Streamline PCIe 6.0 Switch Design with Effective Verification Strategies

The demand for PCIe 6.0 switches has surged due to the exponential growth in global data traffic. PCIe 6.0 switches play a crucial role in enabling high-performance computing (HPC) systems, particularly in data centers, for applications that demand massive bandwidth and ultra-low latency. Yet, ensuring these switches meet strict criteria for performance, power efficiency, and cost presents a formidable challenge. The complexity of designing these switches can be mitigated through thorough testing and verification processes.

Traditional verification methods, like data integrity and virtual channel arbitration testing used for PCIe 5.0 switches, remain valuable. However, PCIe 6.0 demands a more comprehensive approach. One needs advanced verification strategies that delve deeper than basic functionality. This includes generating backpressure traffic to identify potential performance bottlenecks and ensure the switch operates optimally in real-world scenarios. By proactively addressing these challenges, one can guarantee low latency and high bandwidth design for demanding high-performance computing applications.

Flit Mode vs. Non-Flit Mode Interoperability

The introduction of Flit Mode in PCIe 6.0 presents new verification challenges for switches. As shown in the diagram below, we can break down the key areas to consider.

1. Co-Existence Verification: Switches must seamlessly handle a mix of Flit Mode (FM) and Non-Flit Mode (NFM) traffic across different ports. This verification ensures no performance degradation occurs due to traffic mode heterogeneity.
2. Transaction Layer Packet (TLP) Header Translation: NFM and FM utilize distinct header formats. The switch needs to translate between these formats when ingress and egress ports operate in different modes. Verification focuses on the correctness of this translation process.
3. Replay Mechanism Discrepancies: FM and NFM employ different replay mechanisms. Verification must account for these differences to ensure proper error handling and data integrity during retries.
4. Segment Visibility and Translation: Segment information is only readily accessible in FM links. This adds complexity when routing between FM and NFM TLPs, especially when ingress and egress ports operate in different modes. This must be verified against translation rules, i.e., sometimes, switch will report a TLP Translation Egress Blocked error.
   • Device 3 capability register has a "Segment Captured" bit; there are specific translation rules for switch while translating a TLP from NFM to FM w.r.t segment captured bit.
5. Non-Flit Mode LN Bit Deprecation: The LN (Lightweight Notification) bit, used in NFM, is deprecated in PCIe 6.0. Verification should encompass dedicated translation rules for handling this deprecated bit.
6. 14-bit Tag Support and Translation: 14-bit tags are exclusive to FM. The PCIe specification mandates specific translation rules for converting requests from FM to NFM to account for this difference in tag size.
7. Poisoning Mechanism Translation: If either poisoning mechanism is applied to FM TLP during translation to NFM, the NFM TLP's Error Poisoned bit (EP bit) must be set. Conversely, while translating from NFM to FM, if the EP bit is set in the NFM TLP, it must be preserved in the resulting FM TLP. Verification ensures the switch adheres to these translation rules for poisoning mechanisms.

By thoroughly verifying these interoperability aspects, designers can ensure robust and reliable operation of PCIe 6.0 switches handling mixed Flit Mode and Non-Flit Mode traffic. The below topology diagram illustrates one sample scenario with mixed mode traffic (FM + NFM) on switch ports containing one upstream port and three downstream ports connected to Root Complex (RC) and Endpoints (EP) respectively.

Performance Penalties to Consider

While applying these translation rules and different shared/dedicated credit management, Switch Design may add some performance penalties by adding extra No Operation (NOP) Flits or NOP TLPs in Flit mode and may add extra Logical Idle (IDL) Dwords in Non-Flit mode. This is why performance testing is now more crucial as part of switch design verification signoff criteria.

Switch verification can identify and rectify performance bottlenecks with traffic modeling, latency monitoring and buffer analysis.

1. Traffic Modeling: Develop test scenarios that simulate the real-world traffic pattern with a high volume of Flit mode traffic transitioning to Non-Flit mode and vice versa. Testing strategies need to be evaluated against random factors such as different speeds and different link widths, along with different TLP type traffic generation to make sure all the switch ports are verified with Flit mode and Non-Flit mode.
2. Latency Monitoring: Monitor the latency experienced by packets as they traverse the switch, focusing on paths with frequent mode transitions. Look for abnormal spikes in latency that could indicate inefficiencies in the translation process.
3. Buffer Analysis: Analyze the behavior of the switch's internal buffers during the traffic simulation. Identify any scenarios where buffers are approaching full capacity or overflowing. This can signal potential performance bottlenecks due to excessive translation overhead. i.e., switch ports must buffer and route TLPs, including “reserved” TLP entries, as specified in the protocol specification.

Summary

PCIe 6.0 brings revolutionary changes to the PCIe protocol layered stack, the most disruptive being the introduction of “Flit.” As per specification, switch ingress ports receive a flit packet containing multiple destination TLPs that need to streamline routing logic to send data across relevant egress ports with translation logic (or vice-versa). While applying translation logic with OHC encapsulations, different shared/dedicated credit management and TLP replay mechanism switch routing logic may add performance penalties that cannot be identified by just verifying data integration testing. It is essential to evaluate performance testing strategies to ensure thorough verification of all switch ports with Flit mode and Non-Flit mode traffic. Furthermore, performance testing can help to identify potential bottlenecks or issues that might emerge under different load conditions (i.e payload length).

If you have any queries, feel free to contact us at talk_to_vip_expert@cadence.com

More Information

• For more info on how Cadence PCIe Verification IP enables users to confidently verify switches, see our VIP for PCI Express
• For more information on PCIe in general and on the various PCI standards, see the PCI-SIG website.