mmio latency measurements

[Benjamin Herrenschmidt]

> 1. Is this a valid use of get_cycles()
> 2. Are there any special caveats ?
> 3. Please propose an alternative mechanism, in case get_cycles() won't
>    work

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).
That's the value get_cycles() returns. You can see the calibration value
of the timebase in herz in /proc/cpuinfo.

Ben.