Streams

LiteX's streams are group of signals (Migen's Record) used to create Sink/Source Endpoints on Modules and easily transfer data between them. LiteX's streams are a simplified compromise between AXI-4/Avalon-ST:

• valid: similar to AXI-4's tvalid.
• ready: similar to AXI-4's tready.
• first (used for packets only when needed): similar to Avalon-ST's sop, because it's sometimes useful to just easily know the start of a packet instead of deducing it from last as done in AXI-4.
• last (used for packets) : similar to AXI-4's last.
• payload: a Record with its own layout, similar to AXI-4's tdata that can evolve at each valid/ready transaction.
• param (optional): a Record with its own layout, similar to AXI-4's tuser that can evolve at each start of packet.

LiteX's streams can be directly mapped to AXI-4 and adapted easily to AvalonST with: https://github.com/enjoy-digital/litex/blob/master/litex/soc/interconnect/avalon.py.

alanvgreen has provided a thorough description of LiteX streams and a helpful diagram.