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May 1, 2021

Digital Design Interview Question on PCIe express





Digital Design Interview Question on PCIe express :




What are the PCIe protocol extensions, and how do they improve PCIe interconnect performance?
The PCIe protocol extensions are primarily intended to improve interconnect latency, power and platform efficiency. These protocol extensions pave the way for better access to platform resources by various compute- and I/O-intensive applications as they interact with and through the PCIe interconnect hierarchy. There are multiple protocol extensions and enhancements being developed and they range in scope from data reuse hints, atomic operations, dynamic power adjustment mechanisms, loose transaction ordering, I/O page faults, BAR resizing and so on. Together, these protocol extensions will increase PCIe deployment leadership in emerging and future platform I/O usage models by enabling significant platform efficiencies and performance advantages.




Section 4.2.4.2 - When upconfiguring a Link in the LTSSM Configuration.Linkwidth.Start state, are the Lanes which are being activated required to transmit an EIEOS first when they exit Electrical Idle?

No. Lanes being activated for upconfiguration are not required to align their exit of Electrical Idle with the transmission of any Symbol, Block, or Ordered Set type. Furthermore, the Lanes are not required to exit Electrical Idle before the LTSSM enters the Configuration.Linkwidth.Start state.



What is PCI Express (PCIe) 3.0? What are the requirements for this evolution of the PCIe architecture?

PCIe 3.0 is the next evolution of the ubiquitous and general-purpose PCI Express I/O standard. At 8GT/s bit rate, the interconnect performance bandwidth is doubled over PCIe 2.0, while preserving compatibility with software and mechanical interfaces. The key requirement for evolving the PCIe architecture is to continue to provide performance scaling consistent with bandwidth demand from leading applications with low cost, low power and minimal perturbations at the platform level. One of the main factors in the wide adoption of the PCIe architecture is its sensitivity to high-volume manufacturing materials and tolerances such as FR4 boards, low-cost clock sources, connectors and so on. In providing full compatibility, the same topologies and channel reach as in PCIe 2.0 are supported for both client and server configurations. Another important requirement is the manufacturability of products using the most widely available silicon process technology. For the PCIe 3.0 architecture, PCI-SIG believes a 65nm process or better will be required to optimize on silicon area and power.




Section 4.2.7.3 - PCIe 3.0 Base spec section 4.2.7.4 states that "Receivers shall be tolerant to receive and process SKP Ordered Sets at an average interval between 1180 to 1538 Symbol Times when using 8b/10b encoding and 370 to 375 blocks when using 128b/130b encoding.ÌÒ For 128/130 encoding, if the Transmitter sends one SKP OS after 372 blocks and a second after 376 blocks, the average interval comes out to be 374 blocks and that falls in the valid range. So is this allowed, or must every SKP interval count fall inside the 370 to 375 blocks?

At 8 GT/s, a SKP Ordered Set must be scheduled for transmission at an interval between 370 to 375 blocks. However, the Transmitter must not transmit the scheduled SKP Ordered Set until it completes transmission of any TLP or DLLP it is sending, and sends an EDS packet. Therefore, the interval between SKP OS transmissions may not always fall within a 370 to 375 block interval.

For example, if a SKP Ordered Set remains scheduled for 6 block times before framing rules allow it to be transmitted, the interval since the transmission of the previous SKP OS may be 6 blocks longer than normal, and the interval until the transmission of the next SKP OS may be 6 Blocks shorter than normal. But the Transmitter must schedule a new SKP Ordered Set every 370 to 375 blocks, so the long-term average SKP OS transmission rate will match the scheduling rate.

Receivers must size their elastic buffers to tolerate the worst-case transmission interval between any two SKP Ordered Sets (which will depend on the Max Payload Size and the Link width), but can rely on receiving SKP Ordered Sets at a long term average rate of one SKP Ordered Set for every 370 to 375 blocks. The SKP Ordered Set interval is not checked by the Receiver.




Section 4.2.6.4.2 - According to pg227 of spec, "When using 128b/130b encoding, TS1 or TS2 Ordered Sets are considered consecutive only if Symbols 6-9 match Symbols 6-9 of the previous TS1 or TS2 Ordered Set". When in Recovery.Equalization and if using 128b/130b encoding, is it required that lane/link numbers (symbol 2) match in TS1s to be considered as consecutive or is it need not match?

The Receiver is not required to check the Link and Lane numbers while in Recovery.Equalization.




Section 7.28.3 - When the maximum M-PCIe Link speed supported is 2.5 GT/s, what will be the Link speed following a reset?

The Link Speed following reset will be the result of Configuration process. During the M-PCIe discovery and Configuration process, RRAP is used to discover M-PHY capabilities, analyze and configure configuration attributes accordingly. Depending on the High speed supported by both components, the Link Speed and Rate Series may get configured for HS-G1, HS-G2 or HS-G3 and RATE A or B respectively. For this particular example Link Speed could be either HS-G1 or HS-G2 depending on the supported Link Speeds of the other Component on the LINK.




Section 4.2.6.9 - When in the Disabled state the Upstream Port transitions to Detect when an Electrical Idle exit is detected at the receiver. Is an Electrical Idle exit required to be detected on all Lanes?

An Electrical Idle exit is required to be detected on at least one Lane.




Has there been a new compliance specification developed for PCIe 3.0?

For each revision of its specification, PCI-SIG develops compliance tests and related collateral consistent with the requirements of the new architecture. All of these compliance requirements are incremental in nature and build on the prior generation of the architecture. PCI-SIG anticipates releasing compliance specifications as they mature along with corresponding tests and measurement criteria. Each revision of the PCIe technology maintains its own criteria for product interoperability and admission into the PCI-SIG Integrators List.




Section 4.2.6.6.1.3 - How can I configure the RC, if permissible, to send 4096 FTS to EP while RC transits out of L0s?

Setting the Extended Synch bit in the Link Control register of the two devices on the link will increase the number of FTS Ordered Sets to 4096, but the Extended Synch bit is used only for testing purposes

there is Interview audio recording on PCIe. this is purely google search. 



https://www.youtube.com/watch?v=DJ7ksMXAwsk 


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