[U-Boot] usb host problem

hello all I am working on access files from USB disk . The u-boot has implementing a mass storage operation with USB OHCI. I have added the porting code. All things seems right except MMU.When the MMU is closed,the result is right.But if I turn on the MMU,the returned message is not right. I think maybe the MMU has affected USB host,even I assigned HCCA in no-cached area. What should I do?Does anybody can give me some suggestion?Thanks in advance! My Regards zzwuyu