My test system - Neutrino 6.4 on Intel Quadcore -  has a data card in a PCI express slot. The card handles audio I/O 
over DMA and generates interrupts at a 12 kHz rate.

Now the weird part: 
When I run the driver for the card, I see (using the command "top") that the io-usb process suddenly uses 25% of the 
processor.
The strange thing is: I do not have anything connected to USB.

If I slay io-usb, the driver for my PCIe card no longer works, i.e. the interrupts don't seem to get handled anymore.
I also tried removing all USB drivers from my build file. The system boots ok, but the PCIe card no longer works (same 
behaviour as if I slay io-usb).
(The same interrupt line is also shared by the network card, but if I slay this driver, my PCIe card still works).

The driver accesses the card via the pci_attach_device() et al. functions, so I don't see any need for usb.

--> The only connection I see is that the UHCI part of the usb shares the interrupt with the PCIe card.

Could this be a problem of the io-usb driver?
Has anybody had similar experiences?

~Lorenz