Public bug reported:
Summary: Backlight brightness control regressed between kernel 7.0.0-22 and
7.0.0-28 on a Dell Inspiron 15 3511 with Intel Tiger Lake-LP GT2 [Iris Xe
Graphics]. The physical brightness keys are detected (on-screen slider moves),
but the actual screen backlight does not change at all. Completely broken on
7.0.0-28 and 7.0.0-29. Works perfectly on 7.0.0-22.
Hardware:
Laptop: Dell Inspiron 15 3511
CPU/GPU: Intel Tiger Lake-LP GT2 [Iris Xe Graphics] (device ID 9a49)
OS: Ubuntu 26.04 (resolute), dual boot with Windows
Backlight interface: /sys/class/backlight/intel_backlight
DMC firmware: i915/tgl_dmc_ver2_12.bin (v2.12)
Regression window:
✅ Works: 7.0.0-22-generic
❌ Broken: 7.0.0-28-generic
❌ Broken: 7.0.0-29-generic
The jump from 7.0.0-22 to 7.0.0-28 was introduced by an unattended
upgrade on 2026-08-02.
What was ruled out (extensive testing):
Not a hardware fault — brightness works perfectly in Windows on the same
machine (dual boot), and works in a live Ubuntu session from the original
install ISO (kernel 7.0.0-14).
Not a BIOS/firmware issue — booting the same hardware from a clean live USB
with kernel 7.0.0-14 restored full brightness control, confirming the BIOS
behaves correctly. The BIOS was also updated to the latest available version —
no change.
Not a firmware package regression — downgraded linux-firmware-intel-graphics
from 0ubuntu2.1 back to 0ubuntu2 and confirmed the DMC firmware blob is
identical between the working live session and the installed system (md5:
de018e9fa626be8cf8d388c0bf767410). Brightness still broken on 7.0.0-28/29 with
the old firmware package.
Not a software/daemon interference — tested from a bare virtual terminal
(Ctrl+Alt+F3, no desktop session, no extensions, no daemons running). Still
broken.
ACPI/SMI chain verified firing correctly — dumped and disassembled the DSDT.
The _BCM brightness method routes through GENS → SMBF → ASMI(), which writes
0xE0 to I/O port 0xB2 (a genuine SMI trigger). Verified via the CPU's
IA32_SMI_COUNT MSR (register 0x34, msr-tools) that the SMI counter increments
by exactly 1 on every write to /sys/class/backlight/acpi_video0/brightness —
the ACPI method and SMI are firing correctly. This was tested on the broken
kernel (7.0.0-29); the issue is that the result is ignored.
Kernel isolated as the sole cause — with the firmware package held at the old
version, switching between 7.0.0-22 (works) and 7.0.0-28 (broken) is the only
variable that changes the outcome.
Kernel parameters tested (none fixed it on 7.0.0-28/29):
acpi_backlight=native
acpi_backlight=vendor
acpi_backlight=video
i915.enable_dpcd_backlight=1
i915.enable_dpcd_backlight=2
i915.enable_psr=0
i915.invert_brightness=1
Blacklisted xe module, ensured i915 has exclusive control
Current workaround: Pinned to 7.0.0-22-generic via apt-mark hold.
Brightness fully functional on that kernel.
Reproduces on: Dell Inspiron 15 3511, Tiger Lake Iris Xe, Ubuntu 26.04,
kernel ≥ 7.0.0-28.
Happy to provide full dmesg output, ACPI DSDT dump, or any other
diagnostics from either the working (7.0.0-22) or broken (7.0.0-29)
kernel on request.
This bug were spotted by a human and this report was generated with the
assist of an AI after giving it all of the context.
Thank you for making linux available for the community ❤️.
