Suggestion for tracking down ubuntu-kernel bug

[Sandy Patterson]

Hi,

I have found a small bug
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1768139 and I'd like
to try and narrow the problem some.

* Issue is present in ubuntu but not in mainline (4.15.17 I think is
closest? I have tried a bunch though 4.15.0, 4.15.18, 4.15.16...)
* I compiled mainline kernel 4.15.17 from linux-stable with ubuntu config
from /boot/config-4.15.0-22-generic, that did not show issue.
* I attempted to go backwards in ubuntu-bionic git history and made it back
to 4.15.0-1_4.15.0-1.2 which was the last build that succeeded for me. This
exhibited the issue and every newer kernel I tried.

The crux is that intel_prepare_dp_ddi_buffers appears to be called with an
hdmi type encoder (when both are connected). I compared
the drivers/gpu/drm/i915 folder between mainline and bionic, but couldn't
find anything promising. I'm not sure I know enough about kernel gpu to
trace myself.

Can anyone suggest another test? I can't figure out how to bisect since I
can't get a working and failing build in the same tree.

Sandy Patterson
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