Atm AUX-B transfers can fail with the following error if AUX-A is not
enabled:

[  594.594108] [drm:intel_dp_aux_xfer [i915]] dp_aux_ch timeout status 0x7c2003ff
[  594.615854] [drm:intel_dp_aux_xfer [i915]] *ERROR* dp aux hw did not signal timeout!
[  594.632851] [drm:intel_dp_aux_xfer [i915]] *ERROR* dp aux hw did not signal timeout!
[  594.632915] [drm:intel_dp_aux_xfer [i915]] *ERROR* dp_aux_ch not done status 0xac2003ff
[  594.641786] ------------[ cut here ]------------
[  594.641790] dp_aux_ch not started status 0xac2003ff
[  594.641874] WARNING: CPU: 4 PID: 1366 at drivers/gpu/drm/i915/intel_dp.c:1268 intel_dp_aux_xfer+0x232/0x890 [i915]

Ville noticed this issue already earlier and managed to work around it
by keeping AUX-A always powered whenever AUX-B was used. He also
reported the issue to HW folks and they have now root caused the problem
and updated BSpec with a fix (see internal BSpec/Index/21257,
HSD/1607152412).

I noticed the same error - even with the WA being applied - while doing
AUX transfers with Chamelium being connected with a DP cable to the
source but letting Chamelium imitate an unplug. This is probably some
unstandard way on Chamelium's behalf of disconnecting itself from the
AUX pins. For instance it could still pull on the AUX pins which would
prevent the source from detecting AUX timeouts in the proper way,
leading to the ERRORs or WARNs seen in the logs in the Reference: bug
below.

In case I disconnect the sink properly (the cable itself, not via the
Chamelium unplug xmlrpc command) then the AUX timeout signaling works
properly and so there won't be any ERRORs/WARNs emitted.

Reference: https://bugs.freedesktop.org/show_bug.cgi?id=110718
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190524173532.6444-1-imre.deak@intel.com