Comment 1 for bug 1042903

After an extensive series of tests, we've determined that the following kernel config change is the root cause of the issue:

commit 682f607fd4a57d273db9ecc360589a8fa9cec8e6
Author: Andy Whitcroft <email address hidden>
Date: Fri Jul 20 21:33:43 2012 +0100

    UBUNTU: [Config] Enable CONFIG_DRM_AST/_CIRRUS_QEMU/_MGAG200

The above config change enabled the mgag200 driver which is described as follows:

config DRM_MGAG200
        tristate "Kernel modesetting driver for MGA G200 server engines"
        depends on DRM && PCI && EXPERIMENTAL
        select FB_SYS_FILLRECT
        select FB_SYS_COPYAREA
        select FB_SYS_IMAGEBLIT
        select DRM_KMS_HELPER
        select DRM_TTM
        help
         This is a KMS driver for the MGA G200 server chips, it
         does not support the original MGA G200 or any of the desktop
         chips. It requires 0.3.0 of the modesetting userspace driver,
         and a version of mga driver that will fail on KMS enabled
         devices.

This driver is marked as EXPERIMENTAL. Our default config policy is to enable device specific experimental options to promote testing and feedback. However, should any experimental option produce adverse affects, we will disable them. As such, I've built a test kernel with CONFIG_DRM_MGAG200 disabled and we have confirmed this resolves the issue [1]. I've committed this change to the Quantal kernel git repository. It should be included in the subsequent 3.5.0-13.14 or newer upload. This however may unfortunately not land in time for the Quantal Beta-1 release. However, it will land well in advance of 12.10 being final. A temporary workaround aside from running the test kernel [1] is to blacklist the mgag200 driver.

[1] http://people.canonical.com/~ogasawara/lp1039178/