Intel Skylake + DisplayPort 1.2 MST/Daisy chain on Linux

Question

I am planning to buy hardware for using DisplayPort 1.2's MST feature to daisy chain two monitors in combination with the Intel Core i5 6600 Skylake processor on Ubuntu. Is this supported yet with the Intel Linux driver? Can anyone confirm they have this particular setup working? I've seen others got it to work with an nvidia graphics card (note: found some extra info about this also) and the proprietary drivers, albeit a big buggy. However, I specifically would like to try this with Intel Skylake and not via PCI graphics card.

In the news it is claimed that Intel support for MST has been added by David Airlie in 2014, but a little deeper digging shows that this was for Haswell and there is no reason to believe this is working for Skylake yet.

A small snippet of news hints at better MST handling in kernel 4.2, but I found another Linux user claiming it's not working for him on his Dell XPS13 with kernel 4.3 (Dell XPS13 runs on Skylake). It looks like an important bit of the Skylake support for Linux came with kernel 4.3 (Skylake gpu support), so I would be looking for other experiences with kernels 4.3 and up. At the time of writing this would be kernel 4.3 or 4.4-rc3.

Update:

AMD Radeon also seems to have problems, someone describes both his daisy chained monitors to go into power saving mode after logging in on Ubuntu 15.10 with kernel 4.2.6. Before logging in, MST daisy chaining does work, but with a blurry display on one of the monitors. I wonder if the power saving issue is related to the bug the nvidia user described in 1 where one of his screens goes into power saving mode when in MST mode, if so, then maybe this is a kernel issue unrelated to nvidia and it could potentially affect Intel as well?

Update #2:

I went on a treasure hunt through the kernel code and came up with some new leads.

In order to have MST support we need HBR2 support as well. There is only HBR2 support available from certain Skylake revisions. Currently there seem to be five of those according to the source code and the first 2 revisions have no HBR2 support because of a disabled thing called TPS3.

Secondly, I spotted a FIXME for MST on Skylake in kernel 4.4-rc3. I managed to dig up a bug report that addresses this FIXME at:

https://bugs.freedesktop.org/show_bug.cgi?id=91791

The title of this bug report is a bit misleading, read all the comments to see that it is related. The last comment from November 11th states that the screen goes blank when connected over MST on Skylake. This sounds like the issue that has been reported by the PCI graphic card MST users!

A patch that may or may not work came out of it anyway and it currently is in drm-intel-fixes and in drm-fixes, which means that it will make it into the next release candidate for 4.4 (see release schema). So that sounds like that today it won't work yet on 4.4-rc3, but new chances with 4.4-rc4. I will hold off with buying the new hardware until the new test results come in and leave this question open until it works.

Solution

I was having trouble with this on a Skylake NUC (Intel graphics) running Ubuntu 15.10 (4.2 kernel). I can confirm that upgrading to 16.04 beta (4.4.6 kernel) has solved the problem. Though there still seems to be some issue with the window manager 'forgetting' about the 2nd screen's desktop upon logout/in, at least under the MATE desktop. (Display is active and the cursor travels into the space, but the background image is reset to default, and no windows can be dragged into the space or launched from there. Workaround is to go into the display settings, change something simple like which display is primary, hit apply, and it is fixed.)

Comments

[Admin]

I have a Haswell-based system and it works, but I have an annoying bug: I often see each of the external screens turning off for a split-second at random intervals. Do you also see this on Skylake? I wanted to report a bug for it but the requirements for creating bugs are a bit high, users are supposed to migrate to a nightly build compiled from scratch, and that isn't something I'm willing to do at the moment.

[Admin]

What is the status of DisplayPort MST under Linux in 2018?