linux-kernel - Re: [ath5k-devel] [PATCH v2] ath5k: disable ASPM

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Date:	Tue, 22 Jun 2010 18:50:58 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	"Luis R. Rodriguez" <mcgrof@...il.com>
Cc:	Jussi Kivilinna <jussi.kivilinna@...et.fi>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	David Quan <David.Quan@...eros.com>,
	Bob Copeland <me@...copeland.com>,
	"Luis R. Rodriguez" <mcgrof@...badil.infradead.org>,
	ath5k-devel@...ema.h4ckr.net, linux-wireless@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jonathan May <jonathan.may@...eros.com>
Subject: Re: [ath5k-devel] [PATCH v2] ath5k: disable ASPM

On Tue, Jun 22, 2010 at 10:40:15AM -0700, Luis R. Rodriguez wrote:
> On Tue, Jun 22, 2010 at 10:25 AM, Matthew Garrett <mjg59@...f.ucam.org> wrote:
> > Right, which we have to deal with by having drivers disable ASPM on
> > broken devices.
> 
> Agreed, but then the assumption would be drivers are ASPM bug free
> which is expect to be false with Video and 802.11 given that only a
> handful of vendors do actually get involved with their drivers
> upstream. Safe thing of course is to just disable it, of course, but
> if you are going to use pcie_aspm=force good luck!

People who use "force" deserve whatever they get, but "powersave" really 
ought to work. Fedora's defaulted to that for a while now - we've hit 
issues with aacraid, but that's pretty much it in terms of cases where 
the heuristics don't work. Maxim's problems wouldn't be triggered 
because CONFIG_PCIE_ASPM disables it on pre-1.1 devices regardless of 
the BIOS setup.

> > Having looked into this, Windows will enable ASPM on external
> > controllers unless there's some reason for it not to - where that may be
> > either the appropriate bit in the FADT being set, the device not being
> > PCIe 1.1 or later, there being no _OSC method on the appropriate root
> > bridge or the _OSC method not giving it full control over PCIe, the
> > driver disabling ASPM or the device not advertising it in the first
> > place.
> 
> I was unaware of all this root complex sanity checks on Windows,
> thanks for sharing.

With the patch I've just sent, they should also all be used for Linux as 
well.

> I suspect these tweaks will go away as the industry produces cards
> with both L1 and L0s enabled all the time (devices being produced
> today), but for devices caught in that middle of time between whether
> or not L0s would be *required*  (last 2 years) I suspect we'll run
> into these issues.

If the same problems would appear under Windows then it's not a problem 
that I'm hugely concerned about as yet - we'll wait a bit longer and 
then change the ASPM defaults to be more aggressive under Linux, and if 
it turns out to be a significant problem in the real world we'll have to 
reconsider it. But I don't think we should be depending on userspace 
bashing hardware registers in order to be able to enable power 
management.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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