When there is an interface association descriptor IAD, where multiple interface descriptors belonging to a single function are added to the configuration descriptor bundle using one stuct usb_interface element, the bNumInterfaces is incorrect.

In the current usb_standard.c code, it is assumed that each struct usb_interface adds just one logical interface (one interface having multiple altsettings still counts as one). This assumption is incorrect for the above situation.

An example of such a device class is cdc_acm, which has a 'COMM' interface and a 'DATA' interface, which are bundled using IAD. It still makes sense to have bNumInterfaces act as a nr_elements (array_size) for the interface member of the usb_config_descriptor, but the value needs to be adjusted on output (in build_config_descriptor, just like is done with wTotalLength) when such IAD is encountered.

A workaround is currently to add the COMM and the DATA interface in separate struct usb_interface's, where the first one has the IAD and the second one doesn't. This seems counter-intuitive.