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Flits

We start with the flits, also called flow control units, which is the smallest unit of data that is sent from node to another (e.g. from router to router, of network interface to router). Flits consists of two types of data:

  1. Routing information: Also called the header of the flit, this data is used by the routers to route the flit through the network. The header contains information such as the destination address, the source address, the flit type, etc.

  2. Payload: This is the actual data that is being sent from one node to another. Router do not look at the payload, they just forward it to the next hop.

In traditional NoCs, the links have a fixed width (e.g. 32-bit). Usually, the payload and the information required for routing far exceeds this width. Therefore, the payload is split into multiple data flits and a header flit which encodes the routing information. Based on this information, routers in the network then know how to route the following data flits to the destination.

In modern NoCs, this way of serializing payloads over the network is not very efficient anymore. With higher bandwidth requirements of the endpoints, the serialization becomes more and more a bottleneck, since the frequency of the links is limited. Further, serialization causes additional latency which is undesirable. Lastly, the overhead of the header flits is not negligible, especially when the payload is small.

Therefore, In FlooNoC and modern NoCs in general1, another approach is taken, which differs in two ways:

  1. Flit width: The width of the flits is not fixed anymore. Instead, the flits can be as wide as the payload to send the payload in a single cycle resp. in a single flit.

  2. Parallel header: Instead of sending the header before the payload, the header is sent in parallel to the payload. This way, the link utilization is not degraded by header flits.

Below, we will discuss the header and the payload in more detail.

In FlooNoC the header consists of the following fields:

Field Type Required by Description
dst_id dst_t or route_t Router The destination ID of the flit. In the case of source-based routing, it encodes the entire route from source to destination.
src_id dst_t Chimney The source ID of the flit. Used by the Chimney to set the dst_id for the response
last logic Router Can be used by the Chimneys that a burst of flits is not interleaved with other flits from other endpoints. If not set the router performs wormhole routing i.e. it locks the direction until flit with the last bit set has been sent.
axi_ch axi_ch_e2 Chimney Used by the Chimney to encode the type of packet e.g. which AXI channel
rob_req logic Chimney Flag to signal that the flit might need to be reordered in the Chimney
rob_idx rob_idx_t Chimney The index into the reorder buffer of the Chimney
atop logic Chimney Flag to signal that the flit is an ATOP flit and can bypass the reodering logic.

Additional fields

Additional fields can be added to the header if needed. The routers just ignore those fields and forward them to the next router/Chimney.

SystemVerilog macros

FlooNoC provides System Verilog macros in typedef.svh to generate the header fields such as FLOO_TYPEDEF_HDR_T.

Payload

The payload itself can be any data that needs to be sent from one node to another. The payload is not interpreted by the routers, they just forward it to the next hop. The payload can be of any width. Usually, the payload consists of an AXI request or response of one of the 5 channels.

Types of flits

The entire flit is constructed by concatenating the header and the payload. One can define many types of flits i.e. one for every payload. The only requirement is that the flit includes the following fields:

Field Type Description
hdr hdr_t The header of the flit, which is identical across the whole network
payload payload_t3 The payload of the flit, which can be of any width
rsvd logic[RsvdBits-1:0] Optional padding bits if the flit is smaller than the link width

Additional fields

Again, additional fields can be added if needed. The only requirement is that the width of the flit matches exactly the width of the link.

SystemVerilog macros

FlooNoC provides System Verilog macros in typedef.svh to generate the flits such as FLOO_TYPEDEF_FLIT_T for flits with the rsvd field and generic flits FLOO_TYPEDEF_GENERIC_FLIT_T without a rsvd field.

  1. The AMBA CHI protocol also encodes the entire payload (cachelines in this case) in a single flit including all the information required for routing. ↩

  2. The type is arbitrary and depends on the type of Chimney used but it is usually an enum. For instance, the single-AXI chimney encodes only 5 channels in axi_ch_e, while the narrow-wide AXI chimney encodes 10 channels in nw_ch_e. This can also be extended. ↩

  3. The payload type can be anything. For instance, sending an AXI AW is done by defining a axi_aw_chan_t struct for the payload type. ↩