I have working 10bpc (no banding) on GTX 580 via dihtering on 8bpc monitor in dual gpu setup. Also I have real 30bit on Quadro 4000 over DP on 10bpc monitor. I have also tested Xinerama with Quadro + gtx with 4 monitors, 2 per gpu and it also allows 30 bit on all displays and dithered 30bit from gtx looks the same as native or dithered 30bit from Quadro.
Tested with OpenSUSE Factory 13.2, KDE4 using QT native hack, Krita with OpenGL renderer and Photoshop 16bpc gradient picture as test sample.
MSI GTX 580 3gb: Eizo FS2331 (8bpc dithering, DVI) / Eizo EV2335W (8bpc dithering, DVI) / Samsung SyncSomeCrap whatever (8bpc dithering, HDMI)
PNY Quadro 4000: Eizo FS2331 (8bpc dithering, DVI) / Eizo CX240 (10bpc native, DP)
They all look the same (in terms of banding) and there’s noticable difference between dithering enabled and disabled.
messed up colors in KDE before applying Qt native hack
Gwenview should work ONLY in software transitions mode when Qt hack is working (no matter if 10bpc or 8bpc)
Digikam thumbnails should be still messed up despite Qt native hack
Krita with OpenGL renderer should display no banding, whereas with disabled OpenGL banding is visible again.
Compiz should work only in Xrender mode with tiny glitches but in general usable.
Maybe NVidia removed support of 10bpc from gpus with DP output to promote Quadro xD
[EDIT:] It was meant to be joke, but actually it seems to be pretty possible - I’ve just talked with friend who also has exactly the same OpenSUSE release, the same soft - basically from software aspect it’s identical, but he has GTX 680 (with DP output, whereas 580 has only HDMI and DVI) and it seems to not work on his machine, so yeah, It seems that everything from 6xx above doesn’t work but everything from 580 below does. Of course you won’t get “real” 30bit then because those cards didn’t have DP yet, BUT - dithering works so well that I wouldn’t really care about it - especially that you can use monitor with 8bit panel then and it’ll still work fine.
if we’d assume that mentioned before 9xx and 7xx were representative it means that 5xx, 6xx, 7xx and 9xx are covered for now.
If anyone has gpu from other series please also tell if it works as it seems to be really unclear which cards support 10bpc and which don’t
Just to cheer up people disappointed with lack of 10bpc support with DP cards - I bought Quadro just to have 30bit color on Linux in fact (I was skeptical about gtx 30bit support, it turns out to be unnecessary as I have 580 anyways) and I looked for second hand Quadro 4000 (now, just few months ago so I bought pretty old card) for like 150$, they’re really cheap like most of used professional grade hardware. As it performs in OpenGL better than gtx580 I find it pretty good option and in fact quite cheap, you can always have second gpu for gaming (or for CUDA like in my case).
I guess it’s really reasonable option both from economical aspect (PC runs much cooler than when I used gtx for X server and Quadro consumes less power) and convenience (if someone wants to use card for CUDA, he needs second gpu for X anyways because when card is computing CUDA, X has like 1fps drops and is pretty unusable). So yeah - cheap, used Quadro + gtx is pretty nice option for Linux if someone is looking for both 30bit color and high performance computing/gaming