Communication Infrastructure

1. The 3 interconnection networks

The TSAR architecture defines three logically independent VCI compliant networks, that are fully separated for dead-lock prevention :

• The Direct Network implements the 40 bits TSAR physical address space that is visible by the software. It transports the direct READ, WRITE, LL, & SC transactions from any VCI initiator (typically a L1 cache controller or another hardware coprocessor with a DMA capability) to any VCI target (typically a memory cache controller, or a memory mapped peripheral).

• The Coherence Network implements a separated 40 bits physical address space, used to transport the coherence transactions : MULTI_UPDATE, MULTI_INVAL, BROADCAST_INVAL (from memory cache controllers to L1 cache controllers) and CLEANUP (from the L1 cache controllers to the memory cache controllers). This address space is not visible by the software.

• The External Network implements a 34 bits physical address space.This network transports the PUT and GET transactions from the memory cache controller to the external RAM controller, in case of MISS or cache line replacement in the memory cache. This address space is not visible by the software.

2. VCI initiators & targets indexing

As a given hardware component can have several VCI ports (for example the L1 cache has two VCI initiator ports : one port to the direct network, and one port to the coherence network), each VCI port should - in principle - have a different identifier. In the specific case of the TSAR architecture, a VCI target or initiator component will never have several VCI port of the same type (Initiator type or Target type) on the same network. Therefore, each hardware component has – for sake of simplicity - an absolute identifier that is defined by three indexes :

• X_ID is the cluster X-coordinate.
• Y_ID is the cluster Y-coordinate.
• L_ID is the local index inside the cluster.

The X_ID, Y_ID and L_ID are coded on NX, NY, NL bits respectively. NX, NY and NL are global parameters for the TSAR architecture, but NX & NY cannot be larger than 5 (no more than 1024 clusters), and NL cannot be larger than 4 (no more than 16 VCI hardware components per cluster).

In order to simplify the hardware implementation of the memory coherence protocol, the L_ID values are standardized for the memory cache and for the L1 caches :

COMPONENT	LOCAL_INDEX
memory cache	0000
Processor 1 (L1 cache)	0001
Processor 2 (L1 cache)	0011
Processor 3 (L1 cache)	0101
Processor 4 (L1 cache)	0111

2.1 Target identification

The target identification is required to route a command packet. For both the direct and coherence networks, a VCI target is identified by the (NX + NY + NLADR) most significant bits of the VCI ADDRESS field :

X (NX bits)

Y (NY bits)

LADR (NLADR bits)

OFFSET (40 - NX - NY - NL bits)

• According to the NUMA characteristics of the TSAR architecture, there is no transcoding of the X & Y fields, that directly define the target cluster coordinates (X_INDEX, Y_INDEX).
• The network decodes the LADR field to obtain the target LOCAL_INDEX, using a local routing table (implemented as a wired decoder in each local interconnect controller). The local routing tables and the number of bits NLADR to be decoded depend on the cluster.

2.2 Initiator identification

The initiator identification is required to route a response packet. For both the direct and coherence networks, a VCI initiator is identified by the VCI SRCID & RSRCID fields (NX + NY + NL bits) :

X_ID (NX bits)

Y_ID (NY bits)

L_ID (NL bits)

Therefore, the total SRCID width cannot be larger than 14 bits.

3. Direct Network & Coherence Network

These two networks are implemented as two independent virtual networks, in the framework of the DSPIN network on chip general infrastructure :

• The local interconnect is implemented as two physically independent local rings, and the coherence ring supports a broadcast service for single flit VCI commands.

Note : These two physically independant rings will be implemented later as one single physical ring supporting two virtual networks.

• The global interconnect is implemented as one DSPIN network, supporting two virtual sub-networks, and the coherence sub-network supports a broadcast service for single flit VCI commands.

3.1 VCI Address generation

On the direct network, the addresses are controlled by the software.

On the coherence network, the addresses are defined by the hardware with the following policy:

• In a multicast command packet from a memory cache controller to a L1 cache controller, the address is obtained by copying the target L1 cache SRCID in the MSB bits of the VCI ADDRESS (left aligned) : The L1 cache L_ID is actually used as the LADR address field. UPDATE/INVAL requests are distinguished by the bit ADDRESS[2] (0 for INVAL, 1 for UPDATE).

• In a cleanup command packet from a L1 cache controller to a memory cache controller, the address is obtained by copying the (NX + NY) MSB bits of the line address in the VCI ADDRESS field (left aligned). The 0 value for the LADR address field is used to select the memory cache.

