RapidIO: The Next Generation Communication Fabric For Embedded Application

6.5. ROUTING MAINTENANCE PACKETS

While RapidIO end points must have device IDs, it is not required that RapidIO switches have device IDs. For most operations this shouldn't be a problem as switches are not typically the targets of an operation. However, there is a case where switches are the targets of operations. This is when the system is being configured or reconfigured or when the system needs to retrieve information from the registers contained within a switch. Because switches may not have device IDs, there is no way for a typical read or write operation to target a switch.

To remedy this situation, an alternative method of addressing for maintenance packets is defined. Maintenance packets include an additional hop_count field in the packet. This field specifies the number of switches (or hops) into the network, the packet should experience from the issuing processing element to the target destination device. Whenever a switch processing element receives a maintenance packet it examines the hop_count field. If the received hop_count is zero, the transaction is targeted at that switch. If the hop_count field is not zero, it is decremented and the packet is sent out of the switch according to the destination ID field. This method allows easy access to any intervening switches in the path between two addressable processing elements. However, since maintenance response packets are always targeted at an end point, the hop_count field for transactions targeted at non-switch elements, ...