Advanced screen modes

I’ll give some background first.

I have a 3440x1440 monitor, which is very nice for the desktop, but a little too much for some of the newer AAA games. Unfortunately, the EDID info only gives me 3440x1440 (in a whole bunch of refresh rates) and three low res modes (1024x768, 800x600 and 640x480), which are all not right for games.

I tried adding modelines through xrandr, but while --newmode works fine, --addmode always fails with a “BadMatch” error. Even if I try copy the values from the active mode exactly! Since I haven’t fiddled with modelines since the CRT days, I have no idea what could be blocking this. Could the driver be causing this?

Technically, I don’t really want a different resolution. I would be happy to have a cropped version of the display with black borders. This way the screen remains unscaled, which is a big plus for me. I can easily make this happen nvidia-settings or xrandr by messing about with viewportin/viewportout (or equivalent settings). The big problem with this is

  1. Software doesn’t appear to understand that the framebuffer is smaller than the screen
  2. It won’t appear in the screen modes list as a valid fullscreen mode, so it remains unselectable.

Ideally I would like to “create” a screenmode (for instance 1920x1080) mapped onto 3440x1440 with black borders that is selectable is games as a fullscreen mode. Is there any way to make this happen?

Misread your problem.
Sorry for the noise.

No problem!

Also not that I have these “scaled” modes. I did select the obvious one (2560x1440), but while the screen shows the correct image, both the desktop and games assume that they are using a 3440x1440 screen and part of the picture falls off.