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I have GeForce GT 1030 card with 2 Dell U2415 monitors. The connection is 1030's DisplayPort => Monitor 1 DP Input; Monitor 1 DP Output => Monitor 2 DP Input.

The OS is Debian Buster (10), with the default nouveau driver.

I have tried all possible scenarios I can think of w/o any success. What's in common is that when I boot the machine both screens are showing the BIOS/GRUB/OS booting. The action comes when Xorg comes.

  1. Nouveau driver, regular boot. The login screen is seen on the 2nd screen, after login I can see only xfce4 background and the mouse can leave to the left where the main (1st) screen should be, but it is black. Xorg.log

  2. Nouveau driver, nomodeset boot. Both monitors have blinking underscore with Xorg dead. Xorg.log

  3. Nvidia driver (from the Debian's non-free repo). xfce4-display-settings shows the 2 monitors, but the 2nd one is marked as 'Disabled'. If I enable the 2nd screen, the screen on the 1st one blinks and both are seen as active. The mouse can leave the 1st monitor to where the screen of the 2nd should be, but the 2nd is totally black. Selecting 'mirror displays' doesn't help either - still black screen on the 2nd monitor. The 'configure new displays when connected' is checked. Tried to turn off the 2nd screen - it dissapears from both xfce4-display-settings and nvidia X server settings. Turning it on again asks me how to deal with the monitor. Selecting to extend to the right accepts my change, both monitors are seen as active on both xfce4-display-settings and nvidia X server settings. The position of the 2nd one is Absolute +1920+0 as it should be, but it is still black. Xorg.log

  4. Same Nvidia driver, with the xorg.conf provided by the Nvidia X server settings xfce4-display-settings pops up with Unable to start the XFCE display settings. Unable to query the version of the RandR extension being used. xrandr command returns RandR extension missing. Only first screen works, the 2nd one is black Xorg.log

The monitors are tested with the same cables from one Intel NUK using the same OS (Debian 10 Buster) but with the intel's graphic card and the Xinerama works properly out of the box.

What I'm missing which can be done in order to feed the 2 monitors using the GT 1030?

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tl;dr: using nouveau driver, execute:

$ xrandr --output DP-1-8 --auto --output DP-1-1-8 --auto --right-of DP-1-8

For NVIDIA X driver, try to set "Xinerama" option to "0" in xorg.conf ("ServerLayout" section). As for version 302.07 (2012-05-02), support for RandR was added.

According to the question, the second monitor connected through the first, both are tied with DisplayPort cables.

After loading kernel modules, first configuration passes the boot process to default KMS, which creates a display subsection (marked with "=="), detects both displays and lists as DP-1-8, DP-1-1-8 and their EDIDs respectively, initializing the last with 1920px offset. nouveau has nothing to do with managing displays for your WM, it does not throttle the resolution. As long as monitor is displayed, driver does the job, so there is no need to set nouveau.modeset=0 nor nomodeset since nouveau relies on it.

nvidia-driver passes generated configurations and may load it's built-in extensions like Xinerama, which has a conflict perfectly described in log:

[     9.022] (WW) NVIDIA: Xinerama is enabled, so RandR has likely been disabled by the
[     9.022] (WW) NVIDIA:     X server.
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