Coding Guidelines for Free-Running Kernels - 2021.2 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. The while(1) loop structure contains the substance of the kernel code, which repeats as long as the kernel runs.

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  // Special pragma for free-running kernel
 
#pragma HLS DATAFLOW // The kernel is using DATAFLOW optimization
	while(1) {
		...
	}
}