Underclocking the XO 1.5

[James Cameron]

On Tue, Nov 27, 2012 at 12:19:38PM +0100, Arne Babenhauserheide wrote:
> I read the underclocking information on the custom bootloader page??
> und wondered how to achieve the same with the 1.5, since the page
> only gives the codes for the XO-1. How can I get the XO 1.5 to run
> at the lowest possible CPU and Bus speed?

You don't need to, it is automatic.

You can get the XO-1.5 to run at the lowest possible CPU speed by
running as few processes as possible, and writing your code to not
waste CPU cycles.  The CPU will clock down to 400 MHz (1.5 W)
automatically when demand is low.

You can evaluate how successful this is with

# cat /sys/devices/system/cpu/cpu0/cpuidle/state*/time

or

# cat /sys/devices/system/cpu/cpu0/cpuidle/state*/usage

See Documentation/cpuidle in kernel source.

Feel free to write this up on the Wiki.

There's probably also other support in the kernel for manipulating
the clocking, but I haven't identified it.

> Does 1.5 support the under-/overclocking commands of 1.75, too?

No.

The XO-1.75 can certainly be underclocked, but in my measurements this
has no significant effect on power consumption or heat generation, so
you would have to have some other reason for underclocking.

The XO-1.75 core frequency change commands in firmware all start with
pj4- then a number, so you can type that and then tab to get an idea
of them.

Type .fuses on XO-1.75 to display the maximum frequency the core is
certified for.

-- 
James Cameron
http://quozl.linux.org.au/