[REGRESSION] i915: failure to see Dell 30" monitor connected to a Lenovo Haswell docking station

[Theodore Ts'o]

On Tue, Sep 02, 2014 at 02:15:39PM +1000, Dave Airlie wrote:
> On 2 September 2014 14:05, Theodore Ts'o <tytso at mit.edu> wrote:
> > I recently upgraded to v3.17-rc3, and on my Lenovo T540p, I can no
> > longer see the my Dell 30" monitor when it is connected via the
> > docking station using a Displayport connector.  This worked using 3.16
> > kernel.
> >
> > If I connect to the monitor using the mini-display, by passing the
> > docking station, things work fine (but of course it's annoying not to
> > be able to use the docking station).
> >
> > Is this a known problem?  This is not the first time that we've had
> > regressions with this docking station.   It's vaguely reminsicent of
> >
> >         https://bugs.freedesktop.org/show_bug.cgi?id=71267
> >
> > Except the system isn't hanging; it's just not seeing the monitor at all.
> 
> Have you the Dell 30" set to Displayport 1.2 enabled mode?

No, it DP 1.2 was disabled.  If I enable it, it breaks things when I
try connecting via the MiniDP port (bypassing the dock), and it is
still broken when I try to talk via the DisplayPort in the dock.

If I disable DP 1.2 again, it works via the MiniDP port, but if I try
to connect through the dock (which has a DP Hub which I believe is
MST/DP 1.2 capable), it is still broken.

It does seem that this might be related to 3.17-rc3 trying to talk DP
1.2 if it is available, since I can't control what the DP hub in the
docking station advertises --- is there a commit or some kind of hack
I can try to force talking to the DP hub using DP 1.1?

						- Ted