GTX 970 and three 4K display monitors

I have three Dell 32" 4K monitors which I hooked up to my new GTX 970. When I run the system under windows, the card drives the three monitors in full 4K @ 60Hz mode. Very nice. When I run the system under linux using the latest nvidia driver 352.21, it will only drive two of the three monitors. Here’s the deal, when running the Dell 4K monitor in at full resolution, you need to have the monitors running in display port 1.2 mode. (dp1.2) This causes the monitor to actually behave like two monitors with each half of the monitor running at 1920x2160. So the nvidia driver will then tell you you have 2 “virtual” monitors per real monitor. So with three Dell 32" 4K mointors, I have a totoal of 6 “virtual” or half monitors. We’ll here the deal, in linux, the driver will allocate 4 of these half monitors to one GPU and if I need to drive the other two half monitors, I have to use base mosaic mode. In order to use base mosaic mode I need another card. In short the GTX970 (and I suspect all cards which have 3 dp ports) will drive three 4K monitors in dp1.2 configuration under windows but only two under linux. This really sucks…

So, Nvidia, can you confirm that under linux the driver will only support 4 monitors per GPU?

Also, if I get another card, will base mosaic or SLI mosaic work under with the GTX 970 or do I have to have a quadro class card as I’ve seen several posts on the internet indicate?

thanks.

#1

Yes, that’s the case currently I’m afraid. Each stream in a DP 1.2 multistream configuration uses a separate head, and the GPU has four heads total. Windows gets around this using some tricks that aren’t supported in the Linux driver just yet.

The requirements for BaseMosaic are described in the README: http://us.download.nvidia.com/XFree86/Linux-x86/352.21/README/xconfigoptions.html#BaseMosaic

Hi Aaron,

when you say “Windows gets around this using some tricks that aren’t supported in the Linux driver just yet”, are you implying that some time down the road the linux driver will support it?

This is important to me since I’m looking at switch to a Radeon graphics card alternative. Not being able to run my 3 monitors under linux at full 4K @ 60Hz is really driving me nuts.

It’s on our roadmap, but I can’t promise any particular timeframe.

Hi Aaron,
Has this issue been resolved? I am building a six screen workstation and want to keep the Linux option open but don’t want to have to buy a third card just to do so.
Thanks for any insight as to the current state of possibility.

STaylor, I’ve been using three dell 4K monitors with a single GTX980ti for several months and they’re all operating at 60hz full resolution. I can’t say that it’s problem free, however. When they go into power save mode in Linux there’s only about a thirty percent chance that they’ll wake back up. I keep searching for other people having the same problem and don’t see a lot of complaints so it might be unique the particular combination of card and monitors and distro that I’m using. Your mileage may vary.

edit: deleted accidental doublepost.

It may also depend on whether the monitors use MST (older ones) or SST (newer ones). The OP’s monitors definitely use MST, as he described each monitor being two virtual monitors. Newer 4K monitors (like the Dell P2415Q model that I have) use SST and therefore appear as only one monitor to the system. This might allow you to connect 3 to one system, but I can’t try since I only have 2.