Multi-Stream Transport 4K Monitors To Become Better Supported On Linux

[phoronix]

Phoronix: Multi-Stream Transport 4K Monitors To Become Better Supported On Linux

For a number of months David Airlie at Red Hat has been working on DisplayPort Multi-Stream Transport (DP MST) handling for Linux. Keith Packard over at Intel is now playing with DP MST too for bettering modern 4K display support on Linux within X.Org Server based environments...

http://www.phoronix.com/vr.php?view=MTg2NTY

[Tom B]

what is the current limitation imposed on 4k monitors? I've been using the Asus 4k monitor on a 750ti since the day it was released and haven't had any trouble... would this change allow you to run two 4k monitors off a single port if the monitors supported chaining (mine doesn't)?

[FireBurn]

Some monitors are actually two bonded together - to save money

[pheldens]

what is the current limitation imposed on 4k monitors? I've been using the Asus 4k monitor on a 750ti since the day it was released and haven't had any trouble... would this change allow you to run two 4k monitors off a single port if the monitors supported chaining (mine doesn't)?

hdmi/dp doesnt have enough bandwidth for 60hz 4k so you have to fake two streams to a dual monitor acting as 1, for this last trick you need support in some linux components which is now maturing.

[carewolf]

hdmi/dp doesnt have enough bandwidth for 60hz 4k so you have to fake two streams to a dual monitor acting as 1, for this last trick you need support in some linux components which is now maturing.

AFAIK If you have a DisplayPort 1.0 or 1.1 it will be limited to 30Hz, unless you use a trick with using two DisplayPorts and two cables, if you have 1.2 you can do multiple connections on the same cable called MST, but since DP1.2 also has more bandwidth you don't really need it for 4k unless your monitor is using old DP 1.1 parts, but people might still use it for 4k+ screens, and if you have DisplayPort 1.3 the bandwidth is even higher and can do up to 5k@60Hz on a single connection.

Anyway MST is good stuff, we also need to for daisychaining DisplayPort screens and stereoscopic displays.

[carewolf]

AFAIK If you have a DisplayPort 1.0 or 1.1 it will be limited to 30Hz, unless you use a trick with using two DisplayPorts and two cables, if you have 1.2 you can do multiple connections on the same cable called MST, but since DP1.2 also has more bandwidth you don't really need it for 4k unless your monitor is using old DP 1.1 parts, but people might still use it for 4k+ screens, and if you have DisplayPort 1.3 the bandwidth is even higher and can do up to 5k@60Hz on a single connection.

Anyway MST is good stuff, we also need to for daisychaining DisplayPort screens and stereoscopic displays.

Apparently though, this is just XRandR metadata for GNOME, so they can handle cheap DP1.2 4k monitors that are actually just upgraded DP1.1 4k monitors and acts as two screen confusing GNOME into thinking they are two separate screens.

[mczak]

hdmi/dp doesnt have enough bandwidth for 60hz 4k so you have to fake two streams to a dual monitor acting as 1, for this last trick you need support in some linux components which is now maturing.

That's not quite true. MST doesn't increase bandwidth over a single cable one bit. All it allows is, as it says, drive multiple monitors from the same cable (hence multi stream). But the total bandwidth for these multiple monitors is exactly the same as it is for a single monitor. It is true that DP before 1.2 did not have the bandwidth for 4k/60hz (not with 24bit rgb, anyway) but this doesn't really have anything to do with MST (though MST was introduced with DP 1.2 too).
The problem is 100% on the monitor side, apparently there were no chips which could actually handle driving 4k/60Hz screens. Hence splitting it in two. Such monitors should disappear completely soon hopefully (well new ones at least).
(FWIW it is no longer true HDMI doesn't have enough bandwidth, HDMI 2.0 offers higher bandwidth (though it seems to be optional, so not all hdmi 2.0 devices may support it). There is little support for this today, on the graphic card side only GM204 does though I'd think most if not all chips to be released in the future will too. I don't know of a single monitor you could buy which supports it, however it seems some have been announced (and some UHD TVs may support it, but especially for them you can't go by the hdmi 2.0 moniker since they may not support the high bandwidth mode and only support 4k/60hz with subsampled yuv.))

[Zan Lynx]

HDMI 2.0 offers higher bandwidth (though it seems to be optional, so not all hdmi 2.0 devices may support it). There is little support for this today, on the graphic card side only GM204 does though I'd think most if not all chips to be released in the future will too. I don't know of a single monitor you could buy which supports it, however it seems some have been announced (and some UHD TVs may support it, but especially for them you can't go by the hdmi 2.0 moniker since they may not support the high bandwidth mode and only support 4k/60hz with subsampled yuv.))

HDMI 1.4 already supports 60Hz 4K with the 4/2/0 trick. I think that you have to use the correct input port on the TV. I know that it works because I use it with my Windows desktop and an Nvidia card and LG 4K TV to play games.

So HDMI 2.0 doing that isn't any better than 1.4. I would not be surprised at manufacturers claiming HDMI 2.0 that only supports 1.4 features. Disappointed, yes. Yet another thing to check the fine print for.

[mczak]

HDMI 1.4 already supports 60Hz 4K with the 4/2/0 trick. I think that you have to use the correct input port on the TV. I know that it works because I use it with my Windows desktop and an Nvidia card and LG 4K TV to play games.

So HDMI 2.0 doing that isn't any better than 1.4. I would not be surprised at manufacturers claiming HDMI 2.0 that only supports 1.4 features. Disappointed, yes. Yet another thing to check the fine print for.

I believe the ycbcr subsampling support was optional (starting with hdmi 1.2) whereas it is now mandatory with 2.0 (2.0 also has a couple other features noone really cares about). So, I don't know if that works with all hdmi 1.4 UHD output devices, though it's possible (shouldn't add any cost supporting it and the feature really makes sense).

[Delgarde]

Apparently though, this is just XRandR metadata for GNOME, so they can handle cheap DP1.2 4k monitors that are actually just upgraded DP1.1 4k monitors and acts as two screen confusing GNOME into thinking they are two separate screens.

Not just GNOME. It's pretty much anything user-facing... desktop stuff that there's only one monitor, even if it looks like two.