests]
Signed-off-by: Graham Roff <grahamr@qti.qualcomm.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20251215-kconfig_conditional_deps-v3-1-59519af0a5df@qti.qualcomm.com
[nathan: Minor adjustments to spacing]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Documentation/kbuild/kconfig-language.rst

@@ -118,7 +118,7 @@ applicable everywhere (see syntax).
   This is a shorthand notation for a type definition plus a value.
   Optionally dependencies for this default value can be added with "if".
 
-- dependencies: "depends on" <expr>
+- dependencies: "depends on" <expr> ["if" <expr>]
 
   This defines a dependency for this menu entry. If multiple
   dependencies are defined, they are connected with '&&'. Dependencies
@@ -134,6 +134,16 @@ applicable everywhere (see syntax).
 	bool "foo"
 	default y
 
+  The dependency definition itself may be conditional by appending "if"
+  followed by an expression. For example::
+
+    config FOO
+	tristate
+	depends on BAR if BAZ
+
+  meaning that FOO is constrained by the value of BAR only if BAZ is
+  also set.
+
 - reverse dependencies: "select" <symbol> ["if" <expr>]
 
   While normal dependencies reduce the upper limit of a symbol (see
@@ -602,8 +612,14 @@ Some drivers are able to optionally use a feature from another module
 or build cleanly with that module disabled, but cause a link failure
 when trying to use that loadable module from a built-in driver.
 
-The most common way to express this optional dependency in Kconfig logic
-uses the slightly counterintuitive::
+The recommended way to express this optional dependency in Kconfig logic
+uses the conditional form::
+
+  config FOO
+	tristate "Support for foo hardware"
+	depends on BAR if BAR
+
+This slightly counterintuitive style is also widely used::
 
   config FOO
 	tristate "Support for foo hardware"

scripts/kconfig/lkc.h

@@ -82,7 +82,7 @@ void menu_warn(const struct menu *menu, const char *fmt, ...);
 struct menu *menu_add_menu(void);
 void menu_end_menu(void);
 void menu_add_entry(struct symbol *sym, enum menu_type type);
-void menu_add_dep(struct expr *dep);
+void menu_add_dep(struct expr *dep, struct expr *cond);
 void menu_add_visibility(struct expr *dep);
 struct property *menu_add_prompt(enum prop_type type, const char *prompt,
 				 struct expr *dep);

scripts/kconfig/menu.c

@@ -127,8 +127,18 @@ static struct expr *rewrite_m(struct expr *e)
 	return e;
 }
 
-void menu_add_dep(struct expr *dep)
+void menu_add_dep(struct expr *dep, struct expr *cond)
 {
+	if (cond) {
+		/*
+		 * We have "depends on X if Y" and we want:
+		 *	Y != n --> X
+		 *	Y == n --> y
+		 * That simplifies to: (X || (Y == n))
+		 */
+		dep = expr_alloc_or(dep,
+				expr_trans_compare(cond, E_EQUAL, &symbol_no));
+	}
 	current_entry->dep = expr_alloc_and(current_entry->dep, dep);
 }
 

scripts/kconfig/parser.y

@@ -323,7 +323,7 @@ if_entry: T_IF expr T_EOL
 {
 	printd(DEBUG_PARSE, "%s:%d:if\n", cur_filename, cur_lineno);
 	menu_add_entry(NULL, M_IF);
-	menu_add_dep($2);
+	menu_add_dep($2, NULL);
 	$$ = menu_add_menu();
 };
 
