[ECOS] In-order exection of I/O on PowerPC 60x

Peter Graf p.graf@itknet.de
Thu Mar 23 12:54:00 GMT 2006


Andrew Lunn wrote:
>> When I placed a sync instruction between the critical read accesses,
>> instead of eieio, our problems disappeared. (The sync instruction covers
>> reads even if memory is not marked as "guarded".)
> 
> What is the overhead of a sync compaired to an eieio? Should we just
> change the barrier macro?

The respective freescale docs let me hesitate, and I'm not PowerPC
expert enough anyway. Quote from  (MPCFPE32B/AD REV2, page 8-198):

"The functions performed by the sync instruction will normally take a
significant amount of time to complete, so indiscriminate use of this
instruction may adversely affect performance. In addition, the time
required to execute sync may vary from one execution to another.
The eieio instruction may be more appropriate than sync for many cases."

The other option would be to mark the respective memory regions as
"guarded", but this involves changes on several platforms.

All the best
Peter

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