[PATCH] Documentation: warn against using int, hex, string options as expressions in Kconfig

[Julian Braha]

I've been working on type checking Kconfig, and found some instances of
dead code due to conditions that always evaluate to false.

For example:

config FOO
  int

config BAR
  bool
  default 'y' if FOO

This 'if FOO' will always be false, regardless of FOO's value (even when
FOO=1). The same goes for strings and hex.

See also this related patch:
https://lore.kernel.org/all/20260729210546.394392-1-julianbraha@xxxxxxxxx/

Let's warn users against this usage.

Signed-off-by: Julian Braha <julianbraha@xxxxxxxxx>
---
 Documentation/kbuild/kconfig-language.rst | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index ab2822d7ca1c..aee84b294a56 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -320,6 +320,17 @@ An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
 respectively for calculations). A menu entry becomes visible when its
 expression evaluates to 'm' or 'y'.
 
+Note that options of int, hex, or string type should not be used directly
+as expressions; these will always evaluate to 'n'. Instead, to check if
+one of these options is in use, consider using one of these heuristics::
+
+  <symbol> != 0
+  <symbol> != 0x0
+  <symbol> != ""
+
+because int, hex, and string options will take these values of '0', '0x0',
+and the empty string, respectively, when the dependencies are unmet.
+
 There are two types of symbols: constant and non-constant symbols.
 Non-constant symbols are the most common ones and are defined with the
 'config' statement. Non-constant symbols consist entirely of alphanumeric
-- 
2.55.0