Can not find usb disk in u-boot

Hi ,
I update u-boot to b82f09c3, and now I can enter u-boot console by CTRL-C.
I want to load kernl form usb-disk, but after run usb reset command, it seems can not find any USB storage. I have tried on all 3 USB ports.How can I detect USB storage device in u-boot?
log as bellow:
=> usb reset
resetting USB…
USB0: Register 2000140 NbrPorts 2
Starting the controller
scanning bus 0 for devices… 1 USB Device(s) found
scanning usb for storage devices… 0 Storage Device(s) found