diff --git a/src/class/bth/bth_device.c b/src/class/bth/bth_device.c
index c63b994e..f8215e7d 100755
--- a/src/class/bth/bth_device.c
+++ b/src/class/bth/bth_device.c
@@ -186,6 +186,9 @@ uint16_t btd_open(uint8_t rhport, tusb_desc_interface_t const *itf_desc, uint16_
   return drv_len;
 }
 
+// Invoked when a control transfer occurred on an interface of this class
+// Driver response accordingly to the request and the transfer stage (setup/data/ack)
+// return false to stall control endpoint (e.g unsupported request)
 bool btd_control_xfer_cb(uint8_t rhport, uint8_t stage, tusb_control_request_t const *request)
 {
   (void)rhport;