Coherency controller and method for data hazard handling for copending data access requests

A method of maintaining coherency of data comprising:

• receiving data access requests from at least one initiator device;
• transmitting said data access requests and snoop requests to at least one of said at least one initiator device and at least one recipient device;
• storing pending data access requests;
• generating snoop requests in response to said pending data access requests, said snoop requests determining whether said data to be accessed is stored locally in one or more of said at least one initiator device and said at least one recipient devices;
• in response to receipt of a first part of a write request comprising an indication that a write is to be performed to an address for which a read request is pending, stalling said write request by not responding to said first part of said write request until after the pending read request has completed,
• in response to receipt of a signal indicating completion of said read request, responding to said first part of said write request;
• and
• in response to receipt of a second part of said write request received after the response to said first part of said write request and comprising said data and an indication of a state of said data prior to said write request, writing said data to said at least one recipient device.

A coherency controlling means for maintaining data coherency of data stored in a data processing apparatus comprising at least one initiator device and at least one recipient device, said coherency controlling means comprising:

• at least one port means for receiving data access requests from said at least one initiator device and for transmitting said data access requests and snoop requests to at least one of said at least one initiator device and said at least one recipient device;
• a buffer means for storing pending data access requests;
• snoop request generating means for generating said snoop requests in response to said pending data access requests, said snoop requests determining whether data to be accessed is stored locally in one or more of said at least one initiator device and said at least one recipient devices;
• and
• coherency control means for responding to receipt of a first part of a write request comprising an indication that a write is to be performed to an address for which a read request is pending, by stalling said write request by not responding to said first part of said write request until after the pending read request has completed, wherein said coherency control means is responsive to receipt of a signal indicating completion of said read request to respond to said first part of said write request;
• and
• said coherency control means, responsive to receipt of a second part of said write request received after the response to said first part of said write request and comprising said data and an indication of a state of said data prior to said write request, writing said data to said at least one recipient device.