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Cortex A9 WFE instruction not working as expected

Rich Podraza
Note: This was originally posted on 10th October 2012 at http://forums.arm.com

I'm trying to do a simple test with WFE instruction and EVENTI/O signals. I'm trying to have one A9 enter a wait state with WFE, and have another A9 do some dummy operations while the first is waiting, then do SEV instruction to signal from its SIGNALO to the other A9's SIGNALI. I can verify with my simulation envronment that this signal fires. However, the first A9 is not waiting, it breezes right by the WFE instruction. No assembler warning indicating these instructions aren't supported.

My best clue is in the assembler guide there is the line "If the Event Register is set, WFE clears it and returns immediately," it sounds like this might be happening, but I can't find any reference to "Event Register" in the Cortex A9 TRM, so I'm not sure if this is active by default in my system or what.

Any ideas? Thanks!
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  • Arm Employee Badge
    Note: This was originally posted on 11th October 2012 at http://forums.arm.com

    I think you've already spotted the problem - the event register.  You don't know its initial state.  If it is set, the the core will appear to "wake" immediately.

    WFE/SEV are not themselves suitable for implementing synchronization primitives.  That is what LDREX/STREX are for.

    What I'd suggest in your case is to use a separate flag to indicate the second core has finished.  The first core loops waiting for the flag to be cleared - with a WFE in the loop.  That way if you wake unexpectedly you can see that the second core hasn;t finished yet and then go back to sleep.
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  • Arm Employee Badge
    Note: This was originally posted on 11th October 2012 at http://forums.arm.com

    I think you've already spotted the problem - the event register.  You don't know its initial state.  If it is set, the the core will appear to "wake" immediately.

    WFE/SEV are not themselves suitable for implementing synchronization primitives.  That is what LDREX/STREX are for.

    What I'd suggest in your case is to use a separate flag to indicate the second core has finished.  The first core loops waiting for the flag to be cleared - with a WFE in the loop.  That way if you wake unexpectedly you can see that the second core hasn;t finished yet and then go back to sleep.
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