LG 27" 5K UltraFine Monitor - Boot Display Issue

[afgallo]

Below is what I've capture in my attempt to solve a few issues I've encountered trying to use a new 5K monitor with Omarchy. I consider myself an intermediate Linux user with no real kernel experience so this was heavily "AI assisted" (some of these findings may be completed wrong, you have been warned!). Unfortunately, I still haven't found what I am looking for 🎵

Jokes apart, I haven't found a solution to the problem I am still facing which is a black screen when my PC boots. Everything else seems to work fine and my hope is that this thread can sparkle something that leads us to fix this issue.

Hardware Setup

• Computer: Beelink SER8 (AMD GPU)
• Monitor: LG 27" 5K UltraFine (Model: LG UltraFine 910NTYT0W842)
• Connection: USB-C Thunderbolt cable
• OS: Arch Linux with Omarchy
• Display Manager: Plymouth with encryption unlock screen
• Window Manager: Hyprland

Problem Description

When booting the system with only the LG 5K monitor connected, the encryption unlock screen (Plymouth) does not appear on the monitor - the screen remains black. The encryption prompt only appears when a secondary monitor (Dell U3219Q via HDMI) is connected.

After unlocking the encrypted drive and logging into Hyprland, both monitors work perfectly.

Root Cause Analysis

LG 5K DisplayPort MST Architecture

The LG 27" 5K monitor uses DisplayPort Multi-Stream Transport (MST) technology to achieve 5K resolution over a single Thunderbolt/USB-C cable. This causes the monitor to appear as two separate DisplayPort outputs:

• DP-3: 3840x2160@60Hz (4K mode - duplicate/fallback signal)
• DP-4: 2560x2880@60Hz (native 5K resolution)

Both streams must be active for the monitor to display content properly. Disabling DP-3 causes the physical monitor to go black even though DP-4 remains active in Hyprland.

Boot Sequence Timing Issue

The problem occurs during early boot initialization:

1. Kernel initializes and loads graphics drivers
2. Plymouth starts to display the encryption unlock prompt
3. DisplayPort MST negotiation for the LG monitor takes longer than simple HDMI/DP connections
4. Plymouth grabs the first available display (HDMI on Dell) before the LG's dual MST streams are fully initialized
5. Result: encryption prompt appears on Dell, LG screen stays black

After the system fully boots and Hyprland starts, there's sufficient time for the MST streams to negotiate properly, which is why both monitors work post-login.

Troubleshooting Journey

Attempt 1: Hyprland Monitor Configuration

Hypothesis: Hyprland configuration issue with monitor detection.

Actions Taken:

• Modified /home/$USER/.dotfiles/config/hypr/monitors-$USER.conf
• Tried explicitly configuring DP-4 as primary monitor
• Attempted to disable DP-3 (the 4K duplicate signal)

Result: Failed. Disabling DP-3 caused the LG monitor to go black entirely, confirming that both MST streams are required.

Lesson Learned: The LG 5K requires both DP-3 and DP-4 to be active simultaneously. This is a hardware requirement of the MST implementation, not a software configuration issue.

Attempt 2: Hyprlock Configuration

Hypothesis: Login screen (hyprlock) not targeting correct monitor.

Actions Taken:

• Modified /home/$USER/.dotfiles/config/hypr/hyprlock.conf
• Attempted to force login screen to appear on DP-4

Result: Not applicable. Further investigation revealed the "login screen" mentioned was actually the Plymouth encryption unlock prompt, which occurs before Hyprland/hyprlock even loads.

Lesson Learned: The issue occurs at the kernel/initramfs level, not at the Hyprland level.

Attempt 3: Kernel Boot Parameters

Hypothesis: Force DisplayPort outputs to enable early.

Proposed Actions:

• Add kernel parameters video=DP-3:e video=DP-4:e to force enable both outputs
• Add amdgpu.dc=1 to ensure AMD Display Core is active

Result: Not implemented due to safety concerns.

Reason: DisplayPort output names (DP-3, DP-4) are assigned by Hyprland after the system boots. During early boot (initramfs/Plymouth stage), the kernel may use different naming schemes, making these parameters potentially ineffective or problematic.

Final Solution (Implemented, did not work)

Add AMD GPU Driver to Early Boot

Location: /etc/mkinitcpio.conf

Change:

# Before
MODULES=(btrfs)

# After
MODULES=(amdgpu btrfs)

Rationale:
By explicitly loading the amdgpu driver in the initramfs MODULES array, the AMD graphics driver initializes earlier in the boot sequence. This provides more time for the DisplayPort MST negotiation to complete before Plymouth attempts to display the encryption prompt.

