nixos-4screen/niri-4screen.md

Niri + 4-Monitor Intel (DP) Migration Notes (Ubuntu 24.04+ → NixOS)

OWNER / CONTEXT

• User: sam (IT)
• Goal: Move to NixOS and daily-drive Niri on a 4-monitor setup.
• Priority: Reliability and broad tool compatibility over “polish”.
• Testing style: often SSH in from another machine because local display can go black during compositor/DM experiments.

REMOTE REQUIREMENT (clarified)

• SSH is sufficient even when nobody is logged in locally (sshd runs pre-login).
• Remote GUI login is optional/rare. Do not design around RustDesk-at-greeter on Wayland.
• If remote GUI login is ever needed later, consider adding GNOME+GDM+RDP as a separate capability; keep Niri as main local session.

HARDWARE SUMMARY

• GPU: Intel iGPU (exact model TBD)
• Outputs: 4x DisplayPort to 4x HP LA2205 monitors
• DRM nodes observed on Ubuntu (node numbering may differ on NixOS):
  • Primary KMS card for the 4 DP outputs: /dev/dri/card2
  • Render node: /dev/dri/renderD129
• Notes: There may be multiple /dev/dri/card* devices. Session must pick the correct device driving the 4 DP outputs.

NOTE (2026-01 recon):

• Current Ubuntu hardware scan shows NVIDIA Quadro P620 is driving /dev/fb0 at 1680x1050.
• This suggests the monitors may actually be attached to the Quadro, not the Intel outputs.
• Before finalizing GPU/device selection, verify:
  • ls -l /dev/dri/by-path
  • loginctl seat-status seat0
  • (optional) niri output listing once in session

KNOWN KERNEL / PLATFORM ISSUES

• IOMMU faults / “Operation not permitted” style crashes were avoided on Ubuntu with kernel flags:
  • intel_iommu=off
  • dev_mem_signed_off=1
• These flags may or may not be needed on NixOS; keep them as a known-good baseline and only remove once stable.

UBUNTU WORKING STATE (IMPORTANT BEHAVIORAL FINDINGS)

1. GDM “gear icon” / Wayland sessions

   • GDM did not show Wayland sessions until Wayland was enabled.
   • /etc/gdm3/custom.conf had WaylandEnable=false. Commenting it out fixed session availability after restarting GDM.

2. .desktop Exec path issue

   • Session .desktop pointing Exec to /home/sam/start-niri.sh caused GDM issues.
   • Home perms were drwxr-x--- (750), so greeter user couldn’t traverse /home reliably.
   • Fix: Exec must point to a system path (/usr/bin or /usr/local/bin), not /home.

3. niri-session issue (major root cause of login loop)

   • /usr/bin/niri-session existed but session immediately returned to login.
   • Logs showed: Failed to start niri.service: Unit niri.service not found. Failed to start niri-shutdown.target: Unit niri-shutdown.target not found.
   • Therefore niri-session was not usable as packaged (missing systemd user units).

4. FINAL WORKING FIX ON UBUNTU (proven)

   • /usr/share/wayland-sessions/niri.desktop set to start Niri directly: Exec=/usr/bin/niri --session
   • This bypassed niri-session and made Niri start successfully from GDM.

SESSION START METHOD (proven)

• Known working from a display manager: Exec = niri --session
• Avoid relying on niri-session unless NixOS packaging provides the required systemd user units (niri.service, niri-shutdown.target).

PERMISSIONS / SECURITY WORKAROUNDS USED DURING TESTING

• User group membership on Ubuntu: video, render, seat
• Custom udev rules were created to chmod 666 DRM nodes.
  • Result: /dev/dri/card2 and /dev/dri/renderD129 became world-writable.
  • This is NOT desired long term; prefer logind seat ACLs.
• On NixOS, aim to avoid chmod 666 rules unless absolutely needed for debugging.

NIRI CONFIG NOTES

• Config validated successfully on Ubuntu: ~/.config/niri/config.kdl
• Xwayland started via config:
  • spawn-at-startup "Xwayland" ":" "1"
• Avoid exporting XDG_RUNTIME_DIR manually; let pam/systemd-logind manage it.
• If needed to force GPU device, Niri supports choosing a render DRM device (exact config syntax/version dependent). On Ubuntu, correct render node was renderD129.

NIXOS TARGET STATE (WHAT WE WANT)

• Boot to a login method that reliably starts Niri on the Intel GPU with 4 monitors.
• Must keep a working fallback (at minimum TTY + SSH; optionally a full DE).
• Remote recovery/admin always possible via SSH.

LOGIN / DISPLAY MANAGER STRATEGY OPTIONS (pick one) Option A: greetd + tuigreet (recommended for Niri-first reliability)

• Minimal moving parts, compositor-agnostic.
• Start the session with: niri --session.
• Ideal when “polish doesn’t matter” and reliability does.

Option B: GDM (works; proven on Ubuntu)

• Ensure Wayland sessions enabled.
• Ensure session Exec is not in /home.
• If niri-session is incomplete, start niri --session directly.

DISPLAY MANAGER DECISION NOTE

• If SSH-only remote is the requirement: prefer greetd for simplicity.
• If remote graphical login is ever required: consider GDM + GNOME RDP later as a separate capability. (Not required now.)

SCREENSHARE / PORTALS REQUIREMENTS (broad tool compatibility)

• Enable PipeWire + WirePlumber.
• Ensure xdg-desktop-portal is installed and functional in the user session.
• Choose a portal backend compatible with Niri (often portal-gnome and/or portal gtk; exact best choice may be NixOS-specific).
• If screencast/screen-share fails in apps: check portal backend selection, permissions prompts, and PipeWire.

GPU/DRM PERMISSIONS

• Avoid global chmod 666 udev rules in final config.
• Use logind seat/ACLs; add user to video/render groups if needed.
• When debugging device selection:
  • ls -l /dev/dri /dev/dri/by-path
  • loginctl seat-status seat0

FALLBACK PLAN

• Minimum: TTY + SSH access always available.
• Optional: install a full fallback DE only if needed (GNOME or Plasma).
  • Not required for Niri; just a safety net.

DEBUG / TROUBLESHOOTING CHECKLIST (capture these on failure)

• niri config:
  • niri validate
• user session logs:
  • journalctl --user -b -l --no-pager | tail -n 300
• kernel DRM messages:
  • journalctl -b -k -l --no-pager | grep -iE "drm|i915|kms|atomic|permission" | tail
• device inventory:
  • ls -l /dev/dri /dev/dri/by-path
• session type:
  • echo $XDG_SESSION_TYPE
  • loginctl session-status

ACCEPTANCE CRITERIA (DONE WHEN)

• Niri starts reliably after reboot from the chosen DM
• All 4 monitors are active consistently
• Screen sharing works in at least one browser-based app and one native app
• SSH recovery works even if local display is broken
• No chmod 666 DRM hacks required in the final config (preferred)

OPEN QUESTIONS FOR NIXOS MIGRATION

• Exact Intel GPU model + correct DRM node mapping on NixOS (may differ)
• Whether the kernel flags are still required on NixOS
• Whether NixOS niri packaging includes full systemd integration units (niri.service, niri-shutdown.target)
• Best portal backend combo for Niri screencast on NixOS