[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 #90 from Stefan Agner <stefan at agner.ch> ---
I'm not an expert in this field, but as far as I can interpret the debug
output, its only related to the kernel. It seems to be related to link
training, whatever that exactly means. My main setup is using an HDMI monitor,
so I started with that. I added more and more debug output and ended with what
I have in link-train-debug.patch. 

With that patch HDMI takes about 30 seconds and then I see output. But
sometimes, it doesn't work at all (see
with-link-train-debug-hdmi-3.13.0-ARCH-local-dirty.txt).

However, when I attached a DP monitor at it, switching worked almost instantly
(see with-link-train-debug-dp-3.13.0-ARCH-local-dirty.txt).

I then traced down the "offending" debug print, and replaced it with a delay
(see link-train-delay.patch). It seems to work on my setup. I tested docked and
docking while on, in both cases I get display output almost immediately... 

While I don't understand that stuff really, it seems to be a problem of reading
the link status, at least with that delay, reading it seems to heal the
problem, at least for DP...

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