diff --git a/sys/dev/usb/usb_transfer.h b/sys/dev/usb/usb_transfer.h
index 71157ca71156..f035240b2b47 100644
--- a/sys/dev/usb/usb_transfer.h
+++ b/sys/dev/usb/usb_transfer.h
@@ -27,6 +27,120 @@
 #ifndef _USB_TRANSFER_H_
 #define	_USB_TRANSFER_H_
 
+/*
+ * Definition of internal USB transfer states:
+ * ===========================================
+ *
+ * The main reason there are many USB states is that we are allowed to
+ * cancel USB transfers, then start the USB transfer again and that
+ * this state transaction cannot always be done in a single atomic
+ * operation without blocking the calling thread. One reason for this
+ * is that the USB hardware sometimes needs to wait for DMA
+ * controllers to finish which is done asynchronously and grows the
+ * statemachine.
+ *
+ * When extending the following statemachine there are basically two
+ * things you should think about: Which states should be executed or
+ * modified in case of USB transfer stop and which states should be
+ * executed or modified in case of USB transfer start. Also respect
+ * the "can_cancel_immed" flag which basically tells if you can go
+ * directly from a wait state to the cancelling states.
+ */
+
+enum {
+	/* XFER start execute state */
+
+	/* USB_ST_SETUP = 0 (already defined) */
+
+	/* XFER transferred execute state */
+
+	/* USB_ST_TRANSFERRED = 1 (already defined) */
+
+	/* XFER error execute state */
+
+	/* USB_ST_ERROR = 2 (already defined) */
+
+	/* XFER restart after error execute state */
+
+	USB_ST_RESTART = 8,
+
+	/* XFER transfer idle state */
+
+	USB_ST_WAIT_SETUP,
+
+	/* Other XFER execute states */
+
+	USB_ST_PIPE_OPEN = 16,
+	USB_ST_PIPE_OPEN_ERROR,
+	USB_ST_PIPE_OPEN_RESTART,
+
+	USB_ST_BDMA_LOAD,
+	USB_ST_BDMA_LOAD_ERROR,
+	USB_ST_BDMA_LOAD_RESTART,
+
+	USB_ST_IVAL_DLY,
+	USB_ST_IVAL_DLY_ERROR,
+	USB_ST_IVAL_DLY_RESTART,
+
+	USB_ST_PIPE_STALL,
+	USB_ST_PIPE_STALL_ERROR,
+	USB_ST_PIPE_STALL_RESTART,
+
+	USB_ST_ENTER,
+	USB_ST_ENTER_ERROR,
+	USB_ST_ENTER_RESTART,
+
+	USB_ST_START,
+	USB_ST_START_ERROR,
+	USB_ST_START_RESTART,
+
+	USB_ST_PIPE_CLOSE,
+	USB_ST_PIPE_CLOSE_ERROR,
+	USB_ST_PIPE_CLOSE_RESTART,
+
+	USB_ST_BDMA_DLY,
+	USB_ST_BDMA_DLY_ERROR,
+	USB_ST_BDMA_DLY_RESTART,
+
+	/* XFER transfer wait states */
+
+	USB_ST_WAIT_PIPE_OPEN = 64,
+	USB_ST_WAIT_PIPE_OPEN_ERROR,
+	USB_ST_WAIT_PIPE_OPEN_RESTART,
+
+	USB_ST_WAIT_BDMA_LOAD,
+	USB_ST_WAIT_BDMA_LOAD_ERROR,
+	USB_ST_WAIT_BDMA_LOAD_RESTART,
+
+	USB_ST_WAIT_IVAL_DLY,
+	USB_ST_WAIT_IVAL_DLY_ERROR,
+	USB_ST_WAIT_IVAL_DLY_RESTART,
+
+	USB_ST_WAIT_PIPE_STALL,
+	USB_ST_WAIT_PIPE_STALL_ERROR,
+	USB_ST_WAIT_PIPE_STALL_RESTART,
+
+	USB_ST_WAIT_ENTER,
+	USB_ST_WAIT_ENTER_ERROR,
+	USB_ST_WAIT_ENTER_RESTART,
+
+	USB_ST_WAIT_START,
+	USB_ST_WAIT_START_ERROR,
+	USB_ST_WAIT_START_RESTART,
+
+	USB_ST_WAIT_PIPE_CLOSE,
+	USB_ST_WAIT_PIPE_CLOSE_ERROR,
+	USB_ST_WAIT_PIPE_CLOSE_RESTART,
+
+	USB_ST_WAIT_BDMA_DLY,
+	USB_ST_WAIT_BDMA_DLY_ERROR,
+	USB_ST_WAIT_BDMA_DLY_RESTART,
+
+	USB_ST_WAIT_TRANSFERRED,
+	USB_ST_WAIT_TRANSFERRED_ERROR,
+	USB_ST_WAIT_TRANSFERRED_RESTART,
+};
+
 /*
  * The following structure defines the messages that is used to signal
  * the "done_p" USB process.