Re: [PATCH] pci: Rework ASPM disable code

[Jesse Barnes <jbarnes@virtuousgeek.org>]

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On Thu, 10 Nov 2011 16:38:33 -0500
Matthew Garrett <mjg@redhat.com> wrote:

> Right now we forcibly clear ASPM state on all devices if the BIOS indicates
> that the feature isn't supported. Based on the Microsoft presentation
> "PCI Express In Depth for Windows Vista and Beyond", I'm starting to think
> that this may be an error. The implication is that unless the platform
> grants full control via _OSC, Windows will not touch any PCIe features -
> including ASPM. In that case clearing ASPM state would be an error unless
> the platform has granted us that control.
> 
> This patch reworks the ASPM disabling code such that the actual clearing
> of state is triggered by a successful handoff of PCIe control to the OS.
> The general ASPM code undergoes some changes in order to ensure that the
> ability to clear the bits isn't overridden by ASPM having already been
> disabled. Further, this theoretically now allows for situations where
> only a subset of PCIe roots hand over control, leaving the others in the
> BIOS state.
> 
> It's difficult to know for sure that this is the right thing to do -
> there's zero public documentation on the interaction between all of these
> components. But enough vendors enable ASPM on platforms and then set this
> bit that it seems likely that they're expecting the OS to leave them alone.
> 
> Measured to save around 5W on an idle Thinkpad X220.
> 

Applied to linux-next, thanks.

-- 
Jesse Barnes, Intel Open Source Technology Center

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