Kconfig tristate literals are always lowercase ('n', 'y', 'm') and
uppercase N/Y/M are not Kconfig reserved words. Since undefined
symbols evaluate to 'n', writing 'default Y' or 'default M' silently
produces 'n' instead of 'y'/'m'. 'default N' happens to produce the
right value but is still invalid syntax.

Add a warning for N/Y/M in Kconfig expressions found by following
the same preprocessing logic used by the Kconfig parser itself.

This new warning was inspired by work done for [1].

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216748 [1]
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---

v2:
  - Added Andy's tag
  - Changes thanks to sashiko's review
    - strip quoted strings before inline comments to avoid '#' inside a string
    - use [^)]* instead of .* in macro strip regex to avoid greedy match
      eating tokens between adjacent $(macro) expansions

 scripts/checkpatch.pl | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 0d18771f1b01..105478ba6c60 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -3732,6 +3732,21 @@ sub process {
 			}
 		}
 
+# check for uppercase N/Y/M used as Kconfig tristate literals
+		if ($realfile =~ /Kconfig/ &&
+		    $line =~ /^\+\s*(?:default|def_bool|def_tristate|select|imply|depends\s+on|visible\s+if|range|if)\s+(.+)$/) {
+			my $expr = $1;
+			$expr =~ s/"[^"]*"//g;		# strip "quoted strings"
+			$expr =~ s/'[^']*'//g;		# strip 'quoted strings'
+			$expr =~ s/#.*//;		# strip inline comments
+			$expr =~ s/\$\([^)]*\)//g;	# strip $(macro) expansions
+			for my $tok (split /[^A-Za-z0-9_]+/, $expr) {
+				next unless ($tok eq 'Y' || $tok eq 'M' || $tok eq 'N');
+				WARN("KCONFIG_TRISTATE_UPPERCASE",
+				     "'$tok' is probably not what you want here; Kconfig tristate literals are always lowercase ('n', 'y', 'm')\n" . $herecurr);
+			}
+		}
+
 # check MAINTAINERS entries
 		if ($realfile =~ /^MAINTAINERS$/) {
 # check MAINTAINERS entries for the right form
-- 
2.43.0