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: linux-image-7.0.0-22-generic 7.0.0-22.22
ProcVersionSignature: Ubuntu 7.0.0-22.22-generic 7.0.0
Uname: Linux 7.0.0-22-generic x86_64
ApportVersion: 2.34.1-0ubuntu0.1
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/controlC0: mohammed 4163 F.... wireplumber
/dev/snd/seq: mohammed 4147 F.... pipewire
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Fri Aug 7 11:37:32 2026
InstallationDate: Installed on 2026-05-19 (80 days ago)
InstallationMedia: Ubuntu 26.04 "Resolute Raccoon" - Release amd64 (20260423.1)
MachineType: Dell Inc. Inspiron 15 3511
ProcEnviron:
LANG=en_US.UTF-8
PATH=(custom, no user)
SHELL=/bin/bash
TERM=xterm-256color
XDG_RUNTIME_DIR=<set>
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-7.0.0-22-generic
root=UUID=af9f8c53-fb0e-4c6a-951d-7f651c38ed30 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: 05/27/2026
dmi.bios.release: 1.47
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.47.0
dmi.board.name: 0042CN
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 10
dmi.chassis.vendor: Dell Inc.
dmi.modalias:
dmi:bvnDellInc.:bvr1.47.0:bd05/27/2026:br1.47:svnDellInc.:pnInspiron153511:pvr:rvnDellInc.:rn0042CN:rvrA00:cvnDellInc.:ct10:cvr:sku0AB0:pfaInspiron:
dmi.product.family: Inspiron
dmi.product.name: Inspiron 15 3511
dmi.product.sku: 0AB0
dmi.sys.vendor: Dell Inc.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug resolute wayland-session
** Description changed:
- Ubuntu Kernel Bug Report
- Backlight Brightness Regression — Dell Inspiron 15 3511 (Tiger Lake / Iris Xe)
- For filing via: ubuntu-bug linux on Launchpad
- One-Line Summary
- Screen brightness control completely broken on kernel 7.0.0-28 and
- 7.0.0-29; works perfectly on 7.0.0-22. Dell Inspiron 15 3511, Intel
- Tiger Lake Iris Xe.
-
- Bug Description (paste this into Launchpad)
-
- Summary: Backlight brightness control regressed between kernel 7.0.0-22
- and 7.0.0-28 on a Dell Inspiron 15 3511 with Intel Tiger Lake-LP GT2
- [Iris Xe Graphics]. The physical brightness keys are detected (on-screen
- slider moves), but the actual screen backlight does not change at all.
- Completely broken on 7.0.0-28 and 7.0.0-29. Works perfectly on 7.0.0-22.
+ Summary: Backlight brightness control regressed between kernel 7.0.0-22 and
7.0.0-28 on a Dell Inspiron 15 3511 with Intel Tiger Lake-LP GT2 [Iris Xe
Graphics]. The physical brightness keys are detected (on-screen slider moves),
but the actual screen backlight does not change at all. Completely broken on
7.0.0-28 and 7.0.0-29. Works perfectly on 7.0.0-22.
Hardware:
Laptop: Dell Inspiron 15 3511
CPU/GPU: Intel Tiger Lake-LP GT2 [Iris Xe Graphics] (device ID 9a49)
OS: Ubuntu 26.04 (resolute), dual boot with Windows
Backlight interface: /sys/class/backlight/intel_backlight
DMC firmware: i915/tgl_dmc_ver2_12.bin (v2.12)
Regression window:
✅ Works: 7.0.0-22-generic
❌ Broken: 7.0.0-28-generic
❌ Broken: 7.0.0-29-generic
The jump from 7.0.0-22 to 7.0.0-28 was introduced by an unattended
upgrade on 2026-08-02.
What was ruled out (extensive testing):
Not a hardware fault — brightness works perfectly in Windows on the same
machine (dual boot), and works in a live Ubuntu session from the original
install ISO (kernel 7.0.0-14).
Not a BIOS/firmware issue — booting the same hardware from a clean live USB
with kernel 7.0.0-14 restored full brightness control, confirming the BIOS
behaves correctly. The BIOS was also updated to the latest available version —
no change.
Not a firmware package regression — downgraded linux-firmware-intel-graphics
from 0ubuntu2.1 back to 0ubuntu2 and confirmed the DMC firmware blob is
identical between the working live session and the installed system (md5:
de018e9fa626be8cf8d388c0bf767410). Brightness still broken on 7.0.0-28/29 with
the old firmware package.
Not a software/daemon interference — tested from a bare virtual terminal
(Ctrl+Alt+F3, no desktop session, no extensions, no daemons running). Still
broken.
