So, I am running Arch Linux and have a Radeon 270X using the open Radeon drivers (ie. not AMDGPU or AMDGPU-PRO). Until a few days ago I was running two monitors, one LG Flatron L2000CN at 1600x1200 rotated to portrait mode (connected with DVI), and one Dell U2711b at 2560x1440 (DisplayPort), then the LG started failing (power LED blinked for up to a few minutes before the monitor managed to come out of standby) and finally failed to power on completely.

This leads me to a point where I really need to take care of a problem I've noticed long ago, but haven't bothered with since it did work with the setup I had until now - when the Dell is the only monitor active on my machine and set to full resolution, it flickers like mad when the image changes (and slightly less when it's still). When both monitors are connected and both are enabled, it's fine. When I set it to 1920x1080, it's fine even when alone. When I tried the Dell with two laptops I borrowed from work (an HP ProBook 6450b on Windows 10, and an HP ProBook 6360b on Kubuntu 17.04, both using Intel graphics and connected to the display through DisplayPort), it was fine even when being the only active screen.

I tried creating modesetting lines with cvt and gtf, but only got either the same flickering result that seems to be default, or something that's so garbled it cannot be used at all.

I found a Windows application to tell me info about the screen (while connected to the Windows machine), and even got a modesetting line which I then fed to xrandr (on my main machine), but got no better result than what was already auto-detected.

I found a manual, where some information on supported modes exist, but the listed mode for 2560x1440 did not help me any more than what I'd already tried.

Sadly, I don't have any more screens at that resolution, and can't really bring anything bigger than a couple laptops with me between home and work (else I'd test the machine on my monitor there, or bring that home to test).

I recorded a short video of the flicker.
Any one of you have other ideas beyond "sell the display and buy one that hopefully works with the machine"?

TL;DR Otherwise awesome 27-inch monitor flickers, but only in solo-mode at full resolution, and only seen on one machine.

Might be the refresh rate

just a poke in the dark

It's autodetected to 59.95 Hz, and that's the only one I have available unless I create a new arbitrary modeline.

Thanks for the idea though, currently building a shellscript to try out a bunch of different modelines to see if a slightly different one kicks it into shape.

You ever try it with the DVI port instead? IIRC, you will need a dual-link DVI cable for full resolution.

I'm not sure I've thought about that. I had a Power Macintosh G4 connected to it through DVI, but only got 1600x1200, then again, the GPU in that machine likely doesn't support dual-link.

Before everything else, for comprehensive testing I'd also try the same machine - different OS combo, to eliminate certain possibilities. I'd try a live distro (preferebly a "magic" OS like Ubuntu) at 2560x1440 and see if not fiddling with the modesetting yourself makes any difference.

I'll be trying that. Had some trouble when I tried making a multi-boot USB stick with both and [url=https://aur.archlinux.org/packages/multisystem/]multisystem to try different live distros, so I'll probably just write them to the stick one at a time or try YUMI under Wine.

I can't help you, but that link seems wrong

Oh, apparently it is the link to edit said video on my channel.
Fixed now, the correct one should be this..

Woa, I expected some mild flickering, but instead it's a mess! O_o
For some reason, the worst effect happens only in the upper area..

Btw, does it happen at other resolutions and refresh rates too? (just for curiosity)

I would try running in vesa by disabling modesetting at boot to exclude hardware issue and then try vsync settings ("export vblank_mode=0" into /etc/profile.d/debug.sh, reboot), maybe run KDE with distro that uses same kernel/radeon, before contacting Mesa developers at irc.freenode.net, channel #dri-devel

Your monitor seems to have different outputs, I would also try DVI to make sure. This does not imply monitor problem, but driver problem. I use samsung on hdmi without issues on radeon driver / r9 280.

This looks like buffer redraw issue on vsync to me..

Yeah, the white flicker is primarily in the top half, and the bottom half just blinks on and off.

I am currently running it over DVI, and though I can set the full resolution, it becomes a scaled-down image (which is then, obviously, scaled up to the full screen) and thus looks awful. It also flickers in everything above 1024x768 (haven't checked any refresh rates beyond what's automatically selected), and the only 16:9 resolution I can set beyond building modelines is 2560x1440 - 1920x1080 and 1280x720 are not available (but the 16:10 resolutions of 1920x1200, 1680x1050 and 1280x800 are, together with a bunch of common 4:3 and 5:4 resolutions).

EDIT: It's 1920x1200, not 1980x1200, the latter is certainly not common.

Check if your monitor runs analog outputs on DVI in monitor settings.... the maximum analog output for 270x is: 2560 x 1600.

See: https://superuser.com/questions/680517/cannot-set-max-screen-resolution-of-2560x1440-with-dell-u2711-monitor

The cables are DVI-D. But what's that... oh, hey, look at that shiny diagram. Now the question is, why the hell are my DVI cables only single link? Also, why had I forgotten they were single-link?

Oh well, that answers the DVI question.

Also, the Manjaro KDE live image displays the same flicker. Typing from that now. Checking Kubuntu next.