diff --git a/src/class/cdc/cdc_device.c b/src/class/cdc/cdc_device.c
index a865ba3c5..fc124627d 100644
--- a/src/class/cdc/cdc_device.c
+++ b/src/class/cdc/cdc_device.c
@@ -229,16 +229,14 @@ void cdcd_reset(uint8_t rhport)
   }
 }
 
-tusb_error_t cdcd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, uint16_t *p_length)
+bool cdcd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, uint16_t *p_length)
 {
-  if ( CDC_COMM_SUBCLASS_ABSTRACT_CONTROL_MODEL != itf_desc->bInterfaceSubClass) return TUSB_ERROR_CDC_UNSUPPORTED_SUBCLASS;
+  // Only support ACM subclass
+  TU_ASSERT ( CDC_COMM_SUBCLASS_ABSTRACT_CONTROL_MODEL == itf_desc->bInterfaceSubClass);
 
   // Only support AT commands, no protocol and vendor specific commands.
-  if ( !(tu_within(CDC_COMM_PROTOCOL_NONE, itf_desc->bInterfaceProtocol, CDC_COMM_PROTOCOL_ATCOMMAND_CDMA) ||
-         itf_desc->bInterfaceProtocol == 0xff ) )
-  {
-    return TUSB_ERROR_CDC_UNSUPPORTED_PROTOCOL;
-  }
+  TU_ASSERT(tu_within(CDC_COMM_PROTOCOL_NONE, itf_desc->bInterfaceProtocol, CDC_COMM_PROTOCOL_ATCOMMAND_CDMA) ||
+            itf_desc->bInterfaceProtocol == 0xff);
 
   // Find available interface
   cdcd_interface_t * p_cdc = NULL;
@@ -250,23 +248,25 @@ tusb_error_t cdcd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, u
       break;
     }
   }
+  TU_ASSERT(p_cdc);
 
   //------------- Control Interface -------------//
-  p_cdc->itf_num  = itf_desc->bInterfaceNumber;
+  p_cdc->itf_num = itf_desc->bInterfaceNumber;
 
   uint8_t const * p_desc = tu_desc_next( itf_desc );
   (*p_length) = sizeof(tusb_desc_interface_t);
 
   // Communication Functional Descriptors
-  while( TUSB_DESC_CLASS_SPECIFIC == p_desc[DESC_OFFSET_TYPE] )
+  while ( TUSB_DESC_CLASS_SPECIFIC == tu_desc_type(p_desc) )
   {
-    (*p_length) += p_desc[DESC_OFFSET_LEN];
+    (*p_length) += tu_desc_len(p_desc);
     p_desc = tu_desc_next(p_desc);
   }
 
-  if ( TUSB_DESC_ENDPOINT == p_desc[DESC_OFFSET_TYPE])
-  { // notification endpoint if any
-    TU_ASSERT( dcd_edpt_open(rhport, (tusb_desc_endpoint_t const *) p_desc), TUSB_ERROR_DCD_OPEN_PIPE_FAILED);
+  if ( TUSB_DESC_ENDPOINT == tu_desc_type(p_desc) )
+  {
+    // notification endpoint if any
+    TU_ASSERT( dcd_edpt_open(rhport, (tusb_desc_endpoint_t const *) p_desc) );
 
     p_cdc->ep_notif = ((tusb_desc_endpoint_t const *) p_desc)->bEndpointAddress;
 
@@ -278,21 +278,21 @@ tusb_error_t cdcd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, u
   if ( (TUSB_DESC_INTERFACE == p_desc[DESC_OFFSET_TYPE]) &&
        (TUSB_CLASS_CDC_DATA == ((tusb_desc_interface_t const *) p_desc)->bInterfaceClass) )
   {
-    // next to endpoint descritpor
-    (*p_length) += p_desc[DESC_OFFSET_LEN];
+    // next to endpoint descriptor
+    (*p_length) += tu_desc_len(p_desc);
     p_desc = tu_desc_next(p_desc);
 
