DSU CHI Interface Coherency and Link Deactivation Overview
The DynamIQ Shared Unit (DSU) is a critical component in ARM-based systems, particularly when connected to an interconnect via the Coherent Hub Interface (CHI). The DSU acts as a master interface, managing coherency and communication between multiple cores, caches, and the interconnect. A key aspect of the DSU’s operation is its ability to activate and deactivate the link to the CHI interface. This functionality is essential for power management and system optimization, but it raises important questions about coherency management during link deactivation.
When the DSU deactivates the CHI link, it effectively disconnects from the interconnect, which can lead to potential coherency issues. Specifically, if the DSU remains in a coherency domain while the link is deactivated, the interconnect may continue to send snoop transactions to the DSU. These snoop transactions are critical for maintaining cache coherency across the system, but they cannot be serviced if the CHI link is inactive. This scenario creates a risk of coherency violations, where the DSU’s cache state may become inconsistent with the rest of the system.
The ARM CHI specification (version E) and the DSU technical reference manual (revision r4p1) do not explicitly address whether the DSU must exit the coherency domain when deactivating the CHI link. This lack of clarity can lead to implementation challenges, as system designers must ensure that coherency is maintained even during link deactivation. The absence of explicit guidance suggests that the behavior may be implementation-dependent, requiring careful consideration of the specific system architecture and use case.
SYSCO Pin Toggling and Snoop Transaction Handling
One potential solution to the coherency issue during CHI link deactivation is toggling the SYSCO (System Coherency) pins of the DSU. The SYSCO pins control whether the DSU participates in the coherency domain. By toggling these pins, the DSU can explicitly indicate to the interconnect that it is no longer participating in coherency, thereby preventing the interconnect from sending snoop transactions to the DSU.
However, the timing and sequence of SYSCO pin toggling are critical. If the DSU deactivates the CHI link before toggling the SYSCO pins, there is a window during which the interconnect may still attempt to send snoop transactions. Conversely, if the SYSCO pins are toggled before deactivating the CHI link, the DSU may prematurely exit the coherency domain, potentially leading to coherency violations if any pending transactions are not properly handled.
The ARM documentation does not provide a definitive sequence for SYSCO pin toggling and CHI link deactivation, which leaves room for interpretation. System designers must carefully analyze the interaction between the DSU and the interconnect to determine the appropriate sequence. This analysis should consider factors such as the latency of SYSCO pin toggling, the timing of CHI link deactivation, and the behavior of the interconnect when it detects that a master has exited the coherency domain.
Implementing Coherency Management During CHI Link Deactivation
To address the coherency challenges during CHI link deactivation, system designers can implement a combination of hardware and software solutions. These solutions should ensure that the DSU’s cache state remains consistent with the rest of the system, even when the CHI link is deactivated.
Hardware Solutions
At the hardware level, the DSU can be designed to automatically toggle the SYSCO pins when deactivating the CHI link. This automatic toggling ensures that the DSU exits the coherency domain before the link is deactivated, preventing the interconnect from sending snoop transactions to an inactive link. The hardware logic should also handle any pending transactions, ensuring that they are either completed or aborted before the link is deactivated.
Additionally, the interconnect can be designed to detect when a master has deactivated its link and adjust its behavior accordingly. For example, the interconnect could buffer snoop transactions for a short period, allowing the master to reactivate the link and service the transactions. If the link remains inactive, the interconnect could then discard the transactions or take other appropriate actions.
Software Solutions
At the software level, the system firmware or operating system can manage the sequence of SYSCO pin toggling and CHI link deactivation. This management involves ensuring that all pending transactions are completed before deactivating the link and that the DSU’s cache state is flushed or invalidated as necessary. The software can also monitor the state of the CHI link and take corrective actions if coherency violations are detected.
One approach is to implement a state machine in the firmware that handles the transition between active and inactive states. This state machine would coordinate the toggling of the SYSCO pins, the deactivation of the CHI link, and any necessary cache maintenance operations. The state machine should be designed to handle edge cases, such as unexpected link deactivation or coherency violations, to ensure robust operation.
Verification and Testing
Regardless of the specific implementation, thorough verification and testing are essential to ensure that the coherency management mechanisms work as intended. This verification should include both simulation and hardware testing, with a focus on edge cases and corner scenarios. For example, the system should be tested under high load conditions, where the DSU is frequently activating and deactivating the CHI link, to ensure that coherency is maintained.
The verification process should also include stress testing, where the system is subjected to extreme conditions, such as rapid toggling of the SYSCO pins or frequent link deactivation and reactivation. These tests can help identify any weaknesses in the coherency management mechanisms and provide confidence that the system will operate reliably in real-world conditions.
Summary of Key Considerations
To summarize, the key considerations for managing coherency during CHI link deactivation include:
- SYSCO Pin Toggling Sequence: Ensure that the SYSCO pins are toggled in the correct sequence relative to CHI link deactivation to prevent coherency violations.
- Interconnect Behavior: Design the interconnect to handle link deactivation gracefully, either by buffering snoop transactions or taking other appropriate actions.
- Hardware Logic: Implement hardware logic in the DSU to automatically manage SYSCO pin toggling and handle pending transactions during link deactivation.
- Software Management: Use system firmware or operating system software to coordinate SYSCO pin toggling, CHI link deactivation, and cache maintenance operations.
- Verification and Testing: Conduct thorough verification and testing to ensure that the coherency management mechanisms work as intended under all conditions.
By carefully considering these factors and implementing appropriate solutions, system designers can ensure that the DSU maintains coherency even when the CHI link is deactivated, thereby preventing coherency violations and ensuring reliable system operation.
Conclusion
The behavior of the DSU when connected to an interconnect via the CHI interface is complex, particularly when it comes to coherency management during link deactivation. While the ARM documentation does not provide explicit guidance on this topic, system designers can implement a combination of hardware and software solutions to ensure that coherency is maintained. By carefully managing the sequence of SYSCO pin toggling and CHI link deactivation, and by designing the interconnect to handle link deactivation gracefully, it is possible to prevent coherency violations and ensure reliable system operation. Thorough verification and testing are essential to validate these solutions and ensure that they work as intended under all conditions.
