Coding Guidelines for Free-Running Kernels - 2022.1 English

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

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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
Tip: ap_ctrl_none means there is no control interface for the kernel so typically there is no s_axilite interface generated. However, the presence of either scalar arguments or m_axi interfaces requires the use of an s_axilite interface.

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
Tip: The example shows the definition of the streaming input/output ports in a free-running kernel. However, the streaming connection from the free-running kernel to or from another kernel must be defined during the kernel linking process as described in Specifying Streaming Connections between Compute Units.