Packet Stream Interfaces and Operations - 2023.2 English

Two stream types are provided to denote streaming data, consisting of packetized interleaving of several different streams. These types are listed in the following table.

A data packet consists of a one word (32-bit) packet header, followed by some number of data words where the last data word has the TLAST field denoting the end-of-packet. The following operations are used to read and advance input packet streams and write and advance output packet streams.

    int32 readincr(input_pktstream *w);
    int32 readincr(input_pktstream *w, bool &tlast);
    void writeincr(output_pktstream *w, int32 value);
    void writeincr(output_pktstream *w, int32 value, bool tlast);

The API with the TLAST argument reads or writes the end-of-packet condition, if the packet size is not fixed.

The AI Engine tools provide the built-in function writeHeader to generate a packet header for packets originating from the AI Engine kernel and writes them to the output.

    void writeHeader(output_pktstream *str, unsigned int pcktType, unsigned int ID);
    void writeHeader(output_pktstream *str, unsigned int pcktType, unsigned int ID, bool tlast);

The AI Engine tools also provide the built-in function getPacketid to get the packet ID for the packet stream interface. The index for getPacketid only applies if the packet stream feeds into a pktsplit. In this example, each AI Engine kernel output sees only one logical stream (0 for index).

    uint32_t getPacketid(input_pktstream * in, int index);
    uint32_t getPacketid(output_pktstream * out, int index);   

Change working directory to pktstream_aie. Review the AI Engine kernels (aie/aie_core1.cpp, … , aie/aie_core4.cpp). The code for aie_core1 (aie/aie_core1.cpp) is as follows.

	void aie_core1(input_pktstream *in,output_pktstream *out){
		readincr(in);//read header and discard
		uint32 ID=getPacketid(out,0);//for output pktstream
		writeHeader(out,pktType,ID); //Generate header for output
	
		bool tlast;
		for(int i=0;i<8;i++){
			int32 tmp=readincr(in,tlast);
			tmp+=1;
			writeincr(out,tmp,i==7);//TLAST=1 for last word
		}
	}

It can be seen that the input packet header is discarded. The output header is generated by writeHeader, and the packet ID for the header is obtained by getPacketid. TLAST equals 1 for the last word in the packet.