     // Open endpoint pair with usbd helper
     tusb_desc_endpoint_t const *p_desc_ep = (tusb_desc_endpoint_t const *) p_desc;
-    TU_ASSERT_ERR( usbd_open_edpt_pair(rhport, p_desc_ep, TUSB_XFER_BULK, &p_cdc->ep_out, &p_cdc->ep_in) );
+    TU_ASSERT( usbd_open_edpt_pair(rhport, p_desc_ep, TUSB_XFER_BULK, &p_cdc->ep_out, &p_cdc->ep_in) );
 
     (*p_length) += 2*sizeof(tusb_desc_endpoint_t);
   }
 
   // Prepare for incoming data
-  TU_ASSERT( dcd_edpt_xfer(rhport, p_cdc->ep_out, p_cdc->epout_buf, CFG_TUD_CDC_EPSIZE), TUSB_ERROR_DCD_EDPT_XFER);
+  TU_ASSERT( dcd_edpt_xfer(rhport, p_cdc->ep_out, p_cdc->epout_buf, CFG_TUD_CDC_EPSIZE) );
 
-  return TUSB_ERROR_NONE;
+  return true;
 }
 
 // Invoked when class request DATA stage is finished.
diff --git a/src/class/cdc/cdc_device.h b/src/class/cdc/cdc_device.h
index 9dd837d7d..03aa2485e 100644
--- a/src/class/cdc/cdc_device.h
+++ b/src/class/cdc/cdc_device.h
@@ -112,12 +112,12 @@ ATTR_WEAK void tud_cdc_line_coding_cb(uint8_t itf, cdc_line_coding_t const* p_li
 //--------------------------------------------------------------------+
 #ifdef _TINY_USB_SOURCE_FILE_
 
-void         cdcd_init               (void);
-tusb_error_t cdcd_open               (uint8_t rhport, tusb_desc_interface_t const * p_interface_desc, uint16_t *p_length);
+void cdcd_init               (void);
+bool cdcd_open               (uint8_t rhport, tusb_desc_interface_t const * p_interface_desc, uint16_t *p_length);
 bool cdcd_control_request (uint8_t rhport, tusb_control_request_t const * p_request);
 bool cdcd_control_request_complete (uint8_t rhport, tusb_control_request_t const * p_request);
 tusb_error_t cdcd_xfer_cb            (uint8_t rhport, uint8_t ep_addr, xfer_result_t result, uint32_t xferred_bytes);
-void         cdcd_reset              (uint8_t rhport);
+void cdcd_reset              (uint8_t rhport);
 
 #endif
 
diff --git a/src/class/hid/hid_device.c b/src/class/hid/hid_device.c
index fa76ee015..cfb1a698d 100644
--- a/src/class/hid/hid_device.c
+++ b/src/class/hid/hid_device.c
@@ -318,22 +318,22 @@ void hidd_reset(uint8_t rhport)
   #endif
 }
 
-tusb_error_t hidd_open(uint8_t rhport, tusb_desc_interface_t const * desc_itf, uint16_t *p_len)
+bool hidd_open(uint8_t rhport, tusb_desc_interface_t const * desc_itf, uint16_t *p_len)
 {
   uint8_t const *p_desc = (uint8_t const *) desc_itf;
 
-  // TODO not support HID OUT Endpoint
-  TU_ASSERT(desc_itf->bNumEndpoints == 1, ERR_TUD_INVALID_DESCRIPTOR);
+  // TODO support HID OUT Endpoint
+  TU_ASSERT(desc_itf->bNumEndpoints == 1);
 
   //------------- HID descriptor -------------//
-  p_desc += p_desc[DESC_OFFSET_LEN];
+  p_desc = tu_desc_next(p_desc);
   tusb_hid_descriptor_hid_t const *desc_hid = (tusb_hid_descriptor_hid_t const *) p_desc;
-  TU_ASSERT(HID_DESC_TYPE_HID == desc_hid->bDescriptorType, ERR_TUD_INVALID_DESCRIPTOR);
+  TU_ASSERT(HID_DESC_TYPE_HID == desc_hid->bDescriptorType);
 
