change tud_descriptor_string_cb() to be consistent with other descriptor callback

examples/device/cdc_msc_hid/src/usb_descriptors.c

@@ -174,30 +174,36 @@ char const* string_desc_arr [] =
   "TinyUSB HID"                  // 6: HID
 };
 
-// Invoked when received GET_STRING_DESC request
-// max_char is CFG_TUD_ENDOINT0_SIZE/2 -1, typically max_char = 31 if Endpoint0 size is 64
-// Return number of characters. Note usb string is in 16-bits unicode format
-uint8_t tud_descriptor_string_cb(uint8_t index, uint16_t* desc, uint8_t max_char)
+static uint16_t _desc_str[32];
+
+// Invoked when received GET STRING DESCRIPTOR request
+// Application return pointer to descriptor, whose contents must exist long enough for transfer to complete
+uint16_t const* tud_descriptor_string_cb(uint8_t index)
 {
+  uint8_t chr_count;
+
   if ( index == 0)
   {
-    memcpy(desc, string_desc_arr[0], 2);
-    return 1;
+    memcpy(&_desc_str[1], string_desc_arr[0], 2);
+    chr_count = 1;
   }else
   {
-    if ( !(index < sizeof(string_desc_arr)/sizeof(string_desc_arr[0])) ) return 0;
+    if ( !(index < sizeof(string_desc_arr)/sizeof(string_desc_arr[0])) ) return NULL;
 
     const char* str = string_desc_arr[index];
 
     // Cap at max char
-    uint8_t count = strlen(str);
-    if ( count > max_char ) count = max_char;
+    chr_count = strlen(str);
+    if ( chr_count > 31 ) chr_count = 31;
 
-    for(uint8_t i=0; i<count; i++)
+    for(uint8_t i=0; i<chr_count; i++)
     {
-      *desc++ = str[i];
+      _desc_str[1+i] = str[i];
     }
-
-    return count;
   }
+
+  // first byte is len, second byte is string type
+  _desc_str[0] = TUD_DESC_STR_HEADER(chr_count);
+
+  return _desc_str;
 }

examples/device/cdc_msc_hid_freertos/src/usb_descriptors.c

@@ -174,30 +174,36 @@ char const* string_desc_arr [] =
   "TinyUSB HID"                  // 6: HID
 };
 
-// Invoked when received GET_STRING_DESC request
-// max_char is CFG_TUD_ENDOINT0_SIZE/2 -1, typically max_char = 31 if Endpoint0 size is 64
-// Return number of characters. Note usb string is in 16-bits unicode format
-uint8_t tud_descriptor_string_cb(uint8_t index, uint16_t* desc, uint8_t max_char)
+static uint16_t _desc_str[32];
+
+// Invoked when received GET STRING DESCRIPTOR request
+// Application return pointer to descriptor, whose contents must exist long enough for transfer to complete
+uint16_t const* tud_descriptor_string_cb(uint8_t index)
 {
+  uint8_t chr_count;
+
   if ( index == 0)
   {
-    memcpy(desc, string_desc_arr[0], 2);
-    return 1;
+    memcpy(&_desc_str[1], string_desc_arr[0], 2);
+    chr_count = 1;
   }else
   {
-    if ( !(index < sizeof(string_desc_arr)/sizeof(string_desc_arr[0])) ) return 0;
+    if ( !(index < sizeof(string_desc_arr)/sizeof(string_desc_arr[0])) ) return NULL;
 
     const char* str = string_desc_arr[index];
 
     // Cap at max char
-    uint8_t count = strlen(str);
-    if ( count > max_char ) count = max_char;
+    chr_count = strlen(str);
+    if ( chr_count > 31 ) chr_count = 31;
 
-    for(uint8_t i=0; i<count; i++)
+    for(uint8_t i=0; i<chr_count; i++)
     {
-      *desc++ = str[i];
+      _desc_str[1+i] = str[i];
     }
-
-    return count;
   }
+
+  // first byte is len, second byte is string type
+  _desc_str[0] = TUD_DESC_STR_HEADER(chr_count);
+
+  return _desc_str;
 }

examples/device/hid_generic_inout/src/usb_descriptors.c

@@ -123,30 +123,36 @@ char const* string_desc_arr [] =
   "123456",                      // 3: Serials, should use chip ID
 };
 
