Commit 0e32b39c ("drm/i915: add DP 1.2 MST support (v0.7)") breaking DP audio on T440s

Paul Moore
2015-03-14T03:19:51Z

Ever since v3.17 I've had a problem with displayport audio on my
system, but until lately I haven't had a chance to bisect the problem.
After a few rounds I've isolated the buggy commit to 0e32b39c
("drm/i915: add DP 1.2 MST support (v0.7)").

Simply put, prior to v3.17 everything worked correctly: audio from my
system was routed to my monitor via the DP connection and finally out
to the speakers connected to the monitor. However, starting with
kernel v3.17 this no longer works. The various PulseAudio tools all
indicate that the HDMI/DP audio output is unplugged/not-available.
I've tried a few different xrandr commands found via google and
manpages but nothing short of reverting to v3.16 solves the problem.
I'm hoping that someone with more understanding of the Intel graphics
drivers will be able to make some sense of it all and provide a fix.
I'm more than happy to provide additional information and test any
patches you may have.

For reference, my system is a ThinkPad T440s with a "Intel Corporation
Haswell-ULT Integrated Graphics Controller (rev 0b)" (according to
lspci) connected to a Dell U3011 via a Gefen DP KVM.