Two Types Of Double-Hosted Chains

Two Types Of Double-Hosted Chains

There are two basic arrangements for double-hosted chains: sharing and non-sharin g.

Sharing Double-Hosted Chain

In a sharing double-hosted chain, traffic is allowed to flow from end to end. Either host may target any of the devices in the chain, including the other host. In this arrangement, one host is the master host bridge and the other is the slave host bridge. The determination about which host is master or slave is not defined in the specification, but must be defined before reset occurs. Most likely, the system board layout will determine master/slave host bridges ” possibly through a strapping option on the motherboard. Figure 17-4 on page 431 depicts a sharing double-hosted chain with master and slave host bridges.

Figure 17-4. Sharing Double-Hosted Chain With Master/Slave Host Bridges

If Possible, Assign All Devices To Master Host Bridge

The HyperTransport specification recommends that all resources in a sharing double-hosted chain be assigned to the master host bridge if possible; this eliminates a potential deadlock condition in peer-to-peer transactions. The Slave Command Register Master Host and Default Direction bits in PCI configuration space are used to program tunnel devices with the information needed to recognize the "upstream vs. downstream" directions. This is important because interior devices always issue requests and responses in the upstream direction. They only accept responses in the downstream direction.

If Slave Must Access Devices, It Uses Peer-to-Peer Transfers

As illustrated in Figure 17-4 on page 431, the slave host in a sharing double-hosted chain may be required to access the devices on the link. To do so, it may have its Command Register Act as Slave bit set = 1. When this is done, all packets it issues travel first to the master host bridge where they are reissued back to the target devices as peer-to-peer transactions.

Non-Sharing Double-Hosted Chain

A non-sharing double-hosted chain appears logically as two distinct chains with a host bridge at each end.

Software May Break The Chain

Software chooses a point to break the chain in two parts and then:

1. While the link is idle, the link between the two tunnel devices is broken by programing the End Of Chain (EOC) bits in the appropriate tunnel Link Control registers on each side. The Transmit Off bit in each of the Link Control registers can also be set.

2. The slave host bridge writes to the Slave Command register for each device now under its control to force the Master Host and Default Direction bits in each to point at the slave host bridge.

3. Unique bus numbers are assigned to each segment in a non-sharing double-hosted chain. The bus number is used so that chains may be uniquely identified and so type 1 configuration cycles may be forwarded and/or converted to type 0 cycles by bridges.

4. If peer-to-peer transactions are not required, software link partitioning can also be used for load balancing.

Figure 17-5 on page 433 depicts a non-sharing double-hosted chain.

Figure 17-5. Non-Sharing Double-Hosted Chain

Additional Notes About Double-Hosted Chains

Initialization In A Double-Hosted Chain

One of the responsibilities of a master host bridge in a double-hosted chain is to help with initialization after reset. Following low-level link initialization, the slave host bridge "sleeps" pending set up by the master. The basic steps in master initialization include:

1. The master host bridge sets the Slave Command CSR master host bit to point towards the master host bridge in all slave devices it finds. This bit is set automatically whenever the Slave Command CSR is written.

2. When the master host bridge discovers the slave host bridge, it sets the Host Command CSR Double Ended bit in the both its own and the slave's Host Command register. This informs the slave (when it wakes up) that it is in a double-hosted chain and that it is not required to configure devices below it.

3. If the Double Ended bit is not set in the slave, it will initialize its end of the double ended chain when it awakens.

Type 0 Configuration Cycles In A Double-Hosted Chain

Because all host bridges tend to own UnitID 0, a configuration cycle carrying a device number field of "0" in a double-hosted chain might be misinterpreted. The direction a type 0 configuration cycle request is traveling determines which host bridge is the target. If configuration software wishes to prevent a host bridge (e.g. the slave host) in a double-hosted chain from accessing another host's configuration space, the Host Command Register host hide bit may be set = 1.