kconfig: Document SYMBOL_OPTIONAL logic

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@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/scripts/kconfig/menu.c b/scripts/kconfig/menu.c
index 47bec3434fef..7634d567e779 100644
--- a/scripts/kconfig/menu.c
+++ b/scripts/kconfig/menu.c
@@ -549,6 +549,15 @@ 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,