   //------------- Endpoint Descriptor -------------//
-  p_desc += p_desc[DESC_OFFSET_LEN];
+  p_desc = tu_desc_next(p_desc);
   tusb_desc_endpoint_t const *desc_edpt = (tusb_desc_endpoint_t const *) p_desc;
-  TU_ASSERT(TUSB_DESC_ENDPOINT == desc_edpt->bDescriptorType, ERR_TUD_INVALID_DESCRIPTOR);
+  TU_ASSERT(TUSB_DESC_ENDPOINT == desc_edpt->bDescriptorType);
 
   hidd_interface_t * p_hid = NULL;
 
@@ -374,7 +374,7 @@ tusb_error_t hidd_open(uint8_t rhport, tusb_desc_interface_t const * desc_itf, u
     }
     #endif
 
-    TU_ASSERT(p_hid, ERR_TUD_INVALID_DESCRIPTOR);
+    TU_ASSERT(p_hid);
     p_hid->boot_protocol = true; // default mode is BOOT
   }
   /*------------- Generic (multiple report) -------------*/
@@ -386,12 +386,10 @@ tusb_error_t hidd_open(uint8_t rhport, tusb_desc_interface_t const * desc_itf, u
     p_hid->desc_report   = usbd_desc_set->hid_report.generic;
     p_hid->get_report_cb = tud_hid_generic_get_report_cb;
     p_hid->set_report_cb = tud_hid_generic_set_report_cb;
-
-    TU_ASSERT(p_hid, ERR_TUD_INVALID_DESCRIPTOR);
   }
 
-  TU_VERIFY(p_hid->desc_report, ERR_TUD_INVALID_DESCRIPTOR);
-  TU_ASSERT( dcd_edpt_open(rhport, desc_edpt), ERR_TUD_EDPT_OPEN_FAILED );
+  TU_ASSERT(p_hid->desc_report);
+  TU_ASSERT(dcd_edpt_open(rhport, desc_edpt));
 
   p_hid->itf_num   = desc_itf->bInterfaceNumber;
   p_hid->ep_in     = desc_edpt->bEndpointAddress;
@@ -399,7 +397,7 @@ tusb_error_t hidd_open(uint8_t rhport, tusb_desc_interface_t const * desc_itf, u
 
   *p_len = sizeof(tusb_desc_interface_t) + sizeof(tusb_hid_descriptor_hid_t) + desc_itf->bNumEndpoints*sizeof(tusb_desc_endpoint_t);
 
-  return TUSB_ERROR_NONE;
+  return true;
 }
 
 // Handle class control request
diff --git a/src/class/hid/hid_device.h b/src/class/hid/hid_device.h
index 42c61a47b..2f0047d81 100644
--- a/src/class/hid/hid_device.h
+++ b/src/class/hid/hid_device.h
@@ -377,7 +377,7 @@ ATTR_WEAK void tud_hid_mouse_set_report_cb(uint8_t report_id, hid_report_type_t
 #ifdef _TINY_USB_SOURCE_FILE_
 
 void hidd_init(void);
-tusb_error_t hidd_open(uint8_t rhport, tusb_desc_interface_t const * p_interface_desc, uint16_t *p_length);
+bool hidd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, uint16_t *p_length);
 bool hidd_control_request(uint8_t rhport, tusb_control_request_t const * p_request);
 bool hidd_control_request_complete (uint8_t rhport, tusb_control_request_t const * p_request);
 tusb_error_t hidd_xfer_cb(uint8_t rhport, uint8_t ep_addr, xfer_result_t event, uint32_t xferred_bytes);
diff --git a/src/class/msc/msc_device.c b/src/class/msc/msc_device.c
index 48f23688d..14a7e593a 100644
--- a/src/class/msc/msc_device.c
+++ b/src/class/msc/msc_device.c
@@ -150,25 +150,25 @@ void mscd_reset(uint8_t rhport)
   tu_memclr(&_mscd_itf, sizeof(mscd_interface_t));
 }
 
