[PATCH] kconfig: Document SYMBOL_OPTIONAL logic

[Ulf Magnusson]

Not obvious, especially if you don't already know how choices are
implemented.

No functional changes. Only comments added.

Signed-off-by: Ulf Magnusson <ulfalizer@xxxxxxxxx>
---
 scripts/kconfig/menu.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/scripts/kconfig/menu.c b/scripts/kconfig/menu.c
index 92d3f06cd8a2..372eb5d9fef3 100644
--- a/scripts/kconfig/menu.c
+++ b/scripts/kconfig/menu.c
@@ -548,6 +548,15 @@ void menu_finalize(struct menu *parent)
 		sym->flags |= SYMBOL_WARNED;
 	}
 
+	/*
+	 * For non-optional choices, add a reverse dependency (corresponding to
+	 * a select) of '<visibility> && m'. This prevents the user from
+	 * setting the choice mode to 'n' when the choice is visible.
+	 *
+	 * This would also work for non-choice symbols, but only non-optional
+	 * choices clear SYMBOL_OPTIONAL as of writing. Choices are implemented
+	 * as a type of symbol.
+	 */
 	if (sym && !sym_is_optional(sym) && parent->prompt) {
 		sym->rev_dep.expr = expr_alloc_or(sym->rev_dep.expr,
 				expr_alloc_and(parent->prompt->visible.expr,
-- 
2.14.1