Timekeeping oddities on MacMini G4s
segher at kernel.crashing.org
Mon Feb 6 13:12:34 AEDT 2017
On Mon, Feb 06, 2017 at 10:22:01AM +1100, Benjamin Herrenschmidt wrote:
> > On the plus side, that means that the values are guaranteed not
> > to be core-specific. On the minus side, it means that its count rate is
> > lower, and it's sufficiently "distant" that accessing it is somewhat more
> > expensive.
> Right so there are various configuration options and ways to feed the timebase
> to PowerPC chips depending on the generation and manufacturer. On the old
> 32-bit chips, typically it was either a divisor of the bus frequency or
> externally clocked. Apple typically used the latter.
On all 6xx and most 7xx/7xxx it is 1:4 of the bus clock. And on the
newer machines the clock chip uses clock spreading. So you then cannot
calibrate with a dumb fast routine (the time base ticks pretty slow
anyhow, you cannot calibrate any fast if you want decent results; but
with clock spreading you either have to measure for many seconds, or you
need to find the period of the spreading and work with that).
> > The PowerPC architecture permits the timebase frequency to be variable,
> > but I'm not aware of any implementations that take advantage of that.
> I think it's pretty much accepted that this would be a very bad idea
> and no implementation did it.
> > The
> > Motorola 32-bit implementations in general run it on the "bus clock",
> > which is independent of processor-clock multipliers, and is also common
> > across processor chips in systems with more than one.
> There's also a TBEN external pin iirc which can be used to feed it.
Some implementations have an MSR bit to stop the TB as well (7450 for
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