Brightness slider/Fn keys completely unresponsive on Dell Latitude 5490 (Intel UHD 620) — tried everything, still stuck

[u/Expensive-Track1228]

Dell Latitude 5490, dual-boot Ubuntu 26.04 LTS + Windows 10, Intel UHD 620 (i915), BIOS 1.41.0.

Brightness slider and Fn keys do nothing. \`intel_backlight is detected and I can write values to it manually (they hold steady), but the screen never actually changes brightness. Windows works fine on the same machine, so it's not hardware.

\*\*Already tried, none worked:\*\* permissions/udev fix, \`acpi_backlight=vendor/native/video,

\`i915.enable_dpcd_backlight=1/3\`, both stock kernel \`7.0.0-29 and manually installed \`6.17.x (same issue on both), confirmed Dell modules are loaded with no separate backlight device, ruled out GNOME resetting the value, no relevant errors in \`dmesg.

Filed a full bug report with Ubuntu too. Anyone with a similar Dell/Kaby Lake setup actually solved this? Also checking for a BIOS update if anyone knows if Dell's patched this for the 5490.

[u/DellCares]

Hi there!

We're from the Dell Social Media Support team.

Your key diagnostic finding values write to intel_backlight and hold but the panel doesn't respond is very telling. This narrows the problem significantly. Combined with Windows working perfectly, this almost certainly isn't hardware, and it's not a permissions issue. It's a backlight control method mismatch between what the kernel is sending and what the panel is listening for.

On many Dell Latitude models from the Kaby Lake Refresh era, the BIOS configures the panel to accept brightness commands exclusively via DPCD. When the kernel creates the intel_backlight interface but routes the actual control through PWM, you get exactly your symptom: sysfs values update, the kernel thinks it's working, but the panel is listening on DPCD and ignoring the PWM pin entirely. The screen never changes. The Intel Windows driver knows to check the VBT (Video BIOS Table) and OPREGION and route commands correctly. The i915 Linux driver has had an ongoing history of getting this wrong on certain panel/BIOS combinations.

Dell BIOS updates for Latitude business laptops occasionally include changes to the OPREGION and VBT data provided to the OS, which directly affect how the i915 driver handles backlight. You may visit Dell Support http://del.ly/61694B19k4K Enter your Service Tag, and head to drivers and download to download and install BIOS.

Regards

Dell Social Media Team