5.7.2. Restrictions on AHBS transactions

Loopback of transactions from the AXIM or AHBP interface onto the AHBS is not supported because it might cause deadlock.

Note

Loopback arrangements are unlikely to be required. The processor has higher bandwidth to TCM than the AHBS interface. This means that software can directly transfer data to and from TCM faster than through the AHBS interface.

This restriction does not preclude arrangements where there is an indirect relationship between a master interface access and an AHBS access. For example, if a write from the AHBP interface requests an external agent to perform transactions on the AHBS interface. In this case do not introduce dependency in the system between the control access that initiates the transaction and the transaction itself.

AHBS interface transactions are not capable of performing MPU lookups. No distinction is made internally between unprivileged and privileged AHBS interface accesses as indicated on HPROTS. The system is entirely responsible for providing TCM protection functionality for AHBS interface accesses as required.

A TCM error mechanism must be used by the external TCM interface logic to indicate back to the AHBS interface that the access was aborted. In this case, the external TCM interface logic also mask writes and obfuscate read data. For more information on the TCM interface protocol, see TCM interface protocol.

The AHBS interface reads that are aborted on the TCM interface return the read data supplied with an error response on HRESPS. The AHBS interface writes are buffered and always return an OK response speculatively. If a write is subsequently aborted on the TCM interface, the AHBS raises an asynchronous abort to the system using the WABORTS signal.

The AHBS does not support exclusive or locked accesses and AHBS interface stores do not affect the state of the internal exclusive access monitor. This makes it unsuitable for systems requiring concurrency controls between the AHBS interface and software.

For more information on the TCM data sharing models supported between software and AHBS interface, see System access to TCM.

Copyright © 2014-2016, 2018 Arm. All rights reserved.ARM DDI 0489F
Non-ConfidentialID121118