Public bug reported:
After upgrading to Ubuntu 26.04.1 LTS, an external HDMI monitor started
flickering when used together with the laptop internal display in a
specific display mode combination.
Hardware:
- Dell Latitude 3400
- Intel UHD Graphics 620 / WhiskeyLake-U GT2 [8086:3ea0]
- Kernel driver: i915
- X11 session
- Xorg modesetting driver + glamor
- Ubuntu 26.04.1 LTS
- Current kernel: 7.0.0-38-generic
Problematic configuration:
- Internal eDP-1: 1368x768 @ 60.05 Hz
- External HDMI-1: 1920x1080 @ 60 Hz
In this configuration, the external HDMI monitor flickers heavily and
the kernel logs:
i915 0000:00:02.0: [drm] *ERROR* CPU pipe B FIFO underrun
This exact setup worked normally before the Ubuntu 26.04 upgrade.
The issue is reproducible with multiple hardware combinations:
- Tested with 2 different external monitors
- Tested with 3 different HDMI cables
- Same behavior
Tests performed:
1. eDP-1 1368x768 @60 + HDMI 1920x1080 @60
Result: flickering, CPU pipe B FIFO underrun
2. eDP-1 1368x768 @60 + HDMI 1920x1080 @30
Result: stable
3. eDP-1 disabled + HDMI 1920x1080 @60
Result: stable
4. eDP-1 1920x1080 @60 + HDMI 1920x1080 @60
Result: stable
5. eDP-1 1600x900 @60 + HDMI 1920x1080 @60
Result: stable
6. eDP-1 1280x720 @60 + HDMI 1920x1080 @60
Result: stable
Changing the internal display scaling mode from "Full aspect" to
"Center" does NOT solve the issue.
The problematic internal display mode reported by xrandr is:
1368x768 (72.486 MHz)
h: 1368 1416 1448 1528
v: 768 771 781 790
refresh: 60.05 Hz
The working 1600x900 mode is:
1600x900 (97.866 MHz)
h: 1600 1648 1680 1760
v: 900 903 908 926
refresh: 60.05 Hz
Historical kernel logs:
- Before the distribution upgrade, kernels 6.8.0-139, 6.8.0-142 and 6.8.0-146
were used.
- No CPU pipe FIFO underrun was found in the reviewed historical logs for those
kernels.
- Ubuntu distribution upgrade occurred on 2026-09-30.
- Kernel 7.0.0-34-generic was booted afterwards.
- First recorded CPU pipe B FIFO underrun appeared on 2026-10-01 while running
7.0.0-34-generic.
- The errors continue with 7.0.0-38-generic.
Current workaround:
- Internal display: 1600x900 @60
- External HDMI display: 1920x1080 @60
Another workaround:
- Internal display: 1368x768 @60
- External HDMI display: 1920x1080 @30
Additional observation:
Cinnamon stores the problematic display profile and reapplies HDMI 1920x1080@60
after HDMI hotplug, so disconnecting and reconnecting the monitor makes the
flickering return.
This appears to be a regression in the Intel i915/DRM display pipeline
after the Ubuntu 26.04 upgrade, triggered specifically by the eDP
1368x768 mode while HDMI is active at 1920x1080@60.
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: linux-image-7.0.0-38-generic 7.0.0-38.38
ProcVersionSignature: Ubuntu 7.0.0-38.38-generic 7.0.14
Uname: Linux 7.0.0-38-generic x86_64
ApportVersion: 2.34.1-0ubuntu0.1
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/controlC0: sobreiro 11396 F.... wireplumber
/dev/snd/seq: sobreiro 11377 F.... pipewire
CasperMD5CheckResult: pass
CurrentDesktop: X-Cinnamon
Date: Sat Oct 10 16:06:18 2026
InstallationDate: Installed on 2026-03-25 (199 days ago)
InstallationMedia: Ubuntu 22.04.4 LTS "Jammy Jellyfish" - Release amd64
(20240220)
Lsusb:
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 27c6:530c Shenzhen Goodix Technology Co.,Ltd.
Fingerprint Reader
Bus 001 Device 003: ID 0bda:5520 Realtek Semiconductor Corp.
Integrated_Webcam_HD
Bus 001 Device 004: ID 0cf3:e007 Qualcomm Atheros Communications
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
MachineType: Dell Inc. Latitude 3400
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-7.0.0-38-generic
root=UUID=f2fc313e-7088-4155-9ae8-ba73e7ae9a15 ro quiet splash
crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M
PulseList: Error: command ['pacmd', 'list'] failed with exit code 1: No
PulseAudio daemon running, or not running as session daemon.
SourcePackage: linux
UpgradeStatus: Upgraded to resolute on 2026-10-01 (10 days ago)
dmi.bios.date: 03/26/2025
dmi.bios.release: 1.38
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.38.0
dmi.board.name: 0XWHC2
dmi.board.vendor: Dell Inc.
dmi.board.version: A05
dmi.chassis.type: 10
dmi.chassis.vendor: Dell Inc.
dmi.modalias:
dmi:bvnDellInc.:bvr1.38.0:bd03/26/2025:br1.38:svnDellInc.:pnLatitude3400:pvr:rvnDellInc.:rn0XWHC2:rvrA05:cvnDellInc.:ct10:cvr:sku08BC:pfaLatitude:
dmi.product.family: Latitude
dmi.product.name: Latitude 3400
dmi.product.sku: 08BC
dmi.sys.vendor: Dell Inc.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug resolute
--
You received this bug notification because you are a member of Ubuntu
Bugs, which is subscribed to Ubuntu.
https://bugs.launchpad.net/bugs/2170457
Title:
Regression: Intel i915 HDMI flickering and CPU pipe B FIFO underrun
with eDP 1368x768 + HDMI 1920x1080@60 after Ubuntu 26.04 upgrade
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2170457/+subscriptions
--
ubuntu-bugs mailing list
[email protected]
https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
