media: rc: document and fix rc_validate_scancode()

For some IR protocols, some scancode values not valid, i.e. they're part
of a different protocol variant.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/rc/rc-main.c

@@ -776,21 +776,35 @@ void rc_keydown_notimeout(struct rc_dev *dev, enum rc_proto protocol,
 EXPORT_SYMBOL_GPL(rc_keydown_notimeout);
 
 /**
- * rc_validate_scancode() - checks that a scancode is valid for a protocol
+ * rc_validate_scancode() - checks that a scancode is valid for a protocol.
+ *	For nec, it should do the opposite of ir_nec_bytes_to_scancode()
  * @proto:	protocol
  * @scancode:	scancode
  */
 bool rc_validate_scancode(enum rc_proto proto, u32 scancode)
 {
 	switch (proto) {
+	/*
+	 * NECX has a 16-bit address; if the lower 8 bits match the upper
+	 * 8 bits inverted, then the address would match regular nec.
+	 */
 	case RC_PROTO_NECX:
 		if ((((scancode >> 16) ^ ~(scancode >> 8)) & 0xff) == 0)
 			return false;
 		break;
+	/*
+	 * NEC32 has a 16 bit address and 16 bit command. If the lower 8 bits
+	 * of the command match the upper 8 bits inverted, then it would
+	 * be either NEC or NECX.
+	 */
 	case RC_PROTO_NEC32:
-		if ((((scancode >> 24) ^ ~(scancode >> 16)) & 0xff) == 0)
+		if ((((scancode >> 8) ^ ~scancode) & 0xff) == 0)
 			return false;
 		break;
+	/*
+	 * If the customer code (top 32-bit) is 0x800f, it is MCE else it
+	 * is regular mode-6a 32 bit
+	 */
 	case RC_PROTO_RC6_MCE:
 		if ((scancode & 0xffff0000) != 0x800f0000)
 			return false;