[Bug 75710] DP monitor does not get activated when snd_hda_intel is loaded

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=75710

--- Comment #5 from Daniel Martin <consume.noise at gmail.com> ---
So, this took me a while (failed at bisecting etc.). One reason was the
assumption that v3.6 is fine, which isn't true. Additionally, I made
observations that help me to see from the dmesg if it's working or if the
monitor will switch into suspend mode (more on this at the end).

I bisected it down to commit (v3.6-rc1):
    2514bc5 drm/i915: prefer wide & slow to fast & narrow in DP configs

Reverting it on top of v3.6 fixes the problem. The used lanes drop and the
clock raises from:
    [drm:intel_dp_compute_config], DP link bw 06 lane count 4 clock 162000 bpp
24
to:
    [drm:intel_dp_compute_config], DP link bw 0a lane count 2 clock 270000 bpp
24

But, that's the complete opposite why this commit/fix has been added:
    https://bugs.freedesktop.org/show_bug.cgi?id=45801
So, reverting it may cause trouble at other maschines.


Then, I have switched to v3.14-rc5 for further testing:

1. Reverting 2514bc5 on top of v3.14-rc5 fixes the problem too. The next
observatins have been done _without_ the revert - plain v3.14-rc5.

2. In my initial tests `xrandr --output DP2 --set audio off` didn't changed the
result. Now, it works reliable. The problem is gone.

3. [drm:ilk_display_irq_handler], Pipe B FIFO underrun
If I see this message after a `xrandr --output DP2 --preferred`, while the
audio property is at its default (auto), then I know that the monitor will
switch on and everything is fine. If I don't see it, the monitor will switch
into standby.


I hope that at least the last observation is helpful.

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