Why is there an ACP interface for many ARM processors?

Dear sirs,

I read ACE specification and ARM processor documents for ACP explanation. I always have some questions about ACP.

As soon as you know, ACP exists in SCU for data coherency.

Q1: The document says that ACP usually connects to a DMA or cryptographic engine, so why does it connect to them?

Q2: who starts ACP port to transfer data from where to where?

Q3: How does ACP implement coherency in multi-core processor?

Thanks.

Cray