1.2.4 Replay Buffer

In the transmit direction, the AXI4 bridge layer first checks the credits available on the far end before sending the TLP. There must be enough credits available for the entire TLP to be sent. The AXI4 bridge layer must then check that the PCIESS is ready to send the TLP. If there is enough credit, it can proceed with sending data. If the credit is insufficient, the TLP must wait until enough credit is available.

The replay buffer is used to fetch TLPs that are:

• Issued by the AXI before their transmission on the PCIe link.
• Transmitted on the PCIe link, as long as they are not acknowledged by the opposite PCIe device, in case a replay is required.

The replay buffer is 8 KB, allowing the transmit buffer to support up to 16 outstanding transmit replay data packets with a maximum payload of 256 bytes each. A maximum of 32 TLPs can be supported by the replay buffer.

Compliance

PCIe Protocol Compliance Test with LeCroy Exerciser:

Device testing for PCIe compliance with a LeCroy exerciser reports a failure for a PTC test related to Replay Buffer. However, the PTC test conducted on the Key-sight exerciser passed successfully. PolarFire FPGA met full compliance at the PCISIG Compliance Workshop #101 and entered into the integrators list. PTC Test 52-20 (LinkRetrainOnRetryFail) fails in the LeCroy testing suite. The root-cause is that the PCIe controller executes some of the second-round replays; prior to, rather than after, link re-training. It does not impact any application functionality as no transaction layer packet (TLP) is lost due to the built-in replay mechanism required by the PCIe base specification. Replays occur both before and after link re-training as long as TLP is not acknowledged.