ACPI/SMI chain verified firing correctly — dumped and disassembled the DSDT.
The _BCM brightness method routes through GENS → SMBF → ASMI(), which writes
0xE0 to I/O port 0xB2 (a genuine SMI trigger). Verified via the CPU's
IA32_SMI_COUNT MSR (register 0x34, msr-tools) that the SMI counter increments
by exactly 1 on every write to /sys/class/backlight/acpi_video0/brightness —
the ACPI method and SMI are firing correctly. This was tested on the broken
kernel (7.0.0-29); the issue is that the result is ignored.
Kernel isolated as the sole cause — with the firmware package held at the old
version, switching between 7.0.0-22 (works) and 7.0.0-28 (broken) is the only
variable that changes the outcome.
Kernel parameters tested (none fixed it on 7.0.0-28/29):
acpi_backlight=native
acpi_backlight=vendor
acpi_backlight=video
i915.enable_dpcd_backlight=1
i915.enable_dpcd_backlight=2
i915.enable_psr=0
i915.invert_brightness=1
Blacklisted xe module, ensured i915 has exclusive control
Current workaround: Pinned to 7.0.0-22-generic via apt-mark hold.
Brightness fully functional on that kernel.
Reproduces on: Dell Inspiron 15 3511, Tiger Lake Iris Xe, Ubuntu 26.04,
kernel ≥ 7.0.0-28.
Happy to provide full dmesg output, ACPI DSDT dump, or any other
diagnostics from either the working (7.0.0-22) or broken (7.0.0-29)
kernel on request.
This bug were spotted by a human and this report was generated with the
assist of an AI after giving it all of the context.
Thank you for making linux available for the community ❤️.
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: linux-image-7.0.0-22-generic 7.0.0-22.22
ProcVersionSignature: Ubuntu 7.0.0-22.22-generic 7.0.0
Uname: Linux 7.0.0-22-generic x86_64
ApportVersion: 2.34.1-0ubuntu0.1
Architecture: amd64
AudioDevicesInUse:
- USER PID ACCESS COMMAND
- /dev/snd/controlC0: mohammed 4163 F.... wireplumber
- /dev/snd/seq: mohammed 4147 F.... pipewire
+ USER PID ACCESS COMMAND
+ /dev/snd/controlC0: mohammed 4163 F.... wireplumber
+ /dev/snd/seq: mohammed 4147 F.... pipewire
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Fri Aug 7 11:37:32 2026
InstallationDate: Installed on 2026-05-19 (80 days ago)
InstallationMedia: Ubuntu 26.04 "Resolute Raccoon" - Release amd64
(20260423.1)
MachineType: Dell Inc. Inspiron 15 3511
ProcEnviron:
- LANG=en_US.UTF-8
- PATH=(custom, no user)
- SHELL=/bin/bash
- TERM=xterm-256color
- XDG_RUNTIME_DIR=<set>
+ LANG=en_US.UTF-8
+ PATH=(custom, no user)
+ SHELL=/bin/bash
+ TERM=xterm-256color
+ XDG_RUNTIME_DIR=<set>
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-7.0.0-22-generic
root=UUID=af9f8c53-fb0e-4c6a-951d-7f651c38ed30 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: 05/27/2026
dmi.bios.release: 1.47
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.47.0
dmi.board.name: 0042CN
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 10
dmi.chassis.vendor: Dell Inc.
dmi.modalias:
dmi:bvnDellInc.:bvr1.47.0:bd05/27/2026:br1.47:svnDellInc.:pnInspiron153511:pvr:rvnDellInc.:rn0042CN:rvrA00:cvnDellInc.:ct10:cvr:sku0AB0:pfaInspiron:
dmi.product.family: Inspiron
dmi.product.name: Inspiron 15 3511
dmi.product.sku: 0AB0
dmi.sys.vendor: Dell Inc.
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https://bugs.launchpad.net/bugs/2163019
Title:
Screen brightness control completely broken on kernel 7.0.0-28 and
7.0.0-29; works perfectly on 7.0.0-22. Dell Inspiron 15 3511, Intel
Tiger Lake Iris Xe.
To manage notifications about this bug go to:
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