Comment 34 for bug 1041790

Looks that corresponds to the bug

commit 1c8b46fc8c865189f562c9ab163d63863759712f
Author: Chris Wilson <email address hidden>
Date: Wed Nov 14 09:15:14 2012 +0000

    drm/i915: Use LRI to update the semaphore registers

    The bspec was recently updated to remove the ability to update the
    semaphore using the MI_SEMAPHORE_BOX command, the ability to wait upon
    the semaphore value remained. Instead the advice is to update the
    register using the MI_LOAD_REGISTER_IMM command. In cursory testing,
    semaphores continue to function - the question is whether this fixes
    some of the deadlocks where the semaphore registers contained stale
    values?

hopefully addresses.

That patch is only available on drm-intel-next at the moment, which is available either at http://cgit.freedesktop.org/~danvet/drm-intel or available as drm-intel-experimental in the ubuntu kernel-ppa.