Confirming this