Re: enabling aspm on ati radeon

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Monday, May 20, 2013 07:34:44 AM Bjorn Helgaas wrote:
> On Fri, May 17, 2013 at 7:20 PM, Phillip Susi <psusi@ubuntu.com> 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@mail.gmail.com
> >
> > 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.

I'm pretty sure it's measurable, at least in some cases.  Phoronix did quite
some fuss about a commit that caused ASPM to not work any more on some systems
some time ago, so he had to be able to measure it (and it was called a "major
power regression").

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.