(In reply to Alex Deucher from comment #1)
> Are you sure the bisect is correct?  This commit just changes debugfs which
> shouldn't be triggered unless you actually write to the file in question. 
> Please attach your xorg log (is using X) and dmesg output.

A fresh bisect in the stable tree has given a new pair of commits.
e1cb3e4801e6896ba93d63222b1052199d2a8c9b has the problem and
899e2aaddbfa0ff96fbaf31f0d9e91427e87dd88 does not have it. (In the stable tree,
30cdbfaa6aa469347db7fcda5949f1ccf7559ecf also has the problem, unlike in the
drm tree that I bisected previously).
A diff of the new commit pair shows 14 patched files. These are TODO, Makefile
and a mixture of .c and .h sources. I am unsure how to proceed with bisecting
these diffs: advice welcome.