i915 driver crashes on T540p if docking station attached

[Theodore Ts'o]

Unfortunately the failure causes a series of recursive faults and I
haven't been able to capture the stack trace, but on 4.2-rcX kernels,
I can reliably cause the system to crash if my T540p is booted with
the docking station attached.

It will also crash if I boot the system first, and then insert the
laptop into the dockstation.

Unfortunately, I can't get a stack trace because there are a huge
number of recursive/double faults, and the system dies so quickly that
nothing ends up in the log files.  If you really need a stack dump I
can try to rig something, but modern Laptops don't have serial
consoles any more, alas, so it's bit of a pain.

I was able to bisect it down to this commit, however: 8c7b5ccb72987:
"drm/i915: Use atomic helpers for computing changed flags:"

Is there any chance Intel could add a Lenovo Dockstation with a
Multistream DP output to part of your test hardware?  Unfortunately it
seems pretty common that I see regressions with my particular
hardware.  Maybe there aren't enough people using Thinkpads any more?  :-(

      	     	    	   	 	       	      - Ted


P.S.  The git bisect log

git bisect start
# bad: [421d125c06c4be4c5005cb69840206bd09b71dd6] builddeb: sign the modules after splitting out the debuginfo files
git bisect bad 421d125c06c4be4c5005cb69840206bd09b71dd6
# good: [b953c0d234bc72e8489d3bf51a276c5c4ec85345] Linux 4.1
git bisect good b953c0d234bc72e8489d3bf51a276c5c4ec85345
# good: [aeaa2122af4e53f3bfd28e8f294557bb95af43fc] drm/i915/skl: Add the INIT power domain to the MISC I/O power well
git bisect good aeaa2122af4e53f3bfd28e8f294557bb95af43fc
# bad: [4d70f38a760ad2879d2ebd84001c92980180f630] drm/i915/bios: remove a redundant NULL pointer check
git bisect bad 4d70f38a760ad2879d2ebd84001c92980180f630
# bad: [27a1b688d9f1fa2abd14bfe6a8729a19fb3b1b25] drm/i915/bxt: Enable WaEnableYV12BugFixInHalfSliceChicken7 for Broxton
git bisect bad 27a1b688d9f1fa2abd14bfe6a8729a19fb3b1b25
# good: [4be0731786de10d0e9ae1d159504c83c6b052647] drm/i915: Add crtc states before calling compute_config()
git bisect good 4be0731786de10d0e9ae1d159504c83c6b052647
# good: [d5432a9d19b61ba6a2b3d88f3026e0ca60eb57a1] drm/i915: Stage new modeset state straight into atomic state
git bisect good d5432a9d19b61ba6a2b3d88f3026e0ca60eb57a1
# bad: [a821fc46bc7bb6d4cf9a5f8d2787fd70231c2c10] drm/i915: Swap atomic state in legacy modeset
git bisect bad a821fc46bc7bb6d4cf9a5f8d2787fd70231c2c10
# bad: [8c7b5ccb729870e606321b3703e2c2e698c49a95] drm/i915: Use atomic helpers for computing changed flags
git bisect bad 8c7b5ccb729870e606321b3703e2c2e698c49a95
# good: [0f63cca2afdc38877e86acfa9821020f6e2213fd] drm/i915: Update crtc state active flag based on DPMS
git bisect good 0f63cca2afdc38877e86acfa9821020f6e2213fd
# good: [840bfe953384a134c8639f2964d9b74bfa671e16] drm/atomic: Make mode_fixup() optional for check_modeset()
git bisect good 840bfe953384a134c8639f2964d9b74bfa671e16
# first bad commit: [8c7b5ccb729870e606321b3703e2c2e698c49a95] drm/i915: Use atomic helpers for computing changed flags