-tusb_error_t mscd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, uint16_t *p_len)
+bool mscd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, uint16_t *p_len)
 {
   // only support SCSI's BOT protocol
-  TU_VERIFY( ( MSC_SUBCLASS_SCSI == itf_desc->bInterfaceSubClass &&
-            MSC_PROTOCOL_BOT  == itf_desc->bInterfaceProtocol ), TUSB_ERROR_MSC_UNSUPPORTED_PROTOCOL );
+  TU_ASSERT(MSC_SUBCLASS_SCSI == itf_desc->bInterfaceSubClass &&
+            MSC_PROTOCOL_BOT  == itf_desc->bInterfaceProtocol);
 
   mscd_interface_t * p_msc = &_mscd_itf;
 
   // Open endpoint pair with usbd helper
   tusb_desc_endpoint_t const *p_desc_ep = (tusb_desc_endpoint_t const *) tu_desc_next( itf_desc );
-  TU_ASSERT_ERR( usbd_open_edpt_pair(rhport, p_desc_ep, TUSB_XFER_BULK, &p_msc->ep_out, &p_msc->ep_in) );
+  TU_ASSERT( usbd_open_edpt_pair(rhport, p_desc_ep, TUSB_XFER_BULK, &p_msc->ep_out, &p_msc->ep_in) );
 
   p_msc->itf_num = itf_desc->bInterfaceNumber;
   (*p_len) = sizeof(tusb_desc_interface_t) + 2*sizeof(tusb_desc_endpoint_t);
 
-  //------------- Queue Endpoint OUT for Command Block Wrapper -------------//
-  TU_ASSERT( dcd_edpt_xfer(rhport, p_msc->ep_out, (uint8_t*) &p_msc->cbw, sizeof(msc_cbw_t)), TUSB_ERROR_DCD_EDPT_XFER );
+  // Prepare for Command Block Wrapper
+  TU_ASSERT( dcd_edpt_xfer(rhport, p_msc->ep_out, (uint8_t*) &p_msc->cbw, sizeof(msc_cbw_t)) );
 
-  return TUSB_ERROR_NONE;
+  return true;
 }
 
 // Handle class control request
diff --git a/src/class/msc/msc_device.h b/src/class/msc/msc_device.h
index 55bfba83c..80e47d84b 100644
--- a/src/class/msc/msc_device.h
+++ b/src/class/msc/msc_device.h
@@ -171,7 +171,7 @@ ATTR_WEAK bool tud_msc_is_writable_cb(uint8_t lun);
 #ifdef _TINY_USB_SOURCE_FILE_
 
 void mscd_init(void);
-tusb_error_t mscd_open(uint8_t rhport, tusb_desc_interface_t const * p_interface_desc, uint16_t *p_length);
+bool mscd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, uint16_t *p_length);
 bool mscd_control_request(uint8_t rhport, tusb_control_request_t const * p_request);
 bool mscd_control_request_complete (uint8_t rhport, tusb_control_request_t const * p_request);
 tusb_error_t mscd_xfer_cb(uint8_t rhport, uint8_t ep_addr, xfer_result_t event, uint32_t xferred_bytes);
diff --git a/src/device/usbd.c b/src/device/usbd.c
index 096437747..bf7fd950a 100644
--- a/src/device/usbd.c
+++ b/src/device/usbd.c
@@ -88,13 +88,13 @@ tud_desc_set_t const* usbd_desc_set = &tud_desc_set;
 typedef struct {
   uint8_t class_code;
 
