8.5. PACKET EXCHANGE PROTOCOL

The RapidIO parallel physical layer specification defines an exchange of packet and acknowledgment control symbols where the destination or an intermediate switch fabric device explicitly acknowledges receipt of every request or response packet.

If a packet cannot be accepted for any reason, an acknowledgment control symbol indicates that the original packet and any already transmitted subsequent packets should be resent. This behavior provides a flow control and transaction ordering mechanism between processing elements. Link flow control and acknowledgement are performed between adjacent devices. This allows flow control and error handling to be managed between each electrically connected device pair rather than between the original source and the final target of the transaction. This allows the system to optimize the performance of each link and also to pinpoint detected errors to specific links rather than the whole paths between devices (which might contain several links). Devices will always transmit an acknowledge control symbol for a received request. The transaction is always acknowledged before the operation of which it is a component is completed.

8.5.1. Packet and Control Alignment

All packets sent over the RapidIO parallel interface are aligned to 32-bit boundaries. This alignment allows devices to work on packets with a larger internal path width, thus allowing lower core operating frequencies. Packets that are not naturally aligned to a 32-bit boundary are padded. Control symbols are nominally 16-bit quantities, but are defined, in the parallel interface, as a 16-bit control symbol followed by a bit-wise inverted copy of itself to align it to the 32-bit boundary. This, in turn, adds error detection capability to the interface. These 32-bit quantities are referred to as aligned control symbols.

The 16-bit wide port is compatible with an 8-bit wide port. If an 8-bit wide port is properly connected to a 16-bit wide port, the port will function as an 8-bit interface between the devices.

8.5.2. Acknowledge Identification

A packet requires an identifier to uniquely identify its acknowledgment. This identifier, known as the acknowledge ID (or ackID), is three bits, allowing for a range of one to eight outstanding unacknowledged request or response packets between adjacent processing elements; however, only up to seven outstanding unacknowledged packets are allowed at any one time. The ackIDs are assigned sequentially (in increasing order, wrapping back to 0 on overflow) to indicate the order of the packet transmission. The acknowledgments themselves appear as control symbols sent back to the transmitter.