diff --git a/drivers/usb/renesas_usbhs/common.c b/drivers/usb/renesas_usbhs/common.c
index 665259a..d8239e5 100644
--- a/drivers/usb/renesas_usbhs/common.c
+++ b/drivers/usb/renesas_usbhs/common.c
@@ -21,6 +21,29 @@
 #include <linux/sysfs.h>
 #include "./common.h"
 
+/*
+ *		image of renesas_usbhs
+ *
+ * ex) gadget case
+
+ * mod.c
+ * mod_gadget.c
+ * mod_host.c		pipe.c		fifo.c
+ *
+ *			+-------+	+-----------+
+ *			| pipe0 |------>| fifo pio  |
+ * +------------+	+-------+	+-----------+
+ * | mod_gadget |=====> | pipe1 |--+
+ * +------------+	+-------+  |	+-----------+
+ *			| pipe2 |  |  +-| fifo dma0 |
+ * +------------+	+-------+  |  |	+-----------+
+ * | mod_host   |	| pipe3 |<-|--+
+ * +------------+	+-------+  |	+-----------+
+ *			| ....  |  +--->| fifo dma1 |
+ *			| ....  |	+-----------+
+ */
+
+
 #define USBHSF_RUNTIME_PWCTRL	(1 << 0)
 
 /* status */
diff --git a/drivers/usb/renesas_usbhs/fifo.c b/drivers/usb/renesas_usbhs/fifo.c
index 021695d..237e8b1 100644
--- a/drivers/usb/renesas_usbhs/fifo.c
+++ b/drivers/usb/renesas_usbhs/fifo.c
@@ -26,7 +26,16 @@
 #define usbhsf_fifo_is_busy(f)	((f)->pipe) /* see usbhs_pipe_select_fifo */
 
 /*
- *		packet info function
+ *		packet initialize
+ */
+void usbhs_pkt_init(struct usbhs_pkt *pkt)
+{
+	pkt->dma = DMA_ADDR_INVALID;
+	INIT_LIST_HEAD(&pkt->node);
+}
+
+/*
+ *		packet control function
  */
 static int usbhsf_null_handle(struct usbhs_pkt *pkt, int *is_done)
 {
@@ -43,12 +52,6 @@
 	.try_run = usbhsf_null_handle,
 };
 
-void usbhs_pkt_init(struct usbhs_pkt *pkt)
-{
-	pkt->dma = DMA_ADDR_INVALID;
-	INIT_LIST_HEAD(&pkt->node);
-}
-
 void usbhs_pkt_push(struct usbhs_pipe *pipe, struct usbhs_pkt *pkt,
 		    struct usbhs_pkt_handle *handler,
 		    void *buf, int len, int zero)
@@ -293,7 +296,7 @@
 }
 
 /*
- *		PIO fifo functions
+ *		PIO push handler
  */
 static int usbhsf_pio_try_push(struct usbhs_pkt *pkt, int *is_done)
 {
@@ -395,6 +398,9 @@
 	.try_run = usbhsf_pio_try_push,
 };
 
+/*
+ *		PIO pop handler
+ */
 static int usbhsf_prepare_pop(struct usbhs_pkt *pkt, int *is_done)
 {
 	struct usbhs_pipe *pipe = pkt->pipe;
@@ -497,7 +503,7 @@
 };
 
 /*
- *		handler function
+ *		DCP ctrol statge handler
  */
 static int usbhsf_ctrl_stage_end(struct usbhs_pkt *pkt, int *is_done)
 {
@@ -614,6 +620,9 @@
 	dma_async_issue_pending(chan);
 }
 
+/*
+ *		DMA push handler
+ */
 static int usbhsf_dma_prepare_push(struct usbhs_pkt *pkt, int *is_done)
 {
 	struct usbhs_pipe *pipe = pkt->pipe;
@@ -686,6 +695,9 @@
 	.dma_done	= usbhsf_dma_push_done,
 };
 
+/*
+ *		DMA pop handler
+ */
 static int usbhsf_dma_try_pop(struct usbhs_pkt *pkt, int *is_done)
 {
 	struct usbhs_pipe *pipe = pkt->pipe;
diff --git a/drivers/usb/renesas_usbhs/mod_gadget.c b/drivers/usb/renesas_usbhs/mod_gadget.c
index 9a8e2e9..cc3ad63 100644
--- a/drivers/usb/renesas_usbhs/mod_gadget.c
+++ b/drivers/usb/renesas_usbhs/mod_gadget.c
@@ -114,7 +114,7 @@
 #define usbhsg_status_has(gp, b) (gp->status &   b)
 
 /*
- *		list push/pop
+ *		queue push/pop
  */
 static void usbhsg_queue_push(struct usbhsg_uep *uep,
 			      struct usbhsg_request *ureq)
@@ -160,6 +160,9 @@
 	usbhsg_queue_pop(uep, ureq, 0);
 }
 
+/*
+ *		dma map/unmap
+ */
 static int usbhsg_dma_map(struct device *dev,
 			  struct usbhs_pkt *pkt,
 			  enum dma_data_direction dir)
@@ -473,6 +476,11 @@
 		uep->pipe		= pipe;
 		pipe->mod_private	= uep;
 
+		/*
+		 * usbhs_fifo_dma_push/pop_handler try to
+		 * use dmaengine if possible.
+		 * It will use pio handler if impossible.
+		 */
 		if (usb_endpoint_dir_in(desc))
 			uep->handler = &usbhs_fifo_dma_push_handler;
 		else