USB Composite Device Interface Specification

The NKS4 coprocessor exposes itself to the Host as a single physical USB 2.0 High-Speed composite device. The hardware platform registers a single Vendor ID (VID) and Product ID (PID) pair and presents three independent logical interfaces to the Host operating system.

Device Descriptor Specification

To ensure proper device enumeration and driver binding by the Host's kernel, the composite device descriptor must conform to the following schema:

Descriptor Field	Hexadecimal Value	System Specification
idVendor	0x0944	Designated Vendor ID for Korg Inc.
idProduct	0x1005	Designated Product ID for NKS4 Coprocessor
bDeviceClass	0x00	Class specification delegated to the interface descriptors
bNumConfigurations	0x01	Single logical configuration supported
bInterfaceSubClass	0xFF	Vendor-specific subclass (mandatory for Host driver matching)

Driver Matching Constraints

The Host's panel control driver OmapNKS4Module.ko implements a kernel usb_device_id match configuration using a match flags mask of 0x0383 (USB_DEVICE_ID_MATCH_VENDOR | USB_DEVICE_ID_MATCH_PRODUCT | USB_DEVICE_ID_MATCH_INT_CLASS | USB_DEVICE_ID_MATCH_INT_SUBCLASS). Consequently, the device's panel interface descriptor must explicitly populate bInterfaceSubClass with 0xFF and bInterfaceProtocol with 0xFF to satisfy the kernel's match filter and trigger the OmapNKS4Probe driver probe routine.

Co-Resident Interface Allocation

The composite device distributes its features across three interfaces. Different kernel drivers bind to these interfaces simultaneously:

• Interface 0: Panel Control & Register Interface, claimed by OmapNKS4Module.ko.
• Interface 1: Multichannel Isochronous Digital Audio Interface, claimed by KorgUsbAudioDriver.ko via usb_driver_claim_interface().
• Interface 2: Asynchronous Bulk USB MIDI Interface, claimed by KorgUsbAudioDriver.ko.

graph LR
    subgraph "USB Device 0x0944:0x1005"
        IF0["Interface 0<br/>Panel (Vendor-specific, SubClass=0xFF)<br/>OmapNKS4Module.ko"]
        IF1["Interface 1<br/>Audio (Isochronous)<br/>KorgUsbAudioDriver.ko"]
        IF2["Interface 2<br/>MIDI (Bulk)<br/>KorgUsbAudioDriver.ko"]
    end

    IF0 --- EP1["EP1 IN<br/>Interrupt<br/>32 bytes, 1ms"]
    IF0 --- EP2["EP2 OUT<br/>Bulk<br/>512 bytes max"]
    IF0 --- EP7["EP3 IN (Panel BULK)<br/>Bulk<br/>512 bytes max"]
    IF1 --- EP3["EP4 OUT<br/>Isochronous<br/>Audio Output"]
    IF1 --- EP4["EP5 IN<br/>Isochronous<br/>Audio Input"]
    IF2 --- EP5["EP6 OUT<br/>Bulk<br/>MIDI Out"]
    IF2 --- EP6["EP7 IN<br/>Bulk<br/>MIDI In"]

Interface Endpoint Topology

The composite layout defines endpoints to handle control packets, real-time events, audio streams, and MIDI frames.

While the endpoint addresses for Interface 1 (Audio) and Interface 2 (MIDI) are dynamically discovered via standard USB descriptor queries, Interface 0 (Panel) enforces rigid endpoint properties to prevent Host driver URB allocation faults:

Interface Rank	Endpoint Address	Transfer Direction	Transfer Type	Maximum Packet Size	Polling Interval	Functional Specification
0	EP1	IN	Interrupt	32 Bytes	1 ms	System clock ticks and panel controller events
0	EP2	OUT	Bulk	512 Bytes	N/A	Configuration commands and raw pixel stream
0	EP3	IN	Bulk	512 Bytes	N/A	Register-read responses (NV2AC / Atmel checks)
1	EP4	OUT	Isochronous	1024 Bytes	125 µs	High-bandwidth digital audio output (DAC stream)1
1	EP5	IN	Isochronous	1024 Bytes	125 µs	High-bandwidth digital audio input (ADC stream)
2	EP6	OUT	Bulk	64 Bytes	N/A	External device MIDI Out packets
2	EP7	IN	Bulk	64 Bytes	N/A	External device MIDI In packets

---

1. The audio output requires a transfer of 4608 bytes per audio period (256 frames × 6 channels × 3 bytes). Under USB High-Speed parameters, this data block is split across multiple 125 µs microframes, utilizing high-bandwidth Isochronous endpoints supporting up to 1024 bytes per packet with multiple transactions per microframe (up to 3 × 1024 bytes). ↩