@@ -422,9 +422,9 @@ help: help_start T_HELPTEXT
 
 /* depends option */
 
-depends: T_DEPENDS T_ON expr T_EOL
+depends: T_DEPENDS T_ON expr if_expr T_EOL
 {
-	menu_add_dep($3);
+	menu_add_dep($3, $4);
 	printd(DEBUG_PARSE, "%s:%d:depends on\n", cur_filename, cur_lineno);
 };
 

scripts/kconfig/tests/conditional_dep/Kconfig

@@ -0,0 +1,32 @@
+# SPDX-License-Identifier: GPL-2.0
+# Test Kconfig file for conditional dependencies.
+
+# Enable module support for tristate testing
+config MODULES
+	bool "Enable loadable module support"
+	modules
+	default y
+
+config FOO
+	bool "FOO symbol"
+
+config BAR
+	bool "BAR symbol"
+
+config TEST_BASIC
+	bool "Test basic conditional dependency"
+	depends on FOO if BAR
+	default y
+
+config TEST_COMPLEX
+	bool "Test complex conditional dependency"
+	depends on (FOO && BAR) if (FOO || BAR)
+	default y
+
+config BAZ
+	tristate "BAZ symbol"
+
+config TEST_OPTIONAL
+	tristate "Test simple optional dependency"
+	depends on BAZ if BAZ
+	default y

scripts/kconfig/tests/conditional_dep/__init__.py

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0
+"""
+Correctly handle conditional dependencies.
+"""
+
+def test(conf):
+    assert conf.oldconfig('test_config1') == 0
+    assert conf.config_matches('expected_config1')
+
+    assert conf.oldconfig('test_config2') == 0
+    assert conf.config_matches('expected_config2')
+
+    assert conf.oldconfig('test_config3') == 0
+    assert conf.config_matches('expected_config3')

scripts/kconfig/tests/conditional_dep/expected_config1

@@ -0,0 +1,11 @@
+#
+# Automatically generated file; DO NOT EDIT.
+# Main menu
+#
+CONFIG_MODULES=y
+CONFIG_FOO=y
+CONFIG_BAR=y
+CONFIG_TEST_BASIC=y
+CONFIG_TEST_COMPLEX=y
+CONFIG_BAZ=m
+CONFIG_TEST_OPTIONAL=m

scripts/kconfig/tests/conditional_dep/expected_config2

@@ -0,0 +1,9 @@
+#
+# Automatically generated file; DO NOT EDIT.
+# Main menu
+#
+CONFIG_MODULES=y
+# CONFIG_FOO is not set
+CONFIG_BAR=y
+CONFIG_BAZ=y
+CONFIG_TEST_OPTIONAL=y

scripts/kconfig/tests/conditional_dep/expected_config3

@@ -0,0 +1,11 @@
+#
+# Automatically generated file; DO NOT EDIT.
+# Main menu
+#
+CONFIG_MODULES=y
+# CONFIG_FOO is not set
+# CONFIG_BAR is not set
+CONFIG_TEST_BASIC=y
+CONFIG_TEST_COMPLEX=y
+# CONFIG_BAZ is not set
+CONFIG_TEST_OPTIONAL=y

scripts/kconfig/tests/conditional_dep/test_config1

@@ -0,0 +1,6 @@
+# Basic check that everything can be configured if selected.
+CONFIG_FOO=y
+CONFIG_BAR=y
+CONFIG_BAZ=m
+# Ensure that TEST_OPTIONAL=y with BAZ=m is converted to TEST_OPTIONAL=m
+CONFIG_TEST_OPTIONAL=y

scripts/kconfig/tests/conditional_dep/test_config2

@@ -0,0 +1,7 @@
+# If FOO is not selected, then TEST_BASIC should fail the conditional
+# dependency since BAR is set.
+# TEST_COMPLEX will fail dependency as it depends on both FOO and BAR
+# if either of those is selected.
+CONFIG_FOO=n
+CONFIG_BAR=y
+CONFIG_BAZ=y

scripts/kconfig/tests/conditional_dep/test_config3

@@ -0,0 +1,6 @@
+# If FOO is not selected, but BAR is also not selected, then TEST_BASIC
+# should pass since the dependency on FOO is conditional on BAR.
+# TEST_COMPLEX should be also set since neither FOO nor BAR are selected
+# so it has no dependencies.
+CONFIG_FOO=n
+CONFIG_BAR=n