-// Invoked when received GET_STRING_DESC request
-// max_char is CFG_TUD_ENDOINT0_SIZE/2 -1, typically max_char = 31 if Endpoint0 size is 64
-// Return number of characters. Note usb string is in 16-bits unicode format
-uint8_t tud_descriptor_string_cb(uint8_t index, uint16_t* desc, uint8_t max_char)
+static uint16_t _desc_str[32];
+
+// Invoked when received GET STRING DESCRIPTOR request
+// Application return pointer to descriptor, whose contents must exist long enough for transfer to complete
+uint16_t const* tud_descriptor_string_cb(uint8_t index)
 {
+  uint8_t chr_count;
+
   if ( index == 0)
   {
-    memcpy(desc, string_desc_arr[0], 2);
-    return 1;
+    memcpy(&_desc_str[1], string_desc_arr[0], 2);
+    chr_count = 1;
   }else
   {
-    if ( !(index < sizeof(string_desc_arr)/sizeof(string_desc_arr[0])) ) return 0;
+    if ( !(index < sizeof(string_desc_arr)/sizeof(string_desc_arr[0])) ) return NULL;
 
     const char* str = string_desc_arr[index];
 
     // Cap at max char
-    uint8_t count = strlen(str);
-    if ( count > max_char ) count = max_char;
+    chr_count = strlen(str);
+    if ( chr_count > 31 ) chr_count = 31;
 
-    for(uint8_t i=0; i<count; i++)
+    for(uint8_t i=0; i<chr_count; i++)
     {
-      *desc++ = str[i];
+      _desc_str[1+i] = str[i];
     }
-
-    return count;
   }
+
+  // first byte is len, second byte is string type
+  _desc_str[0] = TUD_DESC_STR_HEADER(chr_count);
+
+  return _desc_str;
 }

examples/device/msc_dual_lun/src/usb_descriptors.c

@@ -108,30 +108,36 @@ char const* string_desc_arr [] =
   "123456",                      // 3: Serials, should use chip ID
 };
 
-// Invoked when received GET_STRING_DESC request
-// max_char is CFG_TUD_ENDOINT0_SIZE/2 -1, typically max_char = 31 if Endpoint0 size is 64
-// Return number of characters. Note usb string is in 16-bits unicode format
-uint8_t tud_descriptor_string_cb(uint8_t index, uint16_t* desc, uint8_t max_char)
+static uint16_t _desc_str[32];
+
+// Invoked when received GET STRING DESCRIPTOR request
+// Application return pointer to descriptor, whose contents must exist long enough for transfer to complete
+uint16_t const* tud_descriptor_string_cb(uint8_t index)
 {
+  uint8_t chr_count;
+
   if ( index == 0)
   {
-    memcpy(desc, string_desc_arr[0], 2);
-    return 1;
+    memcpy(&_desc_str[1], string_desc_arr[0], 2);
+    chr_count = 1;
   }else
   {
-    if ( !(index < sizeof(string_desc_arr)/sizeof(string_desc_arr[0])) ) return 0;
+    if ( !(index < sizeof(string_desc_arr)/sizeof(string_desc_arr[0])) ) return NULL;
 
     const char* str = string_desc_arr[index];
 
     // Cap at max char
-    uint8_t count = strlen(str);
-    if ( count > max_char ) count = max_char;
+    chr_count = strlen(str);
+    if ( chr_count > 31 ) chr_count = 31;
 
-    for(uint8_t i=0; i<count; i++)
+    for(uint8_t i=0; i<chr_count; i++)
     {
-      *desc++ = str[i];
+      _desc_str[1+i] = str[i];
     }
-
-    return count;
   }
+
+  // first byte is len, second byte is string type
+  _desc_str[0] = TUD_DESC_STR_HEADER(chr_count);
+
+  return _desc_str;
 }

src/class/hid/hid_device.h

@@ -68,9 +68,8 @@ bool tud_hid_mouse_report(uint8_t report_id, uint8_t buttons, int8_t x, int8_t y
 // Callbacks (Weak is optional)
 //--------------------------------------------------------------------+
 
