The kernel/spec.json specification generatees the s2mm_mp kernel. Review this file again. Notice that the s2mm_mp kernel has an implementation StoreStreamToMaster which moves data from AXI-Streams to DDR. The s2mm_mp kernel has four channels: k0,k1,k2, and k3. Each stream writes the data coming from the tx0-tx3 streams to a DDR buffer.