How to switch to a particular resolution? Xorg complains "No valid modes for "DFP-1:2560x1600"; removing." and ignores my setting.

[Adam Nielsen]

> Here is the log: https://people.freebsd.org/~yuri/Xorg.0.log

So from here you can see:

  DFP-0: 330.0 MHz maximum pixel clock
  Samsung U28D590 (DFP-1): 230.0 MHz maximum pixel clock
  EDID maximum pixel clock     : 300.0 MHz

  Validating Mode "3840x2160_30":
    Mode is rejected: PixelClock (297.0 MHz) too high for
    Display Device (Max: 230.0 MHz).

This means that 3840x2160 @ 30 Hz requires a pixel clock of 297 MHz,
your monitor can go up to 300 MHz, but your video card can only support
230 MHz on that output.

Since there isn't enough bandwidth between the video card and the
monitor, this one is removed from the list of available modes.

> > If that doesn't work, what monitor, video card, and cable type are
> > you using? (e.g. HDMI, Dual DVI, or DisplayPort)
> 
> Monitor: Samsung U28D590
> Video card: NVidia GT610

You didn't say what type of cable you are using so my guess would be
it's something like single-link DVI when you need dual-link DVI, or the
monitor requires HDMI 1.4 and the card only supports an older version.

The output says that although DFP-1 is limited to 230 MHz, DFP-0
supports 330 MHz so if you have a different output on the card you
could try that in case only one output supports 4K.

Cheers,
Adam.