[Bug 71267] sna: (Haswell) X-server freezes when enabling DP at docking station

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=71267

--- Comment #21 from Daniel Martin <consume.noise at gmail.com> ---
Created attachment 89947
  --> https://bugs.freedesktop.org/attachment.cgi?id=89947&action=edit
dmesg 3.11 (working! - with preemption)

I've found that the option "Preemptible Kernel (Low-Latency Desktop)" can be
used as a workaround for the problem. It doesn't work always, but usually.
Tested with v3.9..v3.11.

In the dmesg I can see the call to intel_dp_set_signal_levels() and a few
others, which I don't without preemption (attachment 89906). So, I guess the
lack of it before a call to ironlake_panel_vdd_work() is the bug?

As of v3.12 this workaround doesn't work anymore. I've started to bisect it,
but stopped as it was to inaccurate (as stated above, it didn't worked always
for v3.9..v3.11) and time consuming. And imo it's not that
predictable/representable due to preemption anyways. But, if you think that
it's worth looking at it, then I could try to completly bisect it.

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