• In a broadcast_invalidate command packet, the ADDRESS[1:0] bits must be equal to 0x3. The 20 bits ADDRESS[21:2] will be used in a future extension of the DSPIN network to define the bounding box of a limited broadcast

3.2 VCI parameters

All Hardware components connected to the direct network or to the coherence network respect the VCI/OCP communication interface.

The direct network, and the coherence network being time-multiplexed on the DSPIN infrastructure, have identical VCI parameters :

VCI Field	width
ADDRESS	40 bits
WDATA , RDATA	32 bits
PLEN	8 bits
SRCID, RSRCID	14 bits
TRDID, RTRDID	4 bits
PKTID, RPKTID	4 bits
RERROR	2 bits

The TSAR architecture uses three values for the VCI RERROR field, in order to simplify the VCI/DSPIN wrapper, and to reduce the DSPIN Write Response packet length to one flit :

RERROR	code
READ_OK	00
WRITE_OK	10
READ_ERROR	01
WRITE_ERROR	11

3.3 DSPIN Packet format

The VCI command & response packets are translated (actually serialized) to a more convenient DSPIN network format by appropriate wrappers located between the VCI initiator & target components and the DSPIN network. The DSPIN command packet width is 40 bits, and the DSPIN response packet width is 33 bits. The DSPIN interconnexion network use only the following information to route both the command and response packets to the proper destination (to a VCI target for a command packet, to a VCI initiator for a response packet) :

• For both command and response packets, the MSB bit is the EOP flag (End of Packet).
• For a command packet, the LSB bit of the first flit (called BC) define a special broadcast command packet.
• For a non broadcast packet (BC = 0), the first flit contains a 38 bits ADDRESS field that is actually an aligned 32 bits word address, and the (NX+NY+NL) MSB bits of this ADDRESS field are used to route the packet to the proper destination.
• For a broadcast packet (BC = 1), the first flit contains the XMIN, XMAX, YMIN, YMAX fiels (5 bits each), that will be used by the network to limit the broadcast.

3.3.1 DSPIN Read Command packet format

A single flit VCI Read Command packet (this includes LL packets) is translated to a 2 flits DSPIN Read Command packet :

Flit 0 :

0	----------------ADDRESS-----------------------	0
1	38	1

Flit 1 :

1	SRCID	CMD	CST	PLEN	TRDID	PKTID	reserved
1	14	2	2	8	4	4	5

3.3.2 DSPIN write Command packet format

A N flits VCI Write Command packet (this includes SC packets) is translated to a N+2 flits DSPIN Write Command packet :

Flit 0 :

0	----------------ADDRESS-----------------------	0
1	38	1

Flit 1 :

0	SRCID	CMD	CST	PLEN	TRDID	PKTID	reserved
1	14	2	2	8	4	4	5

Flit N :

1	reserved	BE	-------------WDATA----------------
1	3	4	32

3.3.3 DSPIN Broadcast Command packet format

The single flit VCI Write Broadcast is translated to a 2 flits DSPIN Broadcast Command packet. The X_ID and Y_ID fields are the source cluster coordinates. The XMIN,XMAX, YMIN, YMAX fields can be used by the network to limit the broadcast.

Flit 0 :

0	X_ID	Y_ID	TRDID	PKTID	XMI	XMA	YMI	YMA	1
1	5	5	4	4	5	5	5	5	1

Flit 1 :

1	reserved	----------------NLINE-------------------
1	5	34

3.3.4 DSPIN Read Response packet format

A N flits VCI Read Response packet is translated to a N+1 flits DSPIN Read Response packet :

Flit 0 :

0	RSRCID	reserved	RERROR	RTRDID	RPKTID
1	14	6	4	4	4

Flit 1 :

1	---------------RDATA--------------------------
1	32

3.3.5 DSPIN Write response packet format

A single flit VCI Write Response packet is translated to a single flit DSPIN Write Response packet.

Flit 0 :

1	RSRCID	reserved	RERROR	RTRDID	RPKTID
1	14	6	4	4	4

Note : This format is also used for the response packets to a broadcast command, as each VCI response packet to a broadcast command is actually a VCI response packet to a single flit write command.

4. External Network

This network has a specific topology, as the communication scheme is very peculiar: All PUT/GET transactions are from N initiators (one initiator per cluster) to one single target (the external RAM controller).

4.1 VCI parameters

The external network, that is only transporting cache lines does not use all VCI fields. The address is coded on 34 bits (it is actually a cache line index), and the data field is 64 bits, to increase the bandwidth.

VCI Field	width
ADDRESS	34 bits
WDATA , RDATA	64 bits
PLEN	unused
SRCID, RSRCID	10 bits
TRDID, RTRDID	4 bits
PKTID, RPKTID	unused
RERROR	1 bit