linux-kernel - Re: [PATCH] pci: Rework ASPM disable code

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Date:	Fri, 11 Nov 2011 10:49:02 +0100
From:	Rafał Miłecki <zajec5@...il.com>
To:	Matthew Garrett <mjg@...hat.com>
Cc:	jbarnes@...tuousgeek.org, linux-pci@...r.kernel.org,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pci: Rework ASPM disable code

2011/11/10 Matthew Garrett <mjg@...hat.com>:
> 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.
>
> Signed-off-by: Matthew Garrett <mjg@...hat.com>

Does it make sense to CC stable? To get it into 2.6.38+?

-- 
Rafał
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