Specifying Streaming Connections - 2023.1 English

Support for hardware accelerator pipelines that communicate through streams is one of the major advantages of FPGAs, FPGA-based SoCs, and Versal adaptive SoC devices, and have been used in DSP and image processing applications, as well as in communication systems. Kernel ports involved in streaming are defined within the kernel, and are not addressed by the host program. There is no need to send data back to global memory before it is forwarded to another kernel for processing. The connections between the kernels are directly defined during the v++ linking process as described below.

A streaming data output port of one kernel can be connected to the streaming data input port of another kernel, or between a PL kernel and the PLIO of an ADF graph application, during linking using the --connectivity.sc option. This option can be specified at the command line, or from a config file that is specified using the --config option, as described in v++ Command.

To connect the streaming output port of a producer kernel to the streaming input port of a consumer kernel, set up the connection in the v++ config file using the connectivity.stream_connect option as follows:

[connectivity]
#stream_connect=<cu_name>.<output_port>:<cu_name>.<input_port>:[<fifo_depth>]
stream_connect=vadd_1.stream_out:vadd_2.stream_in
stream_connect=vadd_2.stream_in:ai_engine_0.DataIn0

Where:

• <cu_name> is an instance name of the CU as determined by the connectivity.nk option, described in Creating Multiple Instances of a Kernel. Note that the cu_name can be specified in the config file as described in Creating Multiple Instances of a Kernel, or is defined automatically by the tool when not otherwise specified.
  Tip: When specifying connections to Versal AI Engine ports, the <cu_name> will be ai_engine_0 as shown above.
• <output_port> or <input_port> is the streaming port defined in the producer or consumer kernel.
  Important: If the port-width of the output and input ports do not match, the Vitis compiler will automatically insert a data-width converter between the two ports as part of the build process. The inclusion of the data-width converter will either truncate a larger bit-width output to a smaller bit-width input, or expand a smaller bit-width to a larger bit-width.
• [:<fifo_depth>] inserts a FIFO of the specified depth between the two streaming ports to prevent stalls. The value is specified as an integer.