cpu voltage change question

cpu voltage change question

[mihn]

Hi,

I want to change voltage and frequency seperately.
With cpu-freq, i can change frequency, but cannot change voltage AFAIK.

Can I do this using cpu-freq?

If possible, how can I set voltage with the fixed value?
If not, is there any way to do that?

FYI, i'm using intel i7-core(nehalem) and ubuntu 10.10.
FYI2, i found out i can do using IA32_PERF_CTL register, but there's no
detail from intel.
FYI3, can i change P-state table so that i can only change voltage in P
states?

Thanks,
mihn

Re: cpu voltage change question

[Phillip Susi]

On 1/27/2011 4:10 AM, mihn wrote:
> Hi,
> 
> I want to change voltage and frequency seperately.
> With cpu-freq, i can change frequency, but cannot change voltage AFAIK.

The voltage is controlled by the platform and should already be set to a
level appropriate for that frequency.  AFAIK, recent intel cpus have
MSRs that configure the voltage for each P-state, but these are set up
by the bios and shouldn't need changed.

Re: cpu voltage change question

[Henrique de Moraes Holschuh]

On Thu, 27 Jan 2011, Phillip Susi wrote:
> The voltage is controlled by the platform and should already be set to a
> level appropriate for that frequency.  AFAIK, recent intel cpus have
> MSRs that configure the voltage for each P-state, but these are set up
> by the bios and shouldn't need changed.

"Need", certainly not.

But downvolting and downclocing are very effective ways to get some extra
battery time (or at least used to be, I have no idea if they still are that
useful in the newest processors), so you will find that there is a community
of "downclockers" among laptop enthusiasts, much like the "overclock"
community on the gaming enthusiasts :P

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh

Re: cpu voltage change question

[mihn leigh]

Yes, you're right. As you said, MSR register contains the frequency
and voltage information of cpu. By writing the register, we can change
voltage as well (it's the information from intel datasheet). But, i
cannot find the detail of this register.

BTW, what's the reason that MSR register can not be changed in OS
level? is it only possible to change in BIOS?

Thanks.

2011/1/27 Phillip Susi <psusi@cfl.rr.com>:
> On 1/27/2011 4:10 AM, mihn wrote:
>> Hi,
>>
>> I want to change voltage and frequency seperately.
>> With cpu-freq, i can change frequency, but cannot change voltage AFAIK.
>
> The voltage is controlled by the platform and should already be set to a
> level appropriate for that frequency.  AFAIK, recent intel cpus have
> MSRs that configure the voltage for each P-state, but these are set up
> by the bios and shouldn't need changed.
>
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Re: cpu voltage change question

[Henrique de Moraes Holschuh]

On Thu, 27 Jan 2011, mihn leigh wrote:
> Yes, you're right. As you said, MSR register contains the frequency
> and voltage information of cpu. By writing the register, we can change
> voltage as well (it's the information from intel datasheet). But, i
> cannot find the detail of this register.
> 
> BTW, what's the reason that MSR register can not be changed in OS
> level? is it only possible to change in BIOS?

Well, while we do provide users with enough rope to hang themselves as a
regular friendly kernel service (especially if it is limited to root
access), we usually refrain from giving them hand grenades with the safety
pin removed...

And sometimes the hardware does the same, and thus the notion of the
write-once register was born.  I don't think it is the case here, but...

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh

Re: cpu voltage change question

[mihn]

Thanks for the reply.
I'm happy to hear that OS can change MSR value.

Yes. I noticed from intel i7 datasheet that IA32_PERF_CTL MSR register
can change voltage and frequency at the same time.

What i want to know is how to change voltage while keeping the frequency
unchanged.

Say that current frequency is 2.8Ghz, and voltage is 1.5v.
In this case, i just want to increase voltage to 1.6v w/o frequency
changing.

If i know the spec of IA32_PERF_CTL_MSR, then would it be possible?
If not or if there's no way to get the spec of MSR, is there any
possible way to achieve it?

Thanks,
mihn


On Fri, 2011-01-28 at 14:16 +0800, 吴名 wrote:
> OS can change MSR value. If not, how cpufreq do the scaling job.
> You just can't write frequency and voltage separately.
> Because they are not separate.
> 
> wkq
> 
> 2011/1/28 mihn leigh <mihnsk@gmail.com>:
> > Yes, you're right. As you said, MSR register contains the frequency
> > and voltage information of cpu. By writing the register, we can change
> > voltage as well (it's the information from intel datasheet). But, i
> > cannot find the detail of this register.
> >
> > BTW, what's the reason that MSR register can not be changed in OS
> > level? is it only possible to change in BIOS?
> >
> > Thanks.
> >
> > 2011/1/27 Phillip Susi <psusi@cfl.rr.com>:
> >> On 1/27/2011 4:10 AM, mihn wrote:
> >>> Hi,
> >>>
> >>> I want to change voltage and frequency seperately.
> >>> With cpu-freq, i can change frequency, but cannot change voltage AFAIK.
> >>
> >> The voltage is controlled by the platform and should already be set to a
> >> level appropriate for that frequency.  AFAIK, recent intel cpus have
> >> MSRs that configure the voltage for each P-state, but these are set up
> >> by the bios and shouldn't need changed.
> >>
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-acpi" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >


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Re: cpu voltage change question

[Phillip Susi]

On 1/28/2011 4:10 AM, mihn wrote:
> Yes. I noticed from intel i7 datasheet that 	 MSR register
> can change voltage and frequency at the same time.
> 
> What i want to know is how to change voltage while keeping the frequency
> unchanged.

Then you change the voltage and leave the frequency bits where they are
at.  This should be blindingly obvious.

> Say that current frequency is 2.8Ghz, and voltage is 1.5v.
> In this case, i just want to increase voltage to 1.6v w/o frequency
> changing.

This is not a sensible goal.  If it is running stable already at 1.5v,
then you do not want to increase to 1.6v because that will just be a
waste of power.  The reason to increase the voltage is so that you can
increase the frequency.

> If i know the spec of IA32_PERF_CTL_MSR, then would it be possible?
> If not or if there's no way to get the spec of MSR, is there any
> possible way to achieve it?

Good question.  I can not find it either.  The system programming guide
just says that it is 16 bits and model specific.  The data sheet for the
core i5 does not define its contents.

I thought there were supposed to be a few MSRs somewhere that held the
VID codes that should be used to select the voltage for each P-state,
and then you switch P-states by touching the address specified in the
P-state table the bios provides in the DSDT, but the description of this
MSR makes it sound like that is not correct.