Streaming Kernel Coding Guidelines - 2020.2 English

Vitis Unified Software Platform Documentation: Application Acceleration Development (UG1393)

Document ID
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2020.2 English

In a kernel, the streaming interface directly sending or receiving data to another kernel streaming interface is defined by hls::stream with the ap_axiu<D,0,0,0> data type. The ap_axiu<D,0,0,0> data type requires the use of the ap_axi_sdata.h header file.

Important: Host-to-kernel and kernel-to-host streaming requires the use of the qdma_axis data type. Both the ap_axiu and qdma_axis data types are defined inside the ap_axi_sdata.h header file that is distributed with the Vitis software platform installation.

The following example shows the streaming interfaces of the producer and consumer kernels.

// Producer kernel - provides output as a data stream
// The example kernel code does not show any other inputs or outputs.

void kernel1 (.... , hls::stream<ap_axiu<32, 0, 0, 0> >& stream_out) {
  for(int i = 0; i < ...; i++) {
    int a = ...... ;         // Internally generated data
    ap_axiu<32, 0, 0, 0> v;  // temporary storage for ap_axiu = a;              // Writing the data
    stream_out.write(v);         // Writing to the output stream.
// Consumer kernel - reads data stream as input
// The example kernel code does not show any other inputs or outputs.

void kernel2 (hls::stream<ap_axiu<32, 0, 0, 0> >& stream_in, .... ) {
  for(int i = 0; i < ....; i++) {
    ap_axiu<32, 0, 0, 0> v =; // Reading the input stream
    int a =; // Extract the data
    // Do further processing

Because the hls::stream data type is defined, the Vitis HLS tool infers axis interfaces. The following INTERFACE pragmas are shown as an example, but are not added to the code.

#pragma HLS INTERFACE axis port=stream_out
#pragma HLS INTERFACE axis port=stream_in
Tip: These example kernels show the definition of the streaming input/output ports in the kernel signature, and the handling of the input/output stream in the kernel code. The connection of kernel1 to kernel2 must be defined during the kernel linking process as described in Specifying Streaming Connections between Compute Units.

For more information on mapping streaming connections, refer to Building and Running the Application.