tud_hid_descriptor_report_cb() does not provide the length of the report descriptor

[dhalbert]

The HID report descriptor is supplied dynamically by tud_hid_descriptor_report_cb(), but the length is not passed back. The report descriptor does not include its own length. The length has to have been set previously in p_hid->report_desc_len. It is set in hidd_open(), since the length must be set in the HID descriptor (not the HID report descriptor).

I am not sure what the utility of this callback is, then. You could substitute an HID Report Descriptor of identical length, but that's all. Maybe I am missing something.

[hathach]

The callback does not include the length on purpose. The length is already decided by hid descriptor (after hid interface) as part of configuration descriptor. By the time tud_descriptor_configuration_cb() is called, you shouldn't change your report descriptor (or it has to be the same length). I.e all dynamic hid config must be done before that.

[hathach]

I have done some dynamic descriptors for arduino as well, you may want to take a look at

• Core https://github.com/adafruit/Adafruit_TinyUSB_ArduinoCore/blob/master/Adafruit_USBD_Device.cpp#L116
• lib https://github.com/adafruit/Adafruit_TinyUSB_Arduino/blob/master/src/Adafruit_USBD_HID.cpp#L73

[hathach]

@dhalbert Let me know if you still have question, or API does not make sense to you.