linux-kernel - Re: [PATCH] drm/i915: By default, enable RC6 on IVB and SNB when reasonable

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Date:	Wed, 23 Nov 2011 15:03:43 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Daniel Vetter <daniel@...ll.ch>
Cc:	rajesh.sankaran@...el.com, Keith Packard <keithp@...thp.com>,
	Matthew Garrett <mjg@...hat.com>,
	intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/i915: By default, enable RC6 on IVB and SNB when
 reasonable

On Wed, 2011-11-23 at 15:39 +0100, Daniel Vetter wrote:
> At least for the dmar+gfx+semaphores hang I can reproduce, just disabling
> dmar with intel_iommu=igfx_off is not good enough and iirc the same holds
> for the dmar+rc6 hangs reported. 

Um... let me restate that for clarity (and partly for Rajesh's benefit).

The DMAR associated with the integrated graphics is *disabled*.
Turned off. Not active. Ever.

You have a problem when you enable the *other* DMAR units in the system,
which should not be affecting the graphics device in any way.

When you do this, you see 'hangs' with semaphores and RC6. Is there a
better description of these 'hangs' somewhere? Is the hardware
completely locked?

These hangs go away when you disable the DMAR units. Again, that is the
*other* DMAR units in the system that have nothing to do with graphics.

While I'm getting quite used to DMAR-related errata, this one does make
me stop and think 'wtf?'. It just seems so incongruous that disabling an
*unrelated* IOMMU would make the problem go away, and it makes me wonder
if it's actually a timing-related issue which is always there, but
something about the use of DMAR for network/disk/etc. makes it more
likely to trigger?

We definitely need the hardware folks to get to the bottom of this one.

-- 
dwmw2

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