[Intel-gfx] Flashing display after link training failure (Bug 1378)

[Intel-gfx] Flashing display after link training failure (Bug 1378)

[Jason Andryuk]

Hi,

I opened a bug ( https://gitlab.freedesktop.org/drm/intel/-/issues/1378 )
a few months back, but no one seems to have looked at it.  I'm
emailing here to hopefully get a suggestion on a path forward.

tl;dr: when resuming from DPMS, link training channel equalization
fails on the Dell 7200 Whiskey Lake tablet eDP display.  There is a
flash of the screen and then it goes black.  This repeats probably
until the display goes into DPMS again.  The system is sluggish when
this happens.

i915 0000:00:02.0: [drm:intel_dp_start_link_train [i915]] Channel
equalization failed 5 times
i915 0000:00:02.0: [drm:intel_dp_start_link_train [i915]]
[CONNECTOR:95:eDP-1] Link Training failed at link rate = 270000, lane
count = 2

Right now I have a hack to just return success from
intel_dp_link_training_channel_equalization() after the 5 failures and
the display seems to be usable.

The issue seems to be that while Lane 0 succeeds in link training,
Lane 1 does not.  The system boots up with a usable display and seems
to keep working as long as it doesn't go into DPMS.  And to repeat the
previous paragraph, hacking in "success", even though Lane 1 hasn't
succeeded, seems to give a working display after resuming from DPMS.

More details are in the bug.  Any thoughts or suggestions are greatly
appreciated.

Thanks,
Jason
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