2.5 Exclusive and locked accesses

The AXI protocol supports Exclusive bursts but the AHB protocol only supports single (length 1) Exclusive accesses. Therefore, if the AXI5 to AHB5 bridge receives an AXI Exclusive burst, then it translates the burst to normal (Non-exclusive) AHB transfers. If the bridge receives a single AXI Exclusive transaction, then it translates the transaction to an Exclusive AHB transfer.

The AXI5 to AHB5 bridge does not support single sparse Exclusive writes, because splitting the write transaction would create an Exclusive AHB burst. As the preceding Exclusive read might have been answered with HEXOKAY, the bridge always responds with SLVERR for a single sparse Exclusive write.

Note:

The bridge returns SLVERR because although OKAY is a valid Exclusive response, an OKAY response could cause the AXI master to repeat the Exclusive write indefinitely.

The bridge uses the axid values to identify which AXI master issues an Exclusive access. For the AHB transfer, the bridge copies the axid value to hmaster.

The following table shows the AXI5 to AHB5 Exclusive transaction mapping.

Table 2-6 AXI5 to AHB5 bridge Exclusive transaction mapping

AXI Exclusive access type AHB transfer
AXI Exclusive read. Exclusive AHB transfers, for a single AXI transaction.
Normal AHB transfers, for a burst AXI transaction.
AXI non-sparse Exclusive write. Exclusive AHB transfers, for a single AXI transaction.
Normal AHB transfers, for a burst AXI transaction.
AXI sparse Exclusive write. Normal (SLVERR) AHB transfer, for a single AXI transaction.
Normal AHB transfers, for a burst AXI transaction.

Comparison to the XHB-400 behavior

The previous generation AXI to AHB bridge is the CoreLink™ XHB-400, which supports the AMBA® 3 AHB-Lite protocol. The AHB-Lite protocol does not support Exclusive accesses, so to support AHB-Lite Exclusive transfers, the XHB-400 includes the EXREQ and EXRESP signals. If necessary, you can add external glue logic to the AXI5 to AHB5 bridge, so that the logic generates EXREQ and EXRESP, with the limitation that Exclusive bursts are not supported. See the Arm® CoreLink™ XHB-500 Bridge Configuration and Integration Manual for more information about the glue logic.

Locked transfers for AXI Non-modifiable transactions

The AXI protocol requires that Non-modifiable transactions must not be split into multiple transactions or merged with other transactions. However, the AXI5 to AHB5 bridge must split sparse writes and unaligned Non-modifiable reads to generate multiple AHB bursts. The bridge issues these AHB bursts as Non-modifiable with hmastlock HIGH, and it allows no arbitration during the AXI burst. Asserting hmastlock ensures that the bridge or an AHB interconnect does not separate the bursts because they belong to the same lock sequence. The bridge inserts an IDLE transfer after the lock sequence.

While an AXI burst can cross the 1K address range, the AHB protocol requires that all transfers in a locked sequence are to the same slave address region. If a Non-modifiable AXI burst crosses a 1K address boundary, the bridge generates Non-modifiable AHB bursts with hmastlock LOW, and it responds with SLVERR. In this scenario, the bridge allows arbitration.

Note:

Although not recommended by Arm, the bridge can issue transactions that start, end, or contain only locked IDLE transfers, depending on the incoming AXI transaction or the AXI response channel availability.
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