At work I got a shiny new Lenovo Thinkpad T440p with 16GiB RAM, and an Ultra Dock as docking station to connect my 2 normal displays.
I had only one problem after booting: Of my two external displays (one connected via DVI, the other one via display port), only one was detected. Both showed the same picture (mirroring).
This blog post is about the Thinkpad T440p with an Intel graphics card, not an nvidia card.
Intel has by far the best open source graphics drivers, while nvidia only publishes a closed-source driver that never supports the latest linux kernel features. You'll probably have to wait a half or a full year until DP-MST will work with the nvidia graphics card.
DisplayPort: Multi-Stream Transport
It turned out that the Ultra Docks internally use a DisplayPort 1.2 feature called MST, Multi-Stream Transport. It allows one to have hubs and chains of display devices, just like one knows it from USB.
Despite DP 1.2 being specified end of 2009, real hardware did not hit the shelves until a year ago, which explains the missing support in the stock Linux kernel 3.16. This was the problem - I was getting some kind of fallback mode with my mirrored displays.
airlied to the rescue
Redhat employee and graphics driver kernel hacker David Airlie fortunately has some colleagues that also had the same problem as me. He implemented DisplayPort MST support for the Intel Haswell hardware (the T440p has a Intel HD Graphics 4600 graphics chip) in the linux kernel and the X.org intel driver, and published it all in a Fedora 20 package repository . Awesome!
MST on Ubuntu
I use Ubuntu, so the Fedora repository was of not help for me. Fortunately, the patches are available and others can use them.
Just download it from kernel.org, configure the kernel with the same options the default Ubuntu kernel uses, compile and install it. It's really not hard. The Ubuntu wiki git kernel build howto helps a lot. We even get .deb packages for the kernel.
$ sudo bash $ apt-get install build-essential kernel-package fakeroot libncurses5-dev $ cd /usr/local/src $ wget https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.17.1.tar.xz $ tar xJvf linux-3.17.1.tar.xz $ cd linux-3.17.1 $ cp /boot/config-`uname -r` .config $ yes '' | make oldconfig $ make clean $ make -j `getconf _NPROCESSORS_ONLN` deb-pkg LOCALVERSION=-dp-mst $ cd .. $ dpkg -i linux-image-*.deb $ dpkg -i linux-headers-*.deb
That was the first part.
Intel graphics drivers
The necessary patches for the X.org intel driver xf86-video-intel are already in git master, which means we do not have to apply patches ourselves. Re-compiling the graphics driver is also very easy because the driver is only a loadable module for x.org and does not require recompilation of other packages.
Now reboot and enjoy the all the displays you can connect.
This is a screenshot of my T440p's desktop with the laptop's own display on the left, and two other monitors connected on the middle and right.
I still experience some bugs now and then.
$ xrandr --addmode DP2-1 1280x1024
- Waking up the laptop with the display's power turned off and switching it on afterwards often brings them up switched, the order being 1-3-2 instead of the correct 1-2-3.
- Undocking and docking leaves the computer in a weird state that forces me to hard reboot the laptop.