Re: enabling aspm on ati radeon

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On Fri, May 17, 2013 at 7:20 PM, Phillip Susi <psusi@xxxxxxxxxx> wrote:
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> On 05/17/2013 08:40 PM, Bjorn Helgaas wrote:
>> Right; I meant RO per the PCIe spec.  If it's RW on your chip,
>> setpci should be able to write it.
>
> I assume it would do no good to change it at this point; it would need
> to already be set at the time the kernel reads it and decides whether
> or not to enable it, so I'd have to have grub set it before loading
> the kernel.

Yeah, setting the capability bits after the kernel has booted wouldn't
change anything except what lspci reports.

>> If you can measure the power consumption, you might be able to see
>> a difference there.  I've seen people report that, but don't know
>> how they do it.  For example:
>> https://lkml.kernel.org/r/CANUX_P3F5YhbZX3WGU-j1AGpbXb_T9Bis2ErhvKkFMtDvzatVQ@xxxxxxxxxxxxxx
>
> After
>>
> enabling it on both ends ( despite the root port still claiming
> it is not supported ) I still see ~121 watts according to the
> Kill-A-Watt my system is plugged into.  Oh well.

ASPM only saves power when the PCIe link is idle.  I'm not a video
person, so I don't know how video usage translates into link usage,
and I don't know how much power it saves.  My guess is that the amount
is fairly small and might not be measurable with a Kill-A-Watt.

Bjorn
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