How to temporarily disable MMU in EL3?

I am using Trusted Firmware provided by Linaro on Juno Board R1.

Now I want to disable MMU in EL3 and do some custom jobs, and then enable MMU again. As I don't want to disturb other codes in EL3, I guess I have to clear the instruction cache, data cache, TLB. But I always fail to do this. After I disabled MMU and cleared the caches, the system halts.

Is there anything I missed?

Thanks very much for your help!

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