Arch Linux

brcolow

Member

Registered: 2016-03-19

Posts: 8

Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

Hello,

I have a Radeon R9 290 graphics card with one 34 inch monitor connected using HDMI and two Dell UltraSharp U2715H monitors daisy-chained together using DisplayPort, with one Displayport going into the GPU (they are physically to the left and right of my 34 inch monitor). Currently I am able to get all three monitors to work, but the outer two monitors (the Dells), that are daisychained together, will only mirror eachother. Furthermore, it appears that xrandr is only able to see one Displayport display:

Screen 0: minimum 320 x 200, current 6000 x 1440, maximum 16384 x 16384
DisplayPort-0 connected 2560x1440+3440+0 (normal left inverted right x axis y axis) 597mm x 336mm
   2560x1440     59.95*+
   2048x1152     60.00 
   1920x1200     59.88 
   1920x1080     60.00    50.00    59.94    30.00    25.00    24.00    29.97    23.98 
   1920x1080i    60.00    50.00    59.94 
   1600x1200     60.00 
   1680x1050     59.95 
   1280x1024     75.02    60.02 
   1200x960      59.99 
   1152x864      75.00 
   1280x720      60.00    50.00    59.94 
   1024x768      75.08    60.00 
   800x600       75.00    60.32 
   720x576       50.00 
   720x576i      50.00 
   720x480       60.00    59.94 
   720x480i      60.00    59.94 
   640x480       75.00    60.00    59.94 
   720x400       70.08 
HDMI-0 connected primary 3440x1440+0+0 (normal left inverted right x axis y axis) 800mm x 335mm
   3440x1440     49.99 +  29.99*
   2560x1080     60.00 
   1920x1080     60.00    60.00    59.94 
   1680x1050     59.88 
   1600x900      60.00 
   1280x1024     60.02 
   1280x800      59.91 
   1152x864      59.97 
   1280x720      60.00    59.94 
   1024x768      60.00 
   800x600       60.32 
   720x480       60.00    59.94 
   640x480       60.00    59.94 
DVI-0 disconnected (normal left inverted right x axis y axis)
DVI-1 disconnected (normal left inverted right x axis y axis)

The X logs seem to tell the same tale: https://ptpb.pw/ZLFr.log

Given this limitation, I am using only the middle and right monitors which is not horrible, but it is such a waste . (The setup does indeed work on Windows 7, not that that means much).

I have tried toggling DisplayPort 1.2 support on both the Dell monitors from the monitor menu, but it seems to have no effect either way.

Any help/guidance on resolving this issue (getting the two daisy chained monitors to both be seen and configurable, and not mirrored as they are now) is greatly appreciated. Furthermore, if there is any further information that I can provide to diagnose this issue, I will post that ASAP.

Thanks!

Awebb

Member

Registered: 2010-05-06

Posts: 6,311

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

If you search for MST (multi stream transport) you might get better results.

Are you sure the R9 290 supports MST with more than two screens without an MST hub? A short google search for "Radeon R9 290 mst" indicates otherwise.

EDIT: I'm sorry, I misread the fact, that the third screen is on HDMI.

EDIT: I have a suspicion. Unplug the HDMI screen and reboot, then check xrandr.

Last edited by Awebb (2016-03-20 01:54:11)

brcolow

Member

Registered: 2016-03-19

Posts: 8

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

No problem, just to clarify:

[ 1 ]  [   2   ]  [ 3 ]

1 and 3 are Dell UltraSharp U2715H. 1 is connected to GPU via Displayport, then chained to 3 via displayport.
2 is a LG 34UM95-P, connected to GPU via HDMI.

Current situation: 1 and 3 are mirrored. I told xrandr to make it so the displayport output is "to the right of" the HDMI display, so I use the 2 and 3 currently (as 1 just mirrors 3, but it is not a separate display).
Ideal situation: 1, 2 and 3 are all detected as separate monitors and I can configure 1 to be to the left of the HDMI display, and 3 to the right of the HDMI display (3 separate displays).