-  void         (* init           ) (void);
-  tusb_error_t (* open           ) (uint8_t rhport, tusb_desc_interface_t const * desc_intf, uint16_t* p_length);
-  bool         (* control_request ) (uint8_t rhport, tusb_control_request_t const * request);
+  void (* init           ) (void);
+  bool (* open           ) (uint8_t rhport, tusb_desc_interface_t const * desc_intf, uint16_t* p_length);
+  bool (* control_request ) (uint8_t rhport, tusb_control_request_t const * request);
   bool (* control_request_complete ) (uint8_t rhport, tusb_control_request_t const * request);
   tusb_error_t (* xfer_cb        ) (uint8_t rhport, uint8_t ep_addr, xfer_result_t, uint32_t);
-  void         (* sof            ) (uint8_t rhport);
-  void         (* reset          ) (uint8_t);
+  void (* sof            ) (uint8_t rhport);
+  void (* reset          ) (uint8_t);
 } usbd_class_driver_t;
 
 static usbd_class_driver_t const usbd_class_drivers[] =
@@ -458,7 +458,7 @@ static bool process_set_config(uint8_t rhport)
       _usbd_dev.itf2drv[desc_itf->bInterfaceNumber] = drv_id;
 
       uint16_t itf_len=0;
-      TU_ASSERT_ERR( usbd_class_drivers[drv_id].open( rhport, desc_itf, &itf_len ), false );
+      TU_ASSERT( usbd_class_drivers[drv_id].open( rhport, desc_itf, &itf_len ) );
       TU_ASSERT( itf_len >= sizeof(tusb_desc_interface_t) );
 
       mark_interface_endpoint(_usbd_dev.ep2drv, p_desc, itf_len, drv_id);
@@ -624,14 +624,14 @@ void dcd_event_xfer_complete (uint8_t rhport, uint8_t ep_addr, uint32_t xferred_
 //--------------------------------------------------------------------+
 
 // Helper to parse an pair of endpoint descriptors (IN & OUT)
-tusb_error_t usbd_open_edpt_pair(uint8_t rhport, tusb_desc_endpoint_t const* ep_desc, uint8_t xfer_type, uint8_t* ep_out, uint8_t* ep_in)
+bool usbd_open_edpt_pair(uint8_t rhport, tusb_desc_endpoint_t const* ep_desc, uint8_t xfer_type, uint8_t* ep_out, uint8_t* ep_in)
 {
   for(int i=0; i<2; i++)
   {
     TU_ASSERT(TUSB_DESC_ENDPOINT == ep_desc->bDescriptorType &&
-              xfer_type          == ep_desc->bmAttributes.xfer, TUSB_ERROR_DESCRIPTOR_CORRUPTED);
+              xfer_type          == ep_desc->bmAttributes.xfer );
 
-    TU_ASSERT( dcd_edpt_open(rhport, ep_desc), TUSB_ERROR_DCD_OPEN_PIPE_FAILED );
+    TU_ASSERT(dcd_edpt_open(rhport, ep_desc));
 
     if ( tu_edpt_dir(ep_desc->bEndpointAddress) == TUSB_DIR_IN )
     {
@@ -644,7 +644,7 @@ tusb_error_t usbd_open_edpt_pair(uint8_t rhport, tusb_desc_endpoint_t const* ep_
     ep_desc = (tusb_desc_endpoint_t const *) tu_desc_next(ep_desc);
   }
 
-  return TUSB_ERROR_NONE;
+  return true;
 }
 
 // Helper to defer an isr function
diff --git a/src/device/usbd_pvt.h b/src/device/usbd_pvt.h
index cbe5017bb..798a871b1 100644
--- a/src/device/usbd_pvt.h
+++ b/src/device/usbd_pvt.h
@@ -70,7 +70,7 @@ void usbd_control_stall(uint8_t rhport);
 /* Helper
  *------------------------------------------------------------------*/
 // helper to parse an pair of In and Out endpoint descriptors. They must be consecutive
-tusb_error_t usbd_open_edpt_pair(uint8_t rhport, tusb_desc_endpoint_t const* p_desc_ep, uint8_t xfer_type, uint8_t* ep_out, uint8_t* ep_in);
+bool usbd_open_edpt_pair(uint8_t rhport, tusb_desc_endpoint_t const* p_desc_ep, uint8_t xfer_type, uint8_t* ep_out, uint8_t* ep_in);
 
 void usbd_defer_func( osal_task_func_t func, void* param, bool in_isr );