[Bug 98690] [i915][SKL] System freeze when starting X using kernel 4.9-rc1 or later

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=98690

--- Comment #3 from Paul Donohue <freedesktop-bugs at PaulSD.com> ---
This bug seems unusually difficult to bisect.

The bug appears to be present in 4.8-rc1 through 4.8-rc3, and appears to have
been fixed in 4.8-rc4.  4.8-rc4 through 4.8.7 all work fine.  I have not tested
versions earlier than 4.8-rc1.

The 4.9 branch forked from the 4.8 branch after 4.8.0.  The 4.9 branch works
fine through 56e520c7a0a490b63b042b047ec9659fc08762a4.  However,
6b25e21fa6f26d0f0d45f161d169029411c84286 and later are broken.

6b25e21fa6f26d0f0d45f161d169029411c84286 merges
69405d3da98b48633b78a49403e4f9cdb7c6a0f5 into the 4.9 branch. 
69405d3da98b48633b78a49403e4f9cdb7c6a0f5 appears to have been developed in a
branch that was forked at 4.8.0-rc1, so
69405d3da98b48633b78a49403e4f9cdb7c6a0f5 and all prior commits on that branch
do not work (they all have the same bug because they are based on a version
with the bug).

So, something in the 6b25e21fa6f26d0f0d45f161d169029411c84286 merge appears to
have re-introduced the bug from 4.8-rc1 into the 4.9 branch.  But, of course,
git bisect can't help me figure out what in the merge caused this.

My next step is to bisect 4.8-rc3 through 4.8-rc4 to see if I can figure out
which commit fixed the issue.  Knowing what the fix is should make it easier to
spot the problem with the merge.  I'm pretty sure the fix has to be one of the
9 commits between fa8410b355251fd30341662a40ac6b22d3e38468 and
177d91aaea4bcafb29232336bafaa521b85286aa.

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