mmio latency measurements

[Segher Boessenkool]

> On ppc64, there is no cycle-counter per-se, but a HW timebase that 
> ticks
> at a fixes frequency (independently of the CPU frequency nowadays).

On a 970, and presumably on POWER4 and maybe POWER5 as well,
you *can* get cycle counts -- from the performance monitor
counters.  Simply write 0xf00 to MMCR0 and then read the cycle
count from PMC1.


Segher