RFC: Performance Monitor Counters device

[Benjamin Herrenschmidt]

>It does not exist on 7450 etc.
>It exists on 7410 before version 1.3 .
>I have no data on 7400.
>It does not exist on 750 etc.
>
>[As my only G4 is a 7410 version 1.3, I won't be affected by this.  Hurray.]

I had it happening on 7400.

>The problem itself: if two of thermal assist, decrementer, performance
>monitor interrupts happen within 1 cycle of each other, evil things
>happen with SRR0 and SRR1, so that the return address becomes
>unrecoverable.
>
>Suggested solution:
>
>We can forget about thermal assist, as the TAU on all 74xx is broken
>and unsupported.  (Says those same errata sheets).
>If necessary, it's possible to disable the decrementer interrupt and have
>the performance monitor perform its function.
>
>But I won't run into this, so I'm happy for now :)

We can't disable the DEC that easily as it's the primary timer
source of the kernel ;)

Ben.


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