Re: sysctl support for ICTC

As an exercise ;-), I've added sysctl support for controlling and
displaying the contents of the G3 (and G4?) ICTC register.

Heh ! Nice ;) That's something I've wanted to do for years and never
ended up actually coding ;)

What this register does: it slows down the rate at which instruction are
taken out of the L1 instruction cache for execution. Bit 31 (lowets bit)
enables the feature, the next 8 bits (23-31) define the number of clock
cycles to wait between instruction fetches.

Why it is useful: when combined with DPM (dynamic power management), the
effect of reducing the instrucion fetch rate is that DPM will more often
shut down the various execution units while being idle, thus reducing
power consumption and heat dissipation. This is good for battery life in
portable applications, since there's less power consumed in the
processor, and the fan can stay off (if combined with temperature
monitoring).

So, managing ICTC could be part of an intelligent apmd-style daemon
specifically for PPC.

Well, I didn't think about that aspect of things with DPM.

The Darwin code has complete temperature control and uses the ICTC to
regulate it. That could be done from userland too on linux, by also
adding some support for the temp. registers.

If anyone wants to go ahead and implement that daemon, attached is the
ICTC part already ;-) It's a patch against kernel.org 2.2.17. It is
nothing intelligent, rather a copy of the existing l2cr support. It adds
an entry under the proc filesystem:

Well, put it on sourceforge, and if noone picks it up, I'll merge it one
of these days.

Ben.

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