[openib-general] basic IB doubt

[Talpey, Thomas]

At 12:40 PM 8/25/2006, Sean Hefty wrote:
>>    Thomas> How does an adapter guarantee that no bridges or other
>>    Thomas> intervening devices reorder their writes, or for that
>>    Thomas> matter flush them to memory at all!?
>>
>>That's a good point.  The HCA would have to do a read to flush the
>>posted writes, and I'm sure it's not doing that (since it would add
>>horrible latency for no good reason).
>>
>>I guess it's not safe to rely on ordering of RDMA writes after all.
>
>Couldn't the same point then be made that a CQ entry may come before the data
>has been posted?

When the CQ entry arrives, the context that polls it off the queue
must use the dma_sync_*() api to finalize any associated data
transactions (known by the uper layer).

This is basic, and it's the reason that a completion is so important.
The completion, in and of itself, isn't what drives the synchronization.
It's the transfer of control to the processor.

Tom.