I recently ran into the problem that the kernel was asking me for the root device and I could not enter anything via the USB keyboard. I could scroll up and down (via left-shift-page-up/down), but could not respond to the kernel's question.
ws@ saved by day by suggesting to enter autoconfiguration and disable all [oue]hci controllers. The problem seems to be that the kernel has already initialized the USB controllers, thus disabled the PS/2 emulation, yet isn't ready to actually read from the USB keyboard. The machine doesn't have legacy ports and at that stage had a root-elegible autoconfigurable RAIDframe set it couldn't boot from, so without Wolfgang's advice, I would really have been seriously stuck. It would never have come to my mind that I would need to disable USB controllers in order to make a USB keyboard work. Can there anything be done about this?
