Hard Processor System Technical Reference Manual: Agilex™ 5 SoCs

ID 814346
Date 4/01/2024
Public

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4.1.5.3.1. Coherent Read Request

The following figure shows the coherent read request.

Figure 21. Coherent Read Request

In a coherent read request:

  • CMD message is issued from an AIU to a DCE.
  • In DCE, the CMD message is serialized with respect to other CMD messages to the same cache-line, and the snoop filters are accessed to determine what SNP messages, if any, need to be issued to AIU to enforce coherence. The request phase of the SNP message indicates the type of coherence operation to be performed, while the response phase of the SNP message indicates the result of the coherence operation.
  • Depending on the state of the cache-line in the snooped caching agent, the snooped agent forwards data to the requesting agent by issuing a DTR message in response to the SNP message.
  • If, based on the state of the snoop filters, no SNP messages are issued or based on the responses to the SNP messages, no snooped agent forwards data, the DCE issues an MRD message to a DMI.
  • If the DMI is configured with a system cache, then upon receiving the MRD message the cache forwards the data as DTR message if there is a cache hit; otherwise, it issues a memory read transaction and forwards the data in a DTR message once the transaction has completed.

Once the SNP message responses have been gathered, the DCE issues an STR message to the requesting AIU, the response phase of the STR message indicates completion of the transaction to DCE.