diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
index 9f277fec11..1cfed5c348 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
@@ -556,6 +556,22 @@ uint32_t UARTDriver::available() {
     return _readbuf.available();
 }
 
+uint32_t UARTDriver::available_locked(uint32_t key)
+{
+    if (lock_read_key != 0 && key != lock_read_key) {
+        return -1;
+    }
+    if (sdef.is_usb) {
+#ifdef HAVE_USB_SERIAL
+
+        if (((SerialUSBDriver*)sdef.serial)->config->usbp->state != USB_ACTIVE) {
+            return 0;
+        }
+#endif
+    }
+    return _readbuf.available();
+}
+
 uint32_t UARTDriver::txspace()
 {
     if (!_initialised) {
diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.h b/libraries/AP_HAL_ChibiOS/UARTDriver.h
index 10581ff234..ab2b51aa4d 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.h
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.h
@@ -42,6 +42,8 @@ public:
 
 
     uint32_t available() override;
+    uint32_t available_locked(uint32_t key) override;
+
     uint32_t txspace() override;
     int16_t read() override;
     ssize_t read(uint8_t *buffer, uint16_t count) override;