From the usb(4) man page:

The flags argument to the usb device affects the order in which the
device detection happens during cold boot.  Normally, only the USB host
controller and the usb device are detected during the autoconfiguration
when the machine is booted.  The rest of the devices are detected once
the system becomes functional and the kernel thread for the usb device is
started.  Sometimes it is desirable to have a device detected early in
the boot process, e.g., the console keyboard.  To achieve this use a
flags value of 1.




ctribo@dtcc.edu wrote:
>>Number:         28582
>>Category:       kern
>>Synopsis:       root device prompt blocks USB thread and doesn't accept entry from a USB keyboard
>>Confidential:   no
>>Severity:       serious
>>Priority:       medium
>>Responsible:    kern-bug-people
>>State:          open
>>Class:          sw-bug
>>Submitter-Id:   net
>>Arrival-Date:   Wed Dec 08 00:15:00 +0000 2004
>>Originator:     Chris Tribo
>>Release:        2.0
>>Organization:
>>Environment:
> 
> NetBSD columbia.dtcc.edu 2.0 NetBSD 2.0 (GENERIC) #0: Wed Dec  1 17:45:15 EST 2004  root@columbia.dtcc.edu:/usr/obj/sys/arch/i386/compile/GENERIC i386
> 
>>Description:
> 
> When using a USB keyboard, and the kernel prompts you for a root device typing on the usb keyboard does nothing. If I unplug it and plug it back in nothing happens. However if I plug a PS/2 keyboard in and fill out the root device, dump device and filesystem information. When I hit enter the USB events from me unplugging and plugging the keyboard back in fire and then it attempts to mount the file system.
> 
>>How-To-Repeat:
> 
> Boot with a USB keyboard to a kernel that prompts you for the root device and attempt to type it in.
> 
>>Fix:
> 
> My guess is that something is spl'ing itself higher than the USB code when it prompts for a root device.
>