In message 50c9a22505081501124196fdd4@mail.gmail.com you wrote:

i see in the cmd_usb, it seems there is no cmd to support it?

No command to support what?

does it supprted by u-boot now or just need to modify cmd_usb.c?

Maybe you can rephrase your question so I can parse it?

What exactly are you looking for?

USB is just a protocol to attach external devices, so depending on which sort of devices this is there may be methods to download a kernel. For example, if it's a USM memory stick, then you will probably have a file system on it, and you can use one of the file system access methods to list or load a file.

Best regards,

Wolfgang Denk