netdev - Re: [PATCH 0/16] DMA-API debugging facility v2

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Date:	Fri, 6 Feb 2009 08:08:12 -0800
From:	Chris Wright <chrisw@...s-sol.org>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Joerg Roedel <joerg.roedel@....com>,
	fujita.tomonori@....ntt.co.jp, netdev@...r.kernel.org,
	iommu@...ts.linux-foundation.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/16] DMA-API debugging facility v2

* David Woodhouse (dwmw2@...radead.org) wrote:
> What machine did you get that on?

That's a T400 (I'd expect it's same issue as X200).

> Yeah, I saw one of those. If could be a driver bug, of course -- it
> could be unmapping a range before it's actually finished with it. But
> that's unlikely.

One thing I noticed is many of the faults are the page prior to a
coherent range 0x8000 in length, which seems to correspond to tfd buffer.
No obvious off-by-one error anywhere, and not all faults fit that pattern,
so w/out more iwlagn driver knowledge hard to say if that's meaningful
or just mapping coincidence.

> An alternative explanation... The DMA is aborted¹, and the device
> interrupts us to tell us about it at the _same_ time that the IOMMU
> interrupts us to tell us about the fault. We process the device
> interrupt first, unmap that buffer. And then we process the IOMMU
> interrupt... and the buffer is already gone from the list.

I'd have expected the iommu fault to be delivered first, but hey...

> It might be interesting to make this code also remember and print the
> last range that was unmapped, as well as the currently-mapped ranges.

That's what I was thinking too.  Almost need a flight recorder mode to
see if the range was ever mapped/unmapped.

thanks,
-chris
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