Coding Guidelines for Free-Running Kernels - 2022.1 English

As mentioned previously, the free-running kernel only contains hls::stream inputs and outputs. The recommended coding guidelines include:

• Use hls::stream<ap_axiu<D,0,0,0> > if the port is interacting with another stream port from the kernel.
• Use hls::stream<qdma_axis<D,0,0,0> > if the port is interacting with the host.
• Use the hls::stream data type for the function parameter causes Vitis HLS to infer an AXI4-Stream port (axis) for the interface.
• The free-running kernel must also specify the following special INTERFACE pragma.

  #pragma HLS interface ap_ctrl_none port=return

The following code example shows a free-running kernel with one input and one output communicating with another kernel.

void kernel_top(hls::stream<ap_axiu<32, 0, 0, 0> >& input, 
   hls::stream<ap_axiu<32, 0, 0, 0> >& output) {
#pragma HLS interface ap_ctrl_none port=return  // Pragma for free-running kernel
#pragma HLS DATAFLOW // The kernel is using DATAFLOW optimization
   ...
}