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lexflex
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PostPosted: Fri Feb 01, 2013 6:33 am    Post subject: Undervolting AMD Llano Reply with quote

Hi,

I would like to undervolt my cpu at low frequencies.
For this I installed linux-phc, , but this doesn't support my cpu it seems from the website http://www.linux-phc.org/ .

Also,
Code:
modprobe phc-k8

Resulted in a segmentation fault.

Is there any other way to change the CPU voltage profile ?

Thanks for any pointers related to undervolting,

Alex.
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lexflex
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PostPosted: Fri Mar 08, 2013 3:23 pm    Post subject: Reply with quote

Bump, anyone any idea?
Maybe it cant be done using software but only from the bios (but then it is a static voltage), but I can't find a definite answer ( also not an "its not possible"...)
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depontius
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PostPosted: Fri Mar 08, 2013 5:06 pm    Post subject: Reply with quote

Make sure your kernel includes the cpu frequency modulation stuff - ask more questions if you need more info. I typically make 'ondemand' my default governor, and then make sure you build the right hardware interface. I don't know if Llano uses powernow-k8, powernow-k10, cpufreq-acpi, or something else. I believe the governors correctly modulate the voltage.
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lexflex
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PostPosted: Fri Mar 08, 2013 7:24 pm    Post subject: Reply with quote

Thanks for your reply,

Indeed I already have the powernow-k8 daemon and cpufreq scaling active.
I was looking for a way to adjust the voltage-scheme related to the specific frequencies, since it appears stock-voltages are still quite high.
linux-phc seems to does something like this, but for different cpu's...

Alex.
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depontius
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PostPosted: Fri Mar 08, 2013 7:42 pm    Post subject: Reply with quote

There was an additional set of "stuff" added to the kernel in the past 6-9 months. It looks a lot like the frequency modulation, but carries more overtones of voltage regulation about it. It's listed under "PM_DEVFREQ" and looks a lot like the old cpufreq stuff. (or "Generic Dynamic Voltage and Frequency Scaling (DVFS) support (PM_DEVFREQ)") The funny thing is that while there are the usual 4 governors listed, there is a spot for drivers, but non are listed. I also looked in staging and saw nothing there. So I'm not sure what's up with this, or if it's what you're looking for.
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lexflex
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PostPosted: Sat Mar 09, 2013 7:14 am    Post subject: Reply with quote

Thanks,

I didn't notice that before, I will look into that. It seems indeed to have to do with allowing better userspace control of voltages and frequencies, allthough it is still unclear to me how this works ( searching the web reveals some stuff about DVFS , so I'll start reading :-) ).
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_______0
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PostPosted: Sat Mar 09, 2013 6:19 pm    Post subject: Reply with quote

lexflex

I think for llanos the acpi_cpufreq module is better than powernow-k8. Both work but the former apparently is more modern and complete.

I also like this idea about undervolting. If smartphones can do it why not desktops? I don't like the minimal >1.3Ghz or something for desktop. Ideally minimum would be 700mhz or 500mhz.

I know this should work because there are routers running linux at around 200mhz.

Indeed there was a crazy idea about adding an ARM cpu to x86 machines so they can be put in a lower power state.

Why is x86 so inflexible about frequency scalability? I have an AMD proc just with TWO frequencies options :/
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lexflex
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PostPosted: Sun Mar 10, 2013 6:19 pm    Post subject: Reply with quote

Thanks,

_______0 wrote:

I think for llanos the acpi_cpufreq module is better than powernow-k8. Both work but the former apparently is more modern and complete.

Ah, indeed I see that now. Tried to modprobe acpi_cpufreq with my 'old' 3.6 kernel and couldn't load it, but after upgrading to 3.7.10 now acpi_cpufreq is the default ( and loaded automatically instead of powernog-k8 it seems).
Quote:

I also like this idea about undervolting. If smartphones can do it why not desktops? I don't like the minimal >1.3Ghz or something for desktop. Ideally minimum would be 700mhz or 500mhz.
Why is x86 so inflexible about frequency scalability? I have an AMD proc just with TWO frequencies options :/

For my AMD A6-3500, the minimum is 800 MHz using the regular governors, so that is a better already. And it works in smaller steps up to 2.4 GHz.
But still, a bit lower with a lower power-consumption would indeed be nice...

I'll try further to see what I can do about the voltage, and post here if I manage to reduce it....

Alex.
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