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>

1 files changed, 19 insertions, 3 deletions

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index abce88f15d7c..9ff3e530b2b4 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. 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"