Flushing Pipelines - 2022.1 English

Pipelines continue to execute as long as data is available at the input of the pipeline. If there is no data available to process, the pipeline will stall. This is shown in the following figure, where the input data valid signal goes low to indicate there is no more data. Once there is new data available to process, the pipeline will continue operation.

Figure 1. Loop Pipelining with Stall

In some cases, it is desirable to have a pipeline that can be “emptied” or “flushed.” The flush option is provided to perform this. When a pipeline is “flushed” the pipeline stops reading new inputs when none are available (as determined by a data valid signal at the start of the pipeline) but continues processing, shutting down each successive pipeline stage, until the final input has been processed through to the output of the pipeline.

The default style of pipelining implemented by Vitis HLS is defined by the config_compile -pipeline_style command. You can specify stalling pipelines (stp), or free-running flushing pipelines (frp) to be used throughout the design. You can also define a third type of flushable pipeline (flp) with the PIPELINE pragma or directive, using the enable_flush option. This option applies to the specific scope of the pragma or directive only, and does not change the global default assigned by config_compile.