samples: Bluetooth: encrypt llpm sample

It makes most sense to demonstrate llpm on an ecrypted link.
Also setting down conection interval to the smallest
bluetooth allows. 7.5ms. Since encyrpting the link takes
quite a lot of packets back and forth, this will speed up setup.
But also it makes sense that the sample compares the best
bluetooth can do with the best llpm can do.

Signed-off-by: Martin Tverdal <martin.tverdal@nordicsemi.no>

samples/bluetooth/llpm/src/main.c

@@ -27,8 +27,8 @@
 
 #define DEVICE_NAME	CONFIG_BT_DEVICE_NAME
 #define DEVICE_NAME_LEN (sizeof(DEVICE_NAME) - 1)
-#define INTERVAL_MIN    0x50     /* 80 units,  100 ms */
-#define INTERVAL_MAX    0x50     /* 80 units,  100 ms */
+#define INTERVAL_MIN    0x6     /* 6 units,  7.5 ms */
+#define INTERVAL_MAX    0x6     /* 6 units,  7.5 ms */
 #define INTERVAL_LLPM   0x0D01   /* Proprietary  1 ms */
 #define INTERVAL_LLPM_US 1000
 
@@ -195,18 +195,16 @@ static void connected(struct bt_conn *conn, uint8_t err)
 		bt_scan_stop();
 	} else {
 		bt_le_adv_stop();
+		err = bt_conn_set_security(conn, BT_SECURITY_L2);
+		if (err) {
+			printk("Failed to set security: %d\n", err);
+		}
 	}
 
 	printk("Connected as %s\n",
 	       conn_info.role == BT_CONN_ROLE_CENTRAL ? "central" : "peripheral");
 	printk("Conn. interval is %u units (1.25 ms/unit)\n",
 	       conn_info.le.interval);
-
-	err = bt_gatt_dm_start(default_conn, BT_UUID_LATENCY, &discovery_cb,
-			       &latency_client);
-	if (err) {
-		printk("Discover failed (err %d)\n", err);
-	}
 }
 
 static void disconnected(struct bt_conn *conn, uint8_t reason)
@@ -414,10 +412,29 @@ static void test_run(void)
 	}
 }
 
+void security_changed(struct bt_conn *conn, bt_security_t level,
+				 enum bt_security_err err)
+{
+	printk("Security changed: level %i, err: %i\n", level, err);
+
+	if (err != 0) {
+		printk("Failed to encrypt link\n");
+		bt_conn_disconnect(conn, BT_HCI_ERR_PAIRING_NOT_SUPPORTED);
+		return;
+	}
+	/*Start service discovery when link is encrypted*/
+	err = bt_gatt_dm_start(default_conn, BT_UUID_LATENCY, &discovery_cb,
+			       &latency_client);
+	if (err) {
+		printk("Discover failed (err %d)\n", err);
+	}
+}
+
 BT_CONN_CB_DEFINE(conn_callbacks) = {
 	.connected = connected,
 	.disconnected = disconnected,
 	.le_param_updated = le_param_updated,
+	.security_changed = security_changed,
 };
 
 int main(void)