diff --git a/demos/device/device_os_none/main.c b/demos/device/device_os_none/main.c
index b611122e0..897b5f5be 100644
--- a/demos/device/device_os_none/main.c
+++ b/demos/device/device_os_none/main.c
@@ -118,10 +118,10 @@ int main(void)
 //--------------------------------------------------------------------+
 OSAL_TASK_FUNCTION( led_blinking_task ) (void* p_task_para)
 {
-  static uint32_t led_on_mask = 0;
-
   OSAL_TASK_LOOP_BEGIN
 
+  static uint32_t led_on_mask = 0;
+
   osal_task_delay(1000);
 
   board_leds(led_on_mask, 1 - led_on_mask);
diff --git a/tinyusb/class/msc_device.h b/tinyusb/class/msc_device.h
index 2919e68d6..bb20e4b13 100644
--- a/tinyusb/class/msc_device.h
+++ b/tinyusb/class/msc_device.h
@@ -55,7 +55,6 @@
 //--------------------------------------------------------------------+
 // APPLICATION API
 //--------------------------------------------------------------------+
-//bool tusbd_msc_is_configured(uint8_t coreid);
 
 //--------------------------------------------------------------------+
 // APPLICATION CALLBACK API
@@ -64,11 +63,57 @@ void tusbd_msc_mounted_cb(uint8_t coreid);
 void tusbd_msc_unmounted_cb(uint8_t coreid);
 
 
-// return 0 will end up with stall transaction
-uint16_t tusbd_msc_read10_cb (uint8_t coreid, uint8_t lun, void** pp_buffer, uint32_t lba, uint16_t block_count) ATTR_WARN_UNUSED_RESULT;
-uint16_t tusbd_msc_write10_cb(uint8_t coreid, uint8_t lun, void** pp_buffer, uint32_t lba, uint16_t block_count) ATTR_WARN_UNUSED_RESULT;
+/** \brief 			Callback that is invoked when tinyusb stack received \ref SCSI_CMD_READ_10 command from host
+ * \param[in]		coreid	    USB Controller ID
+ * \param[in]		lun         Targeted Logical Unit
+ * \param[out]	pp_buffer   Pointer to buffer which application need to update with the response data's address.
+ *                          Must be accessible by USB controller (see \ref TUSB_CFG_ATTR_USBRAM)
+ * \param[in]		lba         Starting Logical Block Address to be read
+ * \param[in]		block_count Number of requested block
+ * \retval      non-zero    Actual number of block that application processed, must be less than or equal to \a \b block_count.
+ * \retval      zero        Indicate error in retrieving data from application. Tinyusb device stack will \b STALL the corresponding
+ *                          endpoint and return failed status in command status wrapper phase.
+ * \note        Host can request dozens of Kbytes in one command e.g \a \b block_count = 128, it is insufficient to require
+ *              application to have that much of buffer. Instead, application can return the number of blocks it can processed,
+ *              the stack after transferred that amount of data will continue to invoke this callback with adjusted \a \b lba and \a \b block_count.
+ *              \n\n Although this callback is called by tinyusb device task (non-isr context), however as all the classes share
+ *              the same task (to save resource), any delay in this callback will cause delay in reponse on other classes.
+ */
+uint16_t tusbd_msc_read10_cb (uint8_t coreid, uint8_t lun, void** pp_buffer, uint32_t lba, uint16_t block_count);
+
+/** \brief 			Callback that is invoked when tinyusb stack received \ref SCSI_CMD_WRITE_10 command from host
+ * \param[in]		coreid	    USB Controller ID
+ * \param[in]		lun         Targeted Logical Unit
+ * \param[out]	pp_buffer   Pointer to buffer which application need to update with the address to hold data from host
+ *                          Must be accessible by USB controller (see \ref TUSB_CFG_ATTR_USBRAM)
+ * \param[in]		lba         Starting Logical Block Address to be write
+ * \param[in]		block_count Number of requested block
+ * \retval      non-zero    Actual number of block that application can receive and must be less than or equal to \a \b block_count.
+ * \retval      zero        Indicate error in retrieving data from application. Tinyusb device stack will \b STALL the corresponding
+ *                          endpoint and return failed status in command status wrapper phase.
+ * \note        Host can request dozens of Kbytes in one command e.g \a \b block_count = 128, it is insufficient to require
+ *              application to have that much of buffer. Instead, application can return the number of blocks it can processed,
+ *              the stack after transferred that amount of data will continue to invoke this callback with adjusted \a \b lba and \a \b block_count.
+ *              \n\n Although this callback is called by tinyusb device task (non-isr context), however as all the classes share
+ *              the same task (to save resource), any delay in this callback will cause delay in reponse on other classes.
+ */
+uint16_t tusbd_msc_write10_cb(uint8_t coreid, uint8_t lun, void** pp_buffer, uint32_t lba, uint16_t block_count);
 
 // p_length [in,out] allocated/maximum length, application update with actual length
+/** \brief 			Callback that is invoked when tinyusb stack received an SCSI command other than \ref SCSI_CMD_WRITE_10 and
+ *              \ref SCSI_CMD_READ_10 command from host
+ * \param[in]		coreid	    USB Controller ID
+ * \param[in]		lun         Targeted Logical Unit
+ * \param[in]		scsi_cmd    SCSI command contents, application should examine this command block to know which command host requested
+ * \param[out]	pp_buffer   Pointer to buffer which application need to update with the address to transceive data with host
+ *                          Must be accessible by USB controller (see \ref TUSB_CFG_ATTR_USBRAM)
+ * \param[in]		p_length    length
+ * \retval      non-zero    Actual number of block that application can receive and must be less than or equal to \a \b block_count.
+ * \retval      zero        Indicate error in retrieving data from application. Tinyusb device stack will \b STALL the corresponding
+ *                          endpoint and return failed status in command status wrapper phase.
+ * \note        Although this callback is called by tinyusb device task (non-isr context), however as all the classes share
+ *              the same task (to save resource), any delay in this callback will cause delay in reponse on other classes.
+ */
 msc_csw_status_t tusbd_msc_scsi_received_isr (uint8_t coreid, uint8_t lun, uint8_t scsi_cmd[16], void ** pp_buffer, uint16_t* p_length);
 
 /** @} */
diff --git a/tinyusb/class/msc_host.h b/tinyusb/class/msc_host.h
index 66b8f9aa3..dfbf323a8 100644
--- a/tinyusb/class/msc_host.h
+++ b/tinyusb/class/msc_host.h
@@ -72,6 +72,7 @@ bool          tusbh_msc_is_mounted(uint8_t dev_addr) ATTR_PURE ATTR_WARN_UNUSED_
  */
 bool          tusbh_msc_is_busy(uint8_t dev_addr) ATTR_PURE ATTR_WARN_UNUSED_RESULT;
 
+// TODO refractor
 bool          tusbh_msc_is_failed(uint8_t dev_addr) ATTR_PURE ATTR_WARN_UNUSED_RESULT;
 
 /** \brief      Get SCSI vendor's name of MassStorage device