Arria V Avalon-ST Interface for PCIe Solutions User Guide

ID 683733
Date 1/04/2023
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16.2.1. Link Hangs in L0 State

There are many reasons that link may stop transmitting data. The following table lists some possible causes.

Table 85.  Link Hangs in L0

Possible Causes

Symptoms and Root Causes

Workarounds and Solutions

Avalon‑ST signaling violates Avalon-ST protocol

Avalon‑ST protocol violations include the following errors:

  • More than one tx_st_sop per tx_st_eop.
  • Two or more tx_st_eop’s without a corresponding tx_st_sop.
  • rx_st_valid is not asserted with tx_st_sop or tx_st_eop.

These errors are applicable to both simulation and hardware.

Add logic to detect situations where tx_st_ready remains deasserted for more than 100 cycles. Set post‑triggering conditions to check for the Avalon‑ST signaling of last two TLPs to verify correct tx_st_sop and tx_st_eop signaling.

Incorrect payload size

Determine if the length field of the last TLP transmitted by End Point is greater than the InitFC credit advertised by the link partner. For simulation, refer to the log file and simulation dump. For hardware, use a third‑party logic analyzer trace to capture PCIe transactions.

If the payload is greater than the initFC credit advertised, you must either increase the InitFC of the posted request to be greater than the max payload size or reduce the payload size of the requested TLP to be less than the InitFC value.

Flow control credit overflows

Determine if the credit field associated with the current TLP type in the tx_cred bus is less than the requested credit value. When insufficient credits are available, the core waits for the link partner to release the correct credit type. Sufficient credits may be unavailable if the link partner increments credits more than expected, creating a situation where the Intel L-/H-Tile Avalon-ST for PCI Express IP Core credit calculation is out-of-sync with the link partner.

Add logic to detect conditions where the tx_st_ready signal remains deasserted for more than 100 cycles. Set post‑triggering conditions to check the value of the tx_cred_* and tx_st_* interfaces. Add a FIFO status signal to determine if the TXFIFO is full.

Malformed TLP is transmitted

Refer to the error log file to find the last good packet transmitted on the link. Correlate this packet with TLP sent on Avalon‑ST interface. Determine if the last TLP sent has any of the following errors:

  • The actual payload sent does not match the length field.
  • The byte enable signals violate rules for byte enables as specified in the Avalon Interface Specifications.
  • The format and type fields are incorrectly specified.
  • TD field is asserted, indicating the presence of a TLP digest (ECRC), but the ECRC DWORD is not present at the end of TLP.
  • The payload crosses a 4KByte boundary.

Revise the Application Layer logic to correct the error condition.

Insufficient Posted credits released by Root Port

If a Memory Write TLP is transmitted with a payload greater than the maximum payload size, the Root Port may release an incorrect posted data credit to the Endpoint in simulation. As a result, the Endpoint does not have enough credits to send additional Memory Write Requests.

Make sure Application Layer sends Memory Write Requests with a payload less than or equal the value specified by the maximum payload size.

Missing completion packets or dropped packets

The RX Completion TLP might cause the RX FIFO to overflow. Make sure that the total outstanding read data of all pending Memory Read Requests is smaller than the allocated completion credits in RX buffer.

You must ensure that the data for all outstanding read requests does not exceed the completion credits in the RX buffer.