(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.

No longer sure about that. I had been bisecting in
anongit.freedesktop.org/drm/drm. When I switched to
git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git, both 0a1d565 and
30cdbfa show the problem. So I can now try to bisect further.