Ubuntu: Only one monitor usable with dual monitor setup (Displayport MST setup)


I am running a desktop computer with a Radeon HD6870 video card. The card is attached to a Dell U2515H monitor, to which another Dell U2515H monitor is attached (the displays are daisy chained together for a MST setup). This setup works in Windows.

In Ubuntu 15.10, however, the second monitor is not detected at all and it displays the same content as the first monitor.

The old Ask Ubuntu question Displayport 1.2 MST/daisy chain - Dual Monitor setup - Intel graphics contains some promising replies, and I would assume that the current kernel supports MST. However, not out of the box, apparently. How can I get extend the desktop to the second monitor using MST in Ubuntu 15.10?

Update: I have found a news article Radeon DisplayPort MST Queued For Linux 4.1. As Ubuntu 15.10 uses kernel 4.20, it should theoretically work. The article says that "While the open-source Radeon DP MST support will appear in Linux 4.1, it's disabled by default." I'll continue my research and update this question if I find out a solution.

Update 2: Apparently the support is not just there yet, as of November 25th, 2015. I updated my kernel to 4.2.6 and set the kernel parameter "radeon_mst=1", which should have enabled MST support for Radeon according to various instructions in the Internet. However, this resulted in a black screen after boot-up. Researching some more, I found out the following open bugs describing this phenomenon:

https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/1509997 https://bugs.freedesktop.org/show_bug.cgi?id=92790

Apparently the only viable option is to wait for this bug to be fixed. Or use a hardware-based workaround: purchasing a graphic card with two DisplayPort outputs.

Update 3:

Possibly the problem could be solved by using the proprietary binary driver from AMD, fglrx. For me it's not a viable solution, as I have upgraded to Ubuntu 16.04, and the fglrx is not available for that version. I saw some hints of Radeon MST-related changes in Linux kernel 4.6 change logs, so I'm hoping that brings some improvement (with Ubuntu 16.10).


The kernel parameter is "radeon.mst=1" according to other sources. Not sure if both are supported in the end.

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