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Can't get an interface when device class is 0xFF

I wrote a glob of code, understanding very little and riffing off the sample code in the documentation, which works fine when our USB device is declared to have device class 0x00.

Our USB engineer has recently come to the conclusion that we really should be using device class 0xFF. When we do, I can't succeed in getting an "interface" for the device.

It's a Cypress device. When the firmware we download into it has

DeviceDscr:   
                db   DSCR_DEVICE_LEN      ;; Descriptor length
                db   DSCR_DEVICE   ;; Decriptor type
                dw   0002H      ;; Specification Version (BCD)
                db   000H               ;; Device class
                db   000H               ;; Device sub-class
                db   000H               ;; Device sub-sub-class
                db   64         ;; Maximum packet size
                dw   07311H      ;; Vendor ID (ECRM)
                dw   0A1ECH      ;; Product ID (ECRM Device)
                dw   FW_VERSION  ;; Product version ID
                db   1         ;; Manufacturer string index
                db   2         ;; Product string index
                db   0         ;; Serial number string index
                db   1         ;; Number of configurations

;; Interface Descriptor
                db   DSCR_INTRFC_LEN      ;; Descriptor length
                db   DSCR_INTRFC         ;; Descriptor type
                db   0               ;; Zero-based index of this interface
                db   0               ;; Alternate setting
                db   5               ;; Number of end points
                db   0ffH                               ;; Interface class
                db   0ffH                               ;; Interface sub class
                db   0ffH               ;; Interface sub sub class
                db   0               ;; Interface descriptor string index

everything works fine. If we change the "Device class" to 0ffH, I can't access it.

In both cases, whether the device class is 000H or 0ffH, I get about 98% there. I:

--detect that our ECRM device is not on the bus
--acquire the Cypress device and download our firmware into it, causing a new ECRM device to magically appear on the bus
--create a matching dictionary:
    matchingDictionary = IOServiceMatching(kIOUSBDeviceClassName);
    CFDictionarySetValue(matchingDictionary,
        CFSTR(kUSBVendorID),
        CFNumberCreate(kCFAllocatorDefault, kCFNumberSInt32Type, &vendor));
    CFDictionarySetValue(matchingDictionary,
        CFSTR(kUSBProductID),
        CFNumberCreate(kCFAllocatorDefault, kCFNumberSInt32Type, &product));
--successfully call IOServiceGetMatchingService on the dictionary and obtain a valid io_service_t
--successfully call IOCreatePlugInInterfaceForService on that value
--successfully obtain a IOUSBDeviceInterface182 for the device
--fill out an IOUSBFindInterfaceRequest structure:
                memset(&request, 0, sizeof(request)); // superstition
                request.bInterfaceClass = kIOUSBFindInterfaceDontCare;
                request.bInterfaceSubClass = kIOUSBFindInterfaceDontCare;
                request.bInterfaceProtocol = kIOUSBFindInterfaceDontCare;
                request.bAlternateSetting = kIOUSBFindInterfaceDontCare;
--successfully call CreateInterfaceIterator(dev, &request, &iterator) and obtain a valid, nonzero iterator.

Where the two situations differ is at the point where I try to obtain an io_service_t for an interface: usbInterface = IOIteratorNext(iterator).

When the device class is 00H, I obtain a valid usbInterface and I'm off to the races and everything's fine.

When the device class is 0FFH, the first call to IOIteratorNext(iterator) returns 0, i.e. it doesn't find any interfaces.

Since I'm successfully get to the "device" itself, the API can't be objecting too strenuously to the device class being 0xFF. So, why won't the "device" give me an "interface?"

I've looked all over for somewhere, anywhere in the API where I can tell it that the device class, like the interface class, should be a "DontCare," or else a way of I can tell it explicitly to expect the device class to be 0xFF, but I can't find anything and can't find documentation that discusses this.

(It's particular annoying since my Windows-coding colleague, whose level of understanding is about the same as mine, finds that under Windows the 0xFF device class "just works" with no changes needed on his part...)

 
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