@Override
    public String toString() {
        return String.format("<%s device_name=%s device_id=%s port_number=%s>",
                getClass().getSimpleName(), mDevice.getDeviceName(),
                mDevice.getDeviceId(), mPortNumber);
    }
Internal write buffer.
 Guarded by {@link #mWriteBufferLock}.
 Default length = mReadEndpoint.getMaxPacketSize()
usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CommonUsbSerialPort.java