And just to confirm that it is indeed a possible setup, it does work on Windows 7.

Last edited by brcolow (2016-03-20 01:49:12)

Awebb

Member

Registered: 2010-05-06

Posts: 6,311

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

I found a similar thread on reddit, nothing helpful, though: https://www.reddit.com/r/linuxquestions … _280x_and/

brcolow

Member

Registered: 2016-03-19

Posts: 8

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

Yes, I saw the same thread (and some similar ones) via Google but came to the same conclusion as you: nothing helpful.

Awebb

Member

Registered: 2010-05-06

Posts: 6,311

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

Have you tried unplugging the HDMI screen, to see whether the driver is simply incompetent in mixed modes?

brcolow

Member

Registered: 2016-03-19

Posts: 8

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

Just tried it - no difference. Also wanted to add a tidbit of additional info. When the right-most monitor has DisplayPort 1.2 enabled through the monitor's menu, then the two mirrored displays both have great quality. If instead Displayport 1.2 is disabled, the left-most monitor becomes quite fuzzy (sort of like its only doing every third scanline). Just thought I'd mention it in case it helped.

Awebb

Member

Registered: 2010-05-06

Posts: 6,311

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

Do you have anything in /etc/xorg.conf.d/ or just an xorg.conf? Post these.

brcolow

Member

Registered: 2016-03-19

Posts: 8

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

Nothing in directory /etc/X11/xorg.conf.d.

/etc/X11/xorg.conf is an empty file (0 bytes).

Awebb

Member

Registered: 2010-05-06

Posts: 6,311

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

I'm really clutching straws here, but try deleting the xorg.conf file and see if it prevented X from properly auto-configuring itself. If that fails, we have to learn, how to actually configure a daisy chain manually, instead of letting X figure it out.

brcolow

Member

Registered: 2016-03-19

Posts: 8

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

No luck by deleting /etc/X11/xorg.conf.

Where/how can we learn how to configure the daisy chaining? Scour the manpages? But of which piece of software exactly?

Thanks again.

brcolow

Member

Registered: 2016-03-19

Posts: 8

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

So I was able to make some progress on this issue with guidance from the folks in #radeon. Turns out you have to explicitly enable MST by adding "radeon.mst=1" to the GRUB_CMDLINE_LINUX_DEFAULT setting in /etc/default/grub.

After doing so and updating grub and rebooting, xrandr now shows the daisy-chained display!

The only problem is that the right-most monitor (the one that is second in the chain) is a black screen (not getting input). Using the following two commands I was able to get the left and middle screens to be on, looking good, and working:

xrandr --output HDMI-0 --mode 3440x1440 --rate 39.9 --primary // for some reason this display will only work by downscaling the rate, but it still looks good
xrandr --output DisplayPort-0-8 --left-of HDMI-0
xrandr --output DisplayPort-0-1 --right-of HDMI-0 // this is the one that is now black (before it was mirroring the the left-most monitor).

So definitely progress, but shy of perfection.

Any idea what I can do to try and get the daisy-chained monitor to display (I tried toggling DisplayPort 1.2 on and off in its menu, because I read in a couple places that the last monitor in the chain should have DP 1.2 disabled...but it doesn't seem to work either way).

Here is the new output of xrandr --verbose:

http://pastebin.com/jAsuJ0Up

Thanks!

Awebb

Member

Registered: 2010-05-06

Posts: 6,311

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

Did they tell you where to find such information? This could be a source of even more wisdom.

To not get mad by xrandr options, I usually use arandr or lxrandr to quickly change the settings until it works. I think arandr (?) can even output its xrandr line to a script file, so you can observe what it does.

brcolow

Member

Registered: 2016-03-19

Posts: 8

Re: Displayport Daisy-chain Two Dell UltraSharp U2715H - Won't Detect Both

I was directed to post a bug on freedesktop.org. For reference:

https://bugs.freedesktop.org/show_bug.cgi?id=94689

@airlied in #radeon on freenode has been really helpful so far.