linux-kernel - [PATCH] Support conditional deps using "depends on X if Y"

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Message-Id: <20251107-kconfig_conditional_deps-v1-1-aff22199ec0b@qti.qualcomm.com>
Date: Fri, 07 Nov 2025 17:16:34 -0800
From: Graham Roff <grahamr@....qualcomm.com>
To: Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nsc@...nel.org>,
        Jonathan Corbet <corbet@....net>
Cc: linux-kbuild@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Nicolas Pitre <nico@...xnic.net>,
        Graham Roff <grahamr@....qualcomm.com>
Subject: [PATCH] Support conditional deps using "depends on X if Y"

From: Nicolas Pitre <nico@...xnic.net>

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.

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@...xnic.net>

[Graham Roff: Rewrote commit message and redid patch for latest kernel]

Signed-off-by: Graham Roff <grahamr@....qualcomm.com>
---
This patch updates an earlier one that was not merged to work on 
the latest kernel release.

Link: https://lwn.net/ml/linux-kernel/nycvar.YSQ.7.76.2004231102480.2671@knanqh.ubzr/#t

Support for this change has been expressed by a number of developers
since the original patch was proposed back in 2020, and has recently
also been raised as a patch to the Zephyr kconfig system.
One specific use is when mapping the Bluetooth specification to Kconfig,
as it explicitly provides dependencies between features as conditional
on other features. Many other cases exist where the "slightly
counterintuitive" (quoted from the Kconfig specification) expression 
"depends on BAR || !BAR" has been used when a proper "if" condition 
would be more readable.

The earlier patch discussion ended without a real conclusion and should
be revisited now.
---
 Documentation/kbuild/kconfig-language.rst | 12 +++++++++++-
 scripts/kconfig/lkc.h                     |  2 +-
 scripts/kconfig/menu.c                    | 12 +++++++++++-
 scripts/kconfig/parser.y                  |  6 +++---
 4 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index abce88f15d7c..61848f999db8 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/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. If such expression is false (n) then this
+  dependency is ignored. One possible use case is:
+
+    config FOO
+	tristate
+	depends on BAZ if BAZ != n
+
+  meaning that FOO is constrained by the value of BAZ only when it is set.
+
 - reverse dependencies: "select" <symbol> ["if" <expr>]
 
   While normal dependencies reduce the upper limit of a symbol (see
diff --git a/scripts/kconfig/lkc.h b/scripts/kconfig/lkc.h
index 56548efc14d7..798985961215 100644
--- a/scripts/kconfig/lkc.h
+++ b/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);
diff --git a/scripts/kconfig/menu.c b/scripts/kconfig/menu.c
index 0f1a6513987c..b2d8d4e11e07 100644
--- a/scripts/kconfig/menu.c
+++ b/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);
 }
 
diff --git a/scripts/kconfig/parser.y b/scripts/kconfig/parser.y
index 49b79dde1725..6d1bbee38f5d 100644
--- a/scripts/kconfig/parser.y
+++ b/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);
 };
 

---
base-commit: a1388fcb52fcad3e0b06e2cdd0ed757a82a5be30
change-id: 20251106-kconfig_conditional_deps-51f1c903f863

Best regards,
-- 
Graham Roff <grahamr@....qualcomm.com>