[Bug 2170457] Re: Regression: Intel i915 HDMI flickering and CPU pipe B FIFO underrun with eDP 1368x768 + HDMI 1920x1080@60 after Ubuntu 26.04 upgrade

** Tags added: kernel-daily-bug -- You received this bug notification because you are subscribed to linux in Ubuntu. Matching subscriptions: Bgg, Bmail, Nb 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 Status in linux package in Ubuntu: New Bug description: 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. 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