-// Invoked when received GET HID REPORT DESCRIPTOR
-// Application return pointer to descriptor
-// Descriptor contents must exist long enough for transfer to complete
+// Invoked when received GET HID REPORT DESCRIPTOR request
+// Application return pointer to descriptor, whose contents must exist long enough for transfer to complete
 uint8_t const * tud_hid_descriptor_report_cb(void);
 
 // Invoked when received GET_REPORT control request

src/common/tusb_types.h

@@ -403,10 +403,10 @@ static inline uint8_t tu_desc_len(void const* desc)
 }
 
 // Length of the string descriptors in bytes with slen characters
-#define TUD_DESC_STRLEN(_slen)      (2*(_slen) + 2)
+#define TUD_DESC_STRLEN(_chr_count)      (2*(_chr_count) + 2)
 
 // Header of string descriptors with len + string type
-#define TUD_DESC_STR_HEADER(_slen)  ( (uint16_t) ( (TUSB_DESC_STRING << 8 ) | TUD_DESC_STRLEN(_slen)) )
+#define TUD_DESC_STR_HEADER(_chr_count)  ( (uint16_t) ( (TUSB_DESC_STRING << 8 ) | TUD_DESC_STRLEN(_chr_count)) )
 
 // Convert comma-separated string to descriptor unicode format
 #define TUD_DESC_STRCONV( ... )     (const uint16_t[]) { TUD_DESC_STR_HEADER(VA_ARGS_NUM_(__VA_ARGS__)), __VA_ARGS__ }

src/device/usbd.c

@@ -577,16 +577,11 @@ static bool process_get_descriptor(uint8_t rhport, tusb_control_request_t const
         return false;
       }else
       {
-        uint16_t desc_str[CFG_TUD_ENDOINT0_SIZE/2]; // up to endpoint0 size only
-        uint8_t len = 2*tud_descriptor_string_cb(desc_index, desc_str+1, CFG_TUD_ENDOINT0_SIZE/2-1);
+        uint8_t const* desc_str = (uint8_t const*) tud_descriptor_string_cb(desc_index);
+        TU_ASSERT(desc_str);
 
-        TU_ASSERT(len > 0);
-
-        // first byte of descriptor is size, second byte is string type
-        len += 2; // header len
-        desc_str[0] = tu_u16_from_u8(TUSB_DESC_STRING, len);
-
-        return usbd_control_xfer(rhport, p_request, desc_str, len);
+        // first byte of descriptor is its size
+        return usbd_control_xfer(rhport, p_request, (void*) desc_str, desc_str[0]);
       }
     break;
 

src/device/usbd.h

@@ -63,19 +63,17 @@ bool tud_remote_wakeup(void);
 // Application Callbacks (WEAK is optional)
 //--------------------------------------------------------------------+
 
-// Invoked when received GET DEVICE DESCRIPTOR
+// Invoked when received GET DEVICE DESCRIPTOR request
 // Application return pointer to descriptor
 uint8_t const * tud_descriptor_device_cb(void);
 
-// Invoked when received GET CONFIGURATION DESCRIPTOR
-// Application return pointer to descriptor
-// Descriptor contents must exist long enough for transfer to complete
+// Invoked when received GET CONFIGURATION DESCRIPTOR request
+// Application return pointer to descriptor, whose contents must exist long enough for transfer to complete
 uint8_t const * tud_descriptor_configuration_cb(void);
 
-// Invoked when received GET STRING DESC request
-// max_char is CFG_TUD_ENDOINT0_SIZE/2 -1, typically max_char = 31 if Endpoint0 size is 64
-// Return number of characters. Note usb string is in UTF-16 format
-uint8_t tud_descriptor_string_cb(uint8_t index, uint16_t* desc, uint8_t max_char);
+// Invoked when received GET STRING DESCRIPTOR request
+// Application return pointer to descriptor, whose contents must exist long enough for transfer to complete
+uint16_t const* tud_descriptor_string_cb(uint8_t index);
 
 // Invoked when device is mounted (configured)
 ATTR_WEAK void tud_mount_cb(void);