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

[Paul Moore]

On Fri, Mar 13, 2015 at 11:19 PM, Paul Moore <paul at paul-moore.com> wrote:
> 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.

Help?

The problem still exists on v4.0.1 and I know of at least one other
person that has reported the same problem.  As I said above, I'm
willing to do any additional debugging/testing needed, I just could
use some help on resolving this as I am definitely not a
X/graphics-driver expert.

-Paul

-- 
paul moore
www.paul-moore.com