thunderbolt: Introduce USB only (SL4) security level

This new security level works so that it creates one PCIe tunnel to the
connected Thunderbolt dock, removing PCIe links downstream of the dock.
This leaves only the internal USB controller visible.

Display Port tunnels are created normally.

While there make sure security sysfs attribute returns "unknown" for any
future security level.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>

drivers/thunderbolt/domain.c

@@ -117,6 +117,7 @@ static const char * const tb_security_names[] = {
 	[TB_SECURITY_USER] = "user",
 	[TB_SECURITY_SECURE] = "secure",
 	[TB_SECURITY_DPONLY] = "dponly",
+	[TB_SECURITY_USBONLY] = "usbonly",
 };
 
 static ssize_t boot_acl_show(struct device *dev, struct device_attribute *attr,
@@ -227,8 +228,12 @@ static ssize_t security_show(struct device *dev, struct device_attribute *attr,
 			     char *buf)
 {
 	struct tb *tb = container_of(dev, struct tb, dev);
+	const char *name = "unknown";
 
-	return sprintf(buf, "%s\n", tb_security_names[tb->security_level]);
+	if (tb->security_level < ARRAY_SIZE(tb_security_names))
+		name = tb_security_names[tb->security_level];
+
+	return sprintf(buf, "%s\n", name);
 }
 static DEVICE_ATTR_RO(security);