OpenExSys_NoC/doc/router_spec.md

Router Spec

1 Routing Algorithm

Use XY routing, the routing result is calculated one hop ahead, which is look-ahead routing

1.1 XY Routing

Flits firstly go through X axis, then go though Y axis:

  1st phase: Assign next address
  2nd phase: Define new Next-port

1.2 Look-ahead Routing

2 Input Port

2.1 Input VC

1. Use Fixed VC Assignment with Dynamic VC Allocation (FVADA) paper vc allocation mechanism. The input VCs are associated with the flits' output port.
2. XY routing, so some of the output port are not used in some input ports.

• VC in each input port:

input port	input port id	vc id	assigned output port	output port id
N	0	0	S	1
		1	L	4
S	1	0	N	0
		1	L	4
E	2	0	N	0
		1	S	1
		2	W	3
		3	L	4
W	3	0	N	0
		1	S	1
		2	E	2
		3	L	4
L	4	0	N	0
		1	S	1
		2	E	2
		3	W	3

3 Switch Allocation

Use two-stage input-port-first allocation.

3.1 Local allocation

Use round robin arbition. As XY routing, the local arbiters' input port number are not the same.

3.2 Global allocation

Use round robin arbition. As XY routing, the global arbiters' input port number are not the same.

• input output port mapping for each SA global arbiter:

output port	global SA arbiter connected input ports	global SA arbiter connected input ports id	next hop input port
N	S	1	S
	E	2
	W	3
	L	4
S	N	0	N
	E	2
	W	3
	L	4
E	W	3	W
	L	4
W	E	2	E
	L	4
L	N	0	-
	S	1
	E	2
	W	3

4 QoS

4.1 Common QoS

No special vc, all vc head flits ranked by QoS value.

• no extra real-time vc;
• no bypass arbiter for real-time vc;
• all vc involve QoS value compare.

4.2 Common QoS + extra real-time vc QoS

Add special vc for highest priority flits, all vc head flits ranked by QoS value.

• have extra real-time vc;
• no bypass arbiter for real-time vc;
• the real-time vc always win the local sa, and the local sa rr point should not be updated
• the real-time vc join global sa as common QoS, as it has highest QoS value, it can beat flits with other QoS value;
• all vc involve QoS value compare.

4.3 extra real-time vc QoS + bypass switch allocation + no common QoS

Add special vc for highest priority flits, other vc head flits have no QoS support.

• have extra real-time vc;
• have bypass arbiter for real-time vc;
• non-highest-priority vc have no QoS support.
• the real-time vc bypass local sa, and override common rr arbiter's result;
• the real-time vc from all possible inport use a separate rr arbiter at per global sa, and override common rr arbiter's result;

It can have the same real-time support as 4.2, but may have better timing.