Public bug reported:
Summary
-------
External HDMI monitor connected to the Intel iGPU HDMI port never gets a
picture.
Kernel log repeats every ~16 seconds since boot:
[drm] *ERROR* [CONNECTOR:516:HDMI-A-1] Failed to read TMDS config: -6
`xrandr` shows HDMI-1 as "connected" but with an implausible fallback
resolution (6880x2880), consistent with a failed EDID/DDC read rather
than a real mode.
Hardware
--------
- Laptop: Acer Aspire A715-76G
- iGPU: Intel Alder Lake-P UHD Graphics [8086:46a3] (0000:00:02.0) - HDMI port
is wired to this GPU
- dGPU: NVIDIA RTX 2050 (GA107), mux-less Optimus, no physical video output of
its own
- BIOS: InsydeH20, version V1.57 (2026-05-28) - latest available, no relevant
multi-monitor/IGD options exposed
- OS: Ubuntu 26.04
- Kernel: 7.0.0-29-generic (also reproduced on mainline-adjacent `xe`
force-probe of the same kernel)
Steps to reproduce
------------------
1. Cold boot into Ubuntu with an external monitor connected via HDMI.
2. Monitor is detected but shows no signal / no picture.
3. `journalctl -k -b | grep -i -E "tmds|hdmi"` shows repeated
"Failed to read TMDS config: -6" (errno -6 = ENXIO) on
CONNECTOR:516:HDMI-A-1.
What I already ruled out
-------------------------
- Cable: tested with a second HDMI cable, same result.
- Port/monitor: monitor+cable tested and recognized fine; same monitor works
perfectly under Windows 11 on the exact same hardware/cable/port (both cold
boot and warm reboot from a fully logged-in Windows session into Ubuntu).
- prime-select mode: tested both `nvidia` and `on-demand`, no difference
(expected, since the HDMI port is physically on the iGPU, not the dGPU).
- Driver: forced the newer `xe` driver via
`i915.force_probe=!46a3 xe.force_probe=46a3` - identical
"Failed to read TMDS config: -6" error occurs with `xe` as well, just with
the `xe` module name in the log instead of `i915`. This points to a failure
in the shared GMBUS/DDC/i2c transaction layer, not in driver-specific logic.
- BIOS: no Internal Graphics / IGD Multi-Monitor / DVMT / CSM options exist
in this InsydeH20 BIOS to test.
- Newer kernel: attempted to test mainline v7.1.5, but it fails to boot due
to Secure Boot ("bad shim lock signature") since mainline builds are
unsigned; not tested with Secure Boot disabled.
Expected behavior
-----------------
External HDMI monitor should light up on Linux the same way it does on
Windows with identical hardware.
Additional info
----------------
Please find attached: `journalctl -k -b` output, `lspci -nnk` VGA section,
`xrandr` output, and dmesg from apport.
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: linux-image-7.0.0-29-generic 7.0.0-29.29
ProcVersionSignature: Ubuntu 7.0.0-29.29-generic 7.0.12
Uname: Linux 7.0.0-29-generic x86_64
ApportVersion: 2.34.1-0ubuntu0.1
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/controlC0: alex 3919 F.... pipewire
alex 3933 F.... wireplumber
/dev/snd/seq: alex 3919 F.... pipewire
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Sat Aug 15 14:45:00 2026
InstallationDate: Installed on 2026-08-14 (1 days ago)
InstallationMedia: Ubuntu 26.04 "Resolute Raccoon" - Release amd64 (20260423.1)
MachineType: Acer Aspire A715-76G
ProcEnviron:
LANG=en_US.UTF-8
PATH=(custom, no user)
SHELL=/bin/bash
TERM=xterm-256color
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-7.0.0-29-generic
root=UUID=34813e31-7b29-4b9f-8baa-6216c89d517a ro quiet splash
crashkernel=2G-4G:320M,4G-32G:512M,32G-64G:1024M,64G-128G:2048M,128G-:4096M
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 05/28/2026
dmi.bios.release: 1.57
dmi.bios.vendor: INSYDE Corp.
dmi.bios.version: V1.57
dmi.board.asset.tag: Type2 - Board Asset Tag
dmi.board.name: Metis_ADP
dmi.board.vendor: ADL
dmi.board.version: V1.57
dmi.chassis.type: 10
dmi.chassis.vendor: Chassis Manufacturer
dmi.chassis.version: Chassis Version
dmi.ec.firmware.release: 1.4
dmi.modalias:
dmi:bvnINSYDECorp.:bvrV1.57:bd05/28/2026:br1.57:efr1.4:svnAcer:pnAspireA715-76G:pvrV1.57:rvnADL:rnMetis_ADP:rvrV1.57:cvnChassisManufacturer:ct10:cvrChassisVersion:sku0000000000000000:pfaAspire7:
dmi.product.family: Aspire 7
dmi.product.name: Aspire A715-76G
dmi.product.sku: 0000000000000000
dmi.product.version: V1.57
dmi.sys.vendor: Acer
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug resolute
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https://bugs.launchpad.net/bugs/2163568
Title:
[i915][xe] Failed to read TMDS config: -6 on HDMI-A-1 (Alder Lake-P
iGPU, Acer Aspire A715-76G) - external monitor never lights up
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