RapidIO II IP Core 4x Variations Might Issue Unexpected packet-retry Control Symbol

RapidIO II IP core 4x variations might issue a packet-retry control symbol on the RapidIO link after receiving a small number of consecutive, small RapidIO packets. This issue occurs because the IP core reserves 256 bytes of space, the space required for a maximum-sized packet, for every incoming packet, even if the packet is small. In addition, the IP core requires more time to read a packet from its RX buffer than to write a packet to the RX buffer. When the IP core detects the RX buffer is becoming full, it issues a packet-retry control symbol on the RapidIO link.

The IP core does not drop any packets as a result of this issue. However, the issue impacts throughput.

This issue occurs after a stream of consecutive, small packets arrive on the RapidIO link. If the IP core receives 256-byte packets, the issue does not occur.

To avoid this issue, ensure the IP core link partner does not send a sequence of consecutive packets with only eight bytes, 16 bytes, or 32 bytes of payload.

More specifically, Altera has tested the following data patterns:

• If all packets have a payload size of 256 bytes followed by an explicit end-of-packet indication (EOP is provided even in the case of back-to-back packets), the issue does not occur.
• If all packets have payload sizes of 64 bytes, 128 bytes, or 256 bytes, and they are not SWRITE transactions, the issue is less likely to occur.