Bisected regression: hang on i915 between 3.1.0-rc9 and 3.1.0

[Meelis Roos]

3.0 worked fine, 3.1-rc9 worked fine, I think -rc10 too. 3.1 release 
hangs in random places while using X.

Core i5 660, lspci below. Running 64-bit Debian unstable.

lspci -nn

00:00.0 Host bridge [0600]: Intel Corporation Core Processor DRAM Controller [8086:0040] (rev 02)
00:02.0 VGA compatible controller [0300]: Intel Corporation Core Processor Integrated Graphics Controller [8086:0042] (rev 02)
00:16.0 Communication controller [0780]: Intel Corporation 5 Series/3400 Series Chipset HECI Controller [8086:3b64] (rev 06)
00:16.3 Serial controller [0700]: Intel Corporation 5 Series/3400 Series Chipset KT Controller [8086:3b67] (rev 06)
00:19.0 Ethernet controller [0200]: Intel Corporation 82578DM Gigabit Network Connection [8086:10ef] (rev 05)
00:1a.0 USB Controller [0c03]: Intel Corporation 5 Series/3400 Series Chipset USB2 Enhanced Host Controller [8086:3b3c] (rev 05)
00:1b.0 Audio device [0403]: Intel Corporation 5 Series/3400 Series Chipset High Definition Audio [8086:3b56] (rev 05)
00:1c.0 PCI bridge [0604]: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 1 [8086:3b42] (rev 05)
00:1c.4 PCI bridge [0604]: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 5 [8086:3b4a] (rev 05)
00:1c.6 PCI bridge [0604]: Intel Corporation 5 Series/3400 Series Chipset PCI Express Root Port 7 [8086:3b4e] (rev 05)
00:1d.0 USB Controller [0c03]: Intel Corporation 5 Series/3400 Series Chipset USB2 Enhanced Host Controller [8086:3b34] (rev 05)
00:1e.0 PCI bridge [0604]: Intel Corporation 82801 PCI Bridge [8086:244e] (rev a5)
00:1f.0 ISA bridge [0601]: Intel Corporation 5 Series Chipset LPC Interface Controller [8086:3b0a] (rev 05)
00:1f.2 SATA controller [0106]: Intel Corporation 5 Series/3400 Series Chipset 6 port SATA AHCI Controller [8086:3b22] (rev 05)
3f:00.0 Host bridge [0600]: Intel Corporation Core Processor QuickPath Architecture Generic Non-core Registers [8086:2c61] (rev 05)
3f:00.1 Host bridge [0600]: Intel Corporation Core Processor QuickPath Architecture System Address Decoder [8086:2d01] (rev 05)
3f:02.0 Host bridge [0600]: Intel Corporation Core Processor QPI Link 0 [8086:2d10] (rev 05)
3f:02.1 Host bridge [0600]: Intel Corporation Core Processor QPI Physical 0 [8086:2d11] (rev 05)
3f:02.2 Host bridge [0600]: Intel Corporation Core Processor Reserved [8086:2d12] (rev 05)
3f:02.3 Host bridge [0600]: Intel Corporation Core Processor Reserved [8086:2d13] (rev 05)


Bisected it to the following commit. Bisection may not be 100% correct 
because the issue happens at random, and most of the "good" revisions 
got some usage without problems. The "bad" bisects are the ones that 
hang.

6fbcfb3e467adb414e235eeefaeaf51ad12f2461 is the first bad commit
commit 6fbcfb3e467adb414e235eeefaeaf51ad12f2461
Author: David Woodhouse <dwmw2 at infradead.org>
Date:   Sun Sep 25 19:11:14 2011 -0700

    intel-iommu: Workaround IOTLB hang on Ironlake GPU
    
    To work around a hardware issue, we have to submit IOTLB flushes while
    the graphics engine is idle. The graphics driver will (we hope) go to
    great lengths to ensure that it gets that right on the affected
    chipset(s)... so let's not screw it over by deferring the unmap and
    doing it later. That wouldn't be very helpful.
    
    Signed-off-by: David Woodhouse <David.Woodhouse at intel.com>

:040000 040000 3d2943d67945d8321d7aa8faac7497a8e8f1412a afaa155acfd1a7111ee607a971013ed615582ac8 M      drivers


git bisect log:

git bisect start
# good: [899e3ee404961a90b828ad527573aaaac39f0ab1] Linux 3.1-rc10
git bisect good 899e3ee404961a90b828ad527573aaaac39f0ab1
# bad: [c3b92c8787367a8bb53d57d9789b558f1295cc96] Linux 3.1
git bisect bad c3b92c8787367a8bb53d57d9789b558f1295cc96
# good: [505f48b53478d3816d1f3b001815703cfd7afa09] Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net
git bisect good 505f48b53478d3816d1f3b001815703cfd7afa09
# good: [5117cc25fd43add94c04c9d2ace713c69a21bdb1] Merge branch 'hwmon-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/groeck/linux-staging
git bisect good 5117cc25fd43add94c04c9d2ace713c69a21bdb1
# bad: [4399c8bf2b9093696fa8160d79712e7346989c46] intel-iommu: fix superpage support in pfn_to_dma_pte()
git bisect bad 4399c8bf2b9093696fa8160d79712e7346989c46
# bad: [c0771df8d5297bfb9c4fbe8ada085a49cb22ec4f] intel-iommu: Export a flag indicating that the IOMMU is used for iGFX.
git bisect bad c0771df8d5297bfb9c4fbe8ada085a49cb22ec4f
# bad: [6fbcfb3e467adb414e235eeefaeaf51ad12f2461] intel-iommu: 
Workaround IOTLB hang on Ironlake GPUgit bisect bad 6fbcfb3e467adb414e235eeefaeaf51ad12f2461
# good: [3e7abe2556b583e87dabda3e0e6178a67b20d06f] intel-iommu: Fix AB-BA lockdep report
git bisect good 3e7abe2556b583e87dabda3e0e6178a67b20d06f

-- 
Meelis Roos (mroos at linux.ee)