I'm not sure why this is the case, the setting for "Full HD" should be set to Auto (there's no Yes) in my mainboard, but the resolution still is only 1024x768. That wouldn't be a problem if it wasn't only for UEFI setup but since my Linux tty uses the efifb module, it has that resolution too and it doesn't look too good. But what's strange is, one time, it actually displayed it in my monitor's native resolution (1920x1200). I'm not aware that I changed anything, it just worked, but a few reboots later and it was back to 1024x768.
My only guess whould be that the monitor (connected via DisplayPort) doesn't respond to some kind of query for the display resolution since it's in sleep mode, so it assumes 1024x768.

  • GPU: Asus ROG Strix GeForce GTX1070-O8G
  • Monitor: LG Flatron 24EB23PY (connected via DisplayPort)
  • Motherboard: Asrock H170M Pro4S

I've seen this sort of inconsistency in a couple of cases:

  • Plug-in video cards -- These sometimes don't support the monitor's optimum resolution, particularly when using a card intended for BIOS-based computers on an EFI-based machine. In this case, unplugging the video card and using the motherboard's native video output may work better. Switching to an EFI-enabled card (or upgrading the card's firmware, if such an upgrade is available) may help, too.
  • Enabling/disabling the CSM -- The Compatibility Support Module (CSM) is an EFI feature that enables the computer to boot BIOS-mode OSes. Sometimes this interacts with a computer's built-in video hardware, limiting its resolution options. Thus, you might try fiddling with your firmware's CSM feature. (It's often called "legacy boot support" or something similar.) Note, however, that messing with these features can sometimes cause boot problems. It should be possible to get the system booting again if you run into problems, but some people have problems with this because changing the CSM settings may automatically change something else that you must change back manually.

Your hypothesis about a communications glitch with the monitor sounds plausible to me, too, although I've never run into this exact symptom because of such issues myself. You might try swapping out the cable if this is convenient, on the off chance that it's a little flaky.

Beyond that, you might also try playing with my rEFInd boot manager. Specifically, its configuration file (refind.conf) has an option called resolution that sets the video resolution. This resolution should be carried through to the Linux efifb module, so adjusting it in rEFInd may provide the workaround you want. Note that you can try rEFInd by installing it on a USB flash drive or CD-R without installing it on your hard disk. (The rEFInd downloads page provides download images for both these types of external media.) One caveat is that rEFInd can only set resolutions supported by your EFI and video card. If you try to set an unsupported resolution, rEFInd will complain and display a list of supported resolutions.

  • I am actually using rEFInd and it's great! I'll try what you suggested, thanks!
    – 2xsaiko
    May 15 '17 at 19:10
  • 1
    Turning off CSM fixed it, thanks! For the record, there were other options under the main CSM toggle that looked like fine-tuning, among those was a video-related option, turning that from "Legacy only" to "UEFI only" also fixed the issue. But turning off CSM gives faster boot times and I don't really need it, so I'm going to keep it off.
    – 2xsaiko
    May 15 '17 at 19:58

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