INFO
Understanding potential errors in the PCIe protocol helps debug problems and improve reliability. This covers common PCIe failure points.
Data Integrity
CRC Mismatches
- Cyclic redundancy checks (CRC) are used to detect accidental changes to data during transmission. The sender calculates a CRC value and sends it along with the data packet.
- The receiver recalculates the CRC and compares it to the received CRC value. If they do not match, it indicates the data was corrupted in transit.
- CRC mismatches are usually caused by electrical interference or cabling issues. They can lead to application crashes, data loss, and other errors.
End-to-End CRC Failures
- PCIe Gen3 and above use an additional ECRC field to verify data integrity across the entire link.
- The ECRC covers the entire packet, from the header to the data payload. If the receiver’s ECRC check fails, it means data corruption occurred at some point during transmission.
- Failed ECRC indicates potential issues with PCIe devices, connectors, or cabling. It can result in unintended data being delivered to applications.
Flow Control
Credit Starvation
- Flow control credits regulate data flow to prevent receiver buffer overflow. Sending devices acquire credits prior to transmitting.
- If a device runs out of credits, it can no longer transmit data. This credit starvation stalls the flow of data.
- It’s usually caused by the receiver failing to replenish credits as it processes data. Can lead to suboptimal performance.
Buffer Overflows
- When incoming data rate exceeds a component’s ability to process it, the receiver’s buffer may be overloaded.
- Buffer overflows cause data loss, transaction backpressure, and retries. It indicates a mismatch between device capabilities.
- Can occur from incorrect flow control credit configuration or defective hardware.
Link Training ^linktraining
Negotiation Failures
- PCIe devices must perform link training to establish communications. This involves negotiating parameters like lane counts, speeds, etc.
- Failed negotiations prevent the link from initializing properly. Devices can’t communicate without a trained link.
- Usually caused by unsupported settings, faulty hardware, or interoperability issues between devices.
Link Instability
- Even after successful link training, the connection can become unstable over time. This causes intermittent errors or disconnects.
- Link stability is affected by mechanical factors like vibrations, thermal changes, and corrosion.
- Unstable links lead to unreliable data transfers and connectivity drops. Requires retraining the link.
Transactions
Addressing Faults
- PCIe utilizes memory, I/O, and configuration address spaces. Invalid addresses can cause errors.
- Software bugs, memory leaks, overflow issues etc. can cause out-of-range addresses to be used.
- Faulty address decoding or translations in hardware can also generate invalid addresses.
Data Corruption
- Data gets corrupted during transfer if packets get distorted by noise or interference.
- Memory failures and buffer overflows can modify data as its handled. Software bugs can also corrupt data.
- Corrupted data may get detected via CRCs or higher level checks. Silent data corruption is difficult to detect but detrimental.
Poisoned Packets
- Poisoned TLPs indicate that an error occurred during a transaction. This alerts the receiver that the data is corrupted.
- They are generated by PCIe transaction and data link layers upon detecting uncorrectable internal errors.
- Receivers interpret poisoned packets based on context to handle the error appropriately.
Timeouts
Completion Expirations
- PCIe utilizes timers to detect lost or delayed packets and completions. If a completion is not received within the timeout period, an error occurs.
- Timeouts during read requests can stall devices and degrade performance. They indicate interconnect issues or a malfunctioning device.
Stalled Communications
- Flow control uses timers to ensure smooth transfer of credits and data. Timer expirations can reflect problems.
- Stalled data flows and credit returns prevent progress of PCIe traffic. Caused by link errors, buffer deadlocks, or faulty devices.
Protocol
Invalid State Transitions
- The PCIe protocol defines legal state transitions for transaction layers. Invalid state changes are protocol errors.
- Caused by design flaws, faulty state machines, or hardware defects that disrupt expected sequencing.
- Can lead to unrecoverable conditions, and data and behavioral inconsistency between devices.
Sequence Violations
- PCIe transactions follow a specific sequence of messages for each transaction type.
- Violating the expected sequencing, such as a completion arriving before a request, results in errors.
- Usually caused by flawed logic or congestion leading to delayed or reordered packets. Leads to communication failures.
Timing
Clock Skews
- When clock frequencies differ between PCIe components, the timing of signals can deviate over time.
- Clock skews outside the tolerable range lead to setup and hold timing violations. This results in unreliable data capture.
- Caused by imperfect clock distribution, varying oscillator frequencies, and chip-to-chip variations.
Domain Crossing Faults
- Data transfer between different clock domains require special handling to avoid synchronization issues.
- Problems in synchronizer design and incorrect timing assumptions can cause domain crossing failures and data corruption.
- This leads to hard-to-debug problems like intermittent glitches, race conditions, and jitter.
Hardware
Signal Integrity Issues
- Electrical noise, inter-symbol interference, attenuation, and reflection degrade signal quality along PCIe links.
- Poor signal integrity causes data to be misinterpreted, leading to bit errors and CRC failures.
- Results from suboptimal PCB routing, connectors, drivers, receivers and other platform design issues.
Design Defects
- Incorrectly connected interfaces, race conditions, timing violations, and logical design flaws lead to functional errors.
- Subtle issues like metastability and electrical margin failures go undetected during design verification.
- Bad designs corrupt data, stall interfaces, and cause system instability and crashes.
Configuration
Incorrect Settings
- PCI configuration parameters control device behavior and resource allocation. Invalid configurations can cause conflicts.
- Excessive bus mastering, prefetching, or MPS settings degrade performance. Incorrect base address registers disrupt operation.
Performance Limitations
- Poor configuration optimization restricts PCIe bandwidth and throughput for high-speed devices.
- Limited number of lanes, lower link speeds, and narrow buses create bottlenecks.
- Latency-sensitive applications suffer from configuration-related performance bounds.
Signaling
Interrupt Mishandling
- PCIe utilizes interrupts for event notifications, error signaling, power management, hot plug, and other features.
- Dropped, extra, or misrouted interrupts disrupt software and can hang or crash systems.
- Caused by faulty interrupt generation and improper driver handling of interrupt messages.
Error Misprocessing
- PCIe errors are reported via status registers, poisoned TLPs, and ERR packets. These need to be handled properly.
- Uncorrected errors, failure to isolate faulty devices, and ignoring errors lead to data corruption or system failure.
- Robust error processing is required to maintain PCIe integrity and reliability.