diff --git a/examples/device/cdc_msc_hid/src/main.c b/examples/device/cdc_msc_hid/src/main.c
index 1a954f21..f743026c 100644
--- a/examples/device/cdc_msc_hid/src/main.c
+++ b/examples/device/cdc_msc_hid/src/main.c
@@ -228,6 +228,9 @@ void hid_task(void)
   }
 }
 
+// Invoked when received GET_REPORT control request
+// Application must fill buffer report's content and return its length.
+// Return zero will cause the stack to STALL request
 uint16_t tud_hid_get_report_cb(uint8_t report_id, hid_report_type_t report_type, uint8_t* buffer, uint16_t reqlen)
 {
   // TODO not Implemented
@@ -239,6 +242,8 @@ uint16_t tud_hid_get_report_cb(uint8_t report_id, hid_report_type_t report_type,
   return 0;
 }
 
+// Invoked when received SET_REPORT control request or
+// received data on OUT endpoint ( Report ID = 0, Type = 0 )
 void tud_hid_set_report_cb(uint8_t report_id, hid_report_type_t report_type, uint8_t const* buffer, uint16_t bufsize)
 {
   // TODO not Implemented
diff --git a/examples/device/cdc_msc_hid_freertos/src/main.c b/examples/device/cdc_msc_hid_freertos/src/main.c
index e4471242..b912c23e 100644
--- a/examples/device/cdc_msc_hid_freertos/src/main.c
+++ b/examples/device/cdc_msc_hid_freertos/src/main.c
@@ -259,6 +259,9 @@ void hid_task(void* params)
   }
 }
 
+// Invoked when received GET_REPORT control request
+// Application must fill buffer report's content and return its length.
+// Return zero will cause the stack to STALL request
 uint16_t tud_hid_get_report_cb(uint8_t report_id, hid_report_type_t report_type, uint8_t* buffer, uint16_t reqlen)
 {
   // TODO not Implemented
@@ -270,6 +273,8 @@ uint16_t tud_hid_get_report_cb(uint8_t report_id, hid_report_type_t report_type,
   return 0;
 }
 
+// Invoked when received SET_REPORT control request or
+// received data on OUT endpoint ( Report ID = 0, Type = 0 )
 void tud_hid_set_report_cb(uint8_t report_id, hid_report_type_t report_type, uint8_t const* buffer, uint16_t bufsize)
 {
   // TODO not Implemented
diff --git a/examples/device/hid_generic_inout/src/main.c b/examples/device/hid_generic_inout/src/main.c
index c8926fc1..abe31523 100644
--- a/examples/device/hid_generic_inout/src/main.c
+++ b/examples/device/hid_generic_inout/src/main.c
@@ -106,14 +106,6 @@ void tud_resume_cb(void)
 //--------------------------------------------------------------------+
 // USB HID
 //--------------------------------------------------------------------+
-#if CFG_TUD_HID
-
-// Must match with ID declared by HID Report Descriptor, better to be in header file
-enum
-{
-  REPORT_ID_KEYBOARD = 1,
-  REPORT_ID_MOUSE
-};
 
 void hid_task(void)
 {
@@ -172,6 +164,9 @@ void hid_task(void)
   }
 }
 
+// Invoked when received GET_REPORT control request
+// Application must fill buffer report's content and return its length.
+// Return zero will cause the stack to STALL request
 uint16_t tud_hid_get_report_cb(uint8_t report_id, hid_report_type_t report_type, uint8_t* buffer, uint16_t reqlen)
 {
   // TODO not Implemented
@@ -183,6 +178,8 @@ uint16_t tud_hid_get_report_cb(uint8_t report_id, hid_report_type_t report_type,
   return 0;
 }
 
+// Invoked when received SET_REPORT control request or
+// received data on OUT endpoint ( Report ID = 0, Type = 0 )
 void tud_hid_set_report_cb(uint8_t report_id, hid_report_type_t report_type, uint8_t const* buffer, uint16_t bufsize)
 {
   // TODO not Implemented
@@ -192,8 +189,6 @@ void tud_hid_set_report_cb(uint8_t report_id, hid_report_type_t report_type, uin
   (void) bufsize;
 }
 
-#endif
-
 //--------------------------------------------------------------------+
 // BLINKING TASK
 //--------------------------------------------------------------------+