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But I know I have a lurking bug because there is one location which can potentially be written by both cpu - but neither does it very often at all. Fixing is difficult because I can't spin due to real-time constraints.
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In your case (since you have one location, infrequently accessed), would it be simpler to avoid the simultaneous write issue altogether and use altera_avalon_mutex as a special case?
It is a little bit broader topic than the dual port undefined contents being discussed in this thread, but NIOS (Qsys fabric) is missing atomic exchange which would be useful in your case (and others).
It would be relatively minor to get a workable solution to that using a custom instruction and conduits to guarantee the "atomic" aspect. For example, LOCK/UNLOCK primitives to guard your mutex try_lock() code.
Not elegant, but kind of easy to implement.
Honored Contributor
