[Intel-gfx] i915 4.9 regression: DP AUX CH sanitization no longer working on Asus desktops

[Daniel Drake]

Hi,

Numerous Asus desktops and All-in-one computers (e.g. D520MT) have a
regression on Linux 4.9 where the VGA output is shown all-white.

This is a regression caused by:

commit 0ce140d45a8398b501934ac289aef0eb7f47c596
Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
Date:   Tue Oct 11 20:52:47 2016 +0300

    drm/i915: Clean up DDI DDC/AUX CH sanitation


On these platforms, the VGA output is detected as DP (presumably
theres a DP-to-VGA converter on the motherboard). The sanitization
done by the code that was removed here was correctly realising that
port E's DP aux channel was DP_AUX_A, so it disabled DP output on port
A, also showing this message:

   [drm:intel_ddi_init] VBT says port A is not DVI/HDMI/DP compatible,
respect it

But after this cleanup commit, both port A and port E are activated
and the screen shows all-white. Reverting the commit restores usable
VGA display output.

The reason the new implementation doesn't catch the duplicate
configuration is because the new code only considers ports that are
present in the VBT where parse_ddi_port() has run on them (in order to
set that port's info->alternate_aux_channel).

In this case, port A is not present in the VBT so it will not have
info->alternate_aux_channel set, and the new sanitize_aux_ch will run
on port E but will not consider any overlap with port A.

debug logs from an affected kernel:
https://gist.github.com/dsd/7e56c9bca7b2345b678cfacdab30ec55

Should we modify sanitize_aux_ch to look at all aux channels, not only
for the ports specified in the VBT?

Thanks
Daniel