AHB Read-Ahead Behavior and PCIe BAR Configuration Challenges
In systems where an AHB (Advanced High-performance Bus) is bridged to a PCIe (Peripheral Component Interconnect Express) interface, managing read-ahead behavior becomes critical, especially when dealing with prefetchable and non-prefetchable PCIe Base Address Registers (BARs). The core issue arises from the need to suppress read-ahead on the AHB downstream port when the PCIe BAR is configured as non-prefetchable. This is particularly challenging for INCR (Incrementing) burst types, as the AHB protocol does not natively provide a straightforward mechanism to control read-ahead behavior dynamically based on the PCIe BAR type.
The AHB protocol defines several burst types, including SINGLE, INCR4, INCR8, INCR16, and INCR. While SINGLE and fixed-length INCR bursts (INCR4, INCR8, INCR16) can be handled relatively easily, INCR bursts present a unique challenge. INCR bursts are variable-length and do not have a predefined end address, making it difficult to determine when to suppress read-ahead. The current implementation in some systems involves splitting INCR bursts into a series of SINGLE transactions, which is not ideal and may violate the AHB protocol’s intent.
The HPROT signals, which are part of the AHB protocol, are often considered for controlling read-ahead behavior. However, the AMBA specification indicates that HPROT[3:2] primarily applies to write accesses, with HPROT[3] (cacheable/modifiable) being relevant for both reads and writes, and HPROT[2] (bufferable) being more applicable to writes. This limitation complicates the use of HPROT signals for controlling read-ahead during read transactions.
Another proposed solution involves manipulating the HTRANS signal to indicate BUSY states during INCR bursts, effectively suppressing read-ahead by delaying the progression of the burst until the read data is returned. However, this approach requires careful coordination between the upstream PCIe/AHB bridge and the downstream AHB/AHB bridge, and it may introduce latency or complexity.
The absence of a standardized mechanism to pass prefetchability information across AHB bridges further exacerbates the issue. While AMBA 5 introduces HAUSER and HRUSER signals for additional user-defined information, these signals are not available in earlier versions of the AMBA protocol, limiting their applicability in legacy systems.
HPROT Signal Misinterpretation and INCR Burst Handling
The root cause of the issue lies in the misinterpretation and limited applicability of the HPROT signals for controlling read-ahead behavior. While HPROT[3] (cacheable/modifiable) is relevant for both read and write transactions, its use in indicating prefetchability is not explicitly defined in the AMBA specification. This ambiguity leads to inconsistent implementations across different AHB bridges and PCIe/AHB bridges.
The AMBA 5 specification provides some clarity by defining memory types using HPROT combinations. For example, HPROT[3] low indicates "Device" memory types, which do not allow read data prefetching, while HPROT[3] high indicates "Normal" memory types, which do allow prefetching. However, this distinction is not universally adopted in earlier versions of the AMBA protocol, leading to compatibility issues in systems that do not use AMBA 5.
The handling of INCR bursts further complicates the issue. INCR bursts are inherently variable-length, making it difficult to predict the end address and determine when to suppress read-ahead. Splitting INCR bursts into SINGLE transactions, as done in some implementations, is a workaround but not a solution, as it violates the AHB protocol’s intent and may introduce performance bottlenecks.
The lack of sideband signals or standardized mechanisms to pass prefetchability information across AHB bridges is another significant challenge. While HAUSER and HRUSER signals in AMBA 5 provide a potential solution, their absence in earlier versions of the protocol limits their applicability. This forces designers to rely on proprietary solutions or suboptimal workarounds, which may not be scalable or maintainable.
Implementing HPROT-Based Prefetch Control and AHB Protocol Enhancements
To address the issue of suppressing read-ahead for non-prefetchable PCIe BARs, a combination of HPROT signal utilization and AHB protocol enhancements can be employed. The following steps outline a detailed approach to resolving the issue:
-
HPROT Signal Utilization: Leverage HPROT[3] to indicate prefetchability for both read and write transactions. When HPROT[3] is low, treat the transaction as targeting a "Device" memory type and suppress read-ahead. When HPROT[3] is high, treat the transaction as targeting a "Normal" memory type and allow read-ahead. This approach aligns with the AMBA 5 specification and can be implemented in earlier versions of the protocol with minimal modifications.
-
HTRANS Signal Manipulation: For INCR bursts, manipulate the HTRANS signal to indicate BUSY states after every address cycle. This effectively suppresses read-ahead by delaying the progression of the burst until the read data is returned. The downstream AHB/AHB bridge can detect the BUSY states and suppress read-ahead accordingly. This approach requires careful coordination between the upstream and downstream bridges but provides a standardized mechanism for controlling read-ahead behavior.
-
AMBA 5 HAUSER/HRUSER Signals: In systems that support AMBA 5, use the HAUSER and HRUSER signals to pass prefetchability information across AHB bridges. These signals provide a standardized mechanism for conveying additional user-defined information, eliminating the need for proprietary sideband signals. This approach is particularly useful in systems with complex memory hierarchies or multiple AHB bridges.
-
Protocol Enhancements: Propose enhancements to the AHB protocol to explicitly define mechanisms for controlling read-ahead behavior. This could include the introduction of new signals or the extension of existing signals to provide more granular control over prefetchability. These enhancements would address the limitations of the current protocol and provide a standardized solution for suppressing read-ahead in non-prefetchable memory regions.
-
Validation and Testing: Implement the proposed solutions in a test environment and validate their effectiveness using a combination of simulation and hardware testing. Ensure that the solutions do not introduce additional latency or complexity and are compatible with existing AHB and PCIe implementations.
By combining these approaches, it is possible to effectively suppress read-ahead for non-prefetchable PCIe BARs while maintaining compliance with the AHB protocol. The use of HPROT signals, HTRANS manipulation, and AMBA 5 enhancements provides a standardized and scalable solution that can be adapted to a wide range of systems and applications.
Table: HPROT Signal Usage for Prefetch Control
| HPROT[3] | Memory Type | Prefetch Behavior |
|---|---|---|
| 0 | Device | Suppress read-ahead |
| 1 | Normal | Allow read-ahead |
Table: HTRANS Signal States for INCR Bursts
| HTRANS State | Description | Read-Ahead Behavior |
|---|---|---|
| IDLE | No transfer | N/A |
| BUSY | Transfer in progress, delay next | Suppress read-ahead |
| NONSEQ | Single or first of burst | Depends on HPROT[3] |
| SEQ | Sequential transfer in burst | Depends on HPROT[3] |
This comprehensive approach ensures that read-ahead behavior is controlled effectively, addressing the challenges posed by non-prefetchable PCIe BARs and INCR bursts in AHB-based systems.
