Re: Plain DFS (no voltage scaling)

[Mason <slash.tmp@free.fr>]

On 02/02/2016 22:11, Mason wrote:

> I plan to enable the on-demand governor on the tango platform:
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/tango4-smp8758.dtsi
> 
> I found the cpufreq-dt binding doc:
> 
> https://www.kernel.org/doc/Documentation/devicetree/bindings/cpufreq/cpufreq-dt.txt
> https://www.kernel.org/doc/Documentation/devicetree/bindings/opp/opp.txt
> 
> Something is not clear to me:
> 
> If my platform cannot scale the voltage, what information
> should I put in the voltage part of the DT?

Someone pointed out that tweaking the frequency without tweaking
the voltage might be counter-productive.

I measured the power consumption of the entire board (at the power outlet)
for 3 CPU frequencies (all other things being equal, I hope).

idle @ 111 MHz = 4.6 W
idle @ 333 MHz = 4.6 W
idle @ 999 MHz = 4.6 W

load @ 111 MHz = 5.0 W
load @ 333 MHz = 5.7 W
load @ 999 MHz = 7.7 W

When idle, the kernel calls WFI, which "turns off" most of the CPU
(clock gating?) such that the actual frequency does not matter.

At full load (I use cpuburn to jog as many FUs simultaneously as
possible) it looks like each additional MHz requires ~3 mW.

So it would appear that an on-demand governor might not help to
save power.

But I have another use-case in mind: CPU throttling on over-heating.
There's a temperature sensor in the CPU, and I'd like to say:
"if temperature exceeds a user-set threshold, don't run at the max
frequency until the temperature becomes reasonable".

And I think that requires cpufreq?

Regards.