Data processing system and method for handling castout collisions

1. A method of data processing in a data processing system having a first processing unit supported by a first cache memory and a second processing unit supported by a second cache memory, said method comprising:

a memory controller of a system memory receiving a first castout operation from the first cache memory and a second castout operation from the second cache memory, said first and second castout operations both specifying a same address;

said memory controller providing a first acknowledge response to said first castout operation and a second acknowledge response to said second castout operation to indicate that neither of said first and second castout operations should be retried; and

in response to receiving said first and second castout operations, the memory controller performing a single update to the system memory.

2. The method of claim 1 , and further comprising:

in response to receiving said first and second castout operations, said memory controller allocating only one write queue from which updates are made to said system memory.

3. The method of claim 1 , wherein receiving said first and second castout operations comprises receiving said second castout while said first castout operation is queued by said memory controller.

4. The method of claim 1 , wherein:

said receiving comprises said memory controller receiving said first castout operation before said second castout operation; and

said method further comprises said memory controller discarding said second castout operation.

5. The method of claim 1 , wherein performing a single update to system memory comprises merging said first and second castout operations.

6. The method of claim 1 , wherein:

the data processing system includes a first coherency domain including said memory controller and a second coherency domain; and

said performing a single update to said system memory comprises updating a domain indicator in said system memory uniquely associated with said address, said domain indicator indicating whether a memory block in the system memory associated with the address is cached in the second coherency domain.

7. The method of claim 6 , and further comprising updating the memory block in said system memory associated with said address.

8. The method of claim 1 , wherein:

the data processing system includes a first coherency domain including said memory controller and a second coherency domain;

said memory controller includes one or more write queues from which updates are made to said system memory; and

said method further comprises:

said memory controller receiving a third castout operation from said second coherency domain specifying an address allocated to the system memory; and

said memory controller acknowledging said third castout operation and refraining from allocating one of said one or more write queues to said third castout operation.