Hi all:
I'm reading ACE protocol. I have some questions about such scenario:
two cache masters (master 0/1) and main meomry are connected by Coherent interconnect.
master 0 and master 1 are both SharedClean.
cacheline size : 64bytes.
Q1: if master 1 writes to portion of the shareable cacheline（update the 1st byte of cacheline）,
result 1: master 1 will issue CleanUnique and , master 1 cache state become UniqueDirty.
result 2 :master 1 cache state become SharedDirty, master 0 cache state will still be SharedClean.
which result will be right?
Q2: if result 2 in Q1 is right and then master 0 updates the 3rd byte of cacheline, how the 2 masters and interconnect act?
Thanks.
Hi digital_kevin,
I think in your scenario master 1 will issue a CleanUnique transaction and master 0 will receive a CleanInvalid snoop request.The result is your result 1, with the cache line in master 0's cache becoming invalid.
M1: SC -> UD
M0: SC -> I
The result 2 you describe with master 1 ending up with a different copy of the line in its cache compared to master 0 is not possible: with ACE, all dirty copies of a line in all caches must be the same at all times.
See also this discussion and this whitepaper.
Hi vstehle,
Thanks for the replay.
I have read the whitepaper. It seems that I didn't have a good understanding of SharedDirty state.
Could you build a scenario to describe when the state changes to SharedDirty ?
Really appreciate your help!
An example scenario could be: master 0 has a dirty line, master 1 does not have the line and issues a ReadShared transaction, the interconnect will snoop master 0, master 0 will transfer data to master 1.
M0: UD -> SD
M1: I -> SC
Master 0 could also pass dirty to master 1:
M0: UD -> SC
M1: I -> SD
See the AMBA AXI and ACE Protocol Specification for all the state transitions.
Note that not all implementations are able to handle SharedDirty. This is the reason why there is a ReadNotSharedDirty transaction to avoid the second scenario for example.