Comment 0 for bug 1493746

Switching between extended/mirror modes when a HDMI monitor is plugged in causes a system hang on at least some machines, like my SKL-Y ultramobile test system. It needs a backport of an upstream commit from 4.3, modified to work on 4.2-based i915_bpo:

commit f015c5518879fb3e578caaa63806617468a24045
Author: Ville Syrjälä <email address hidden>
Date: Wed Jun 24 22:00:02 2015 +0300

    drm/i915: Split atomic wm update to pre and post variants
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the watermark stuff is Skylake-specific, so can't regress BSW/BDW. This has been tested on a number of SKL machines now, fixing the bug.