Implementation:

sudo sed -i 's/^MODULES=(btrfs)$/MODULES=(amdgpu btrfs)/' /etc/mkinitcpio.conf
sudo mkinitcpio -P

Testing Plan

1. Verify initramfs regeneration completed successfully
2. Physically disconnect the Dell monitor (HDMI)
3. Reboot system with only LG 5K connected via USB-C
4. Check if Plymouth encryption unlock screen appears on LG monitor
5. Verify post-login functionality in Hyprland

Expected Outcome

The encryption unlock screen should now appear on the LG 5K monitor during boot, as the AMD GPU driver will have initialized the DisplayPort MST streams before Plymouth starts.

Actual Outcome

Black screen, can still type password though

Alternative Solutions (If Primary Solution Fails)

Option 1: Add initramfs Boot Delay

Add a delay hook to initramfs to explicitly wait for DisplayPort initialization:

# Create custom hook
/etc/initcpio/hooks/displayport-delay

# Add sleep command to wait for DP negotiation
sleep 2

Trade-off: Slower boot time (2-3 seconds) but guaranteed display initialization.

Option 2: Modify Plymouth to Retry Display Detection

Configure Plymouth to retry display detection if no monitors are found initially.

Location: /etc/plymouth/plymouthd.conf

Trade-off: Requires Plymouth configuration changes and may not be supported by all Plymouth themes.

Option 3: Kernel Parameter with MST-Specific Settings

Instead of forcing specific outputs, enable verbose MST debugging and longer timeout:

amdgpu.mst=1 drm.debug=0x04

Trade-off: More verbose boot logs, potential performance impact during boot.

Current Boot Configuration

/boot/loader/entries/2025-07-15_09-52-27_linux.conf:

options cryptdevice=PARTUUID=0e541d60-d1c1-4234-9941-b4fe99199ae8:root root=/dev/mapper/root zswap.enabled=0 rootflags=subvol=@ rw rootfstype=btrfs splash quiet

No kernel parameter changes were needed.

Detected Monitor Information

Monitor DP-3 (ID 1):
  Resolution: [email protected]
  Description: LG Electronics LG UltraFine 910NTYT0W842

Monitor DP-4 (ID 2):
  Resolution: [email protected]
  Description: LG Electronics LG UltraFine 910NTYT0W842
  Available Modes: [email protected] (only 5K mode available on DP-4)

References

• Hyprland Monitors Configuration: https://wiki.hyprland.org/Configuring/Monitors/
• Arch Linux mkinitcpio: https://wiki.archlinux.org/title/Mkinitcpio
• DisplayPort MST Overview: https://en.wikipedia.org/wiki/DisplayPort#Multi-Stream_Transport
• AMD GPU Driver Documentation: https://wiki.archlinux.org/title/AMDGPU

Conclusions

What Works

• ✅ Hyprland functions perfectly with only LG 5K connected after boot
• ✅ All applications launch normally
• ✅ Both DP-3 and DP-4 streams work correctly in Hyprland
• ✅ Blind password entry at boot (typing on black screen) successfully unlocks the system

What Doesn't Work

• ❌ Plymouth encryption unlock screen does not appear on LG 5K during boot
• ❌ Adding amdgpu to initramfs MODULES did not resolve the Plymouth timing issue

Recommended Workarounds

Option 1: Blind Password Entry (Most Practical)

• Boot with only LG 5K connected
• Screen will be black during encryption prompt
• Type password blindly (you won't see any feedback)
• System unlocks and Hyprland starts normally
• Pros: No additional hardware needed
• Cons: No visual feedback during password entry

Option 2: Secondary Boot Display (Most Reliable)

• Keep a secondary monitor (Dell or any HDMI/DP monitor) connected during boot
• Use secondary monitor only for encryption unlock
• After login, use LG 5K as primary (or both monitors)
• Pros: Visual feedback during password entry, guaranteed to work
• Cons: Requires keeping a second monitor connected

Option 3: Disable Encryption (Not Recommended)

• Remove full-disk encryption to eliminate the boot prompt
• Pros: LG 5K will display from earliest boot stage
• Cons: Significantly reduces security, not recommended

[afgallo]

This discussion has some similarities although I was not looking to get a CalDigit TS4 setup

[ankor]

I have the exact same hardware configuration and this solved it for me: #1894

The PR is not merged yet but installing the bolt package and running the migration script manually works for me. Additionally, peripherals connected through the LG monitor's USB hub now also work. (The peripherals still don't work at the boot password screen though).

[wpcarro]

I'm having issues using my LG UltraFine 5K with my Dell Plus laptop (the monitor turns on/off randomly).

Did you have to do any custom configuration to get Omarchy to play nicely with the external monitor? I tried installing bolt and running the migration you linked to, but the monitor still turns on/off, which is brutal.

FWIW the monitor and laptop worked fine when I was running Windows 11, so I'm assuming this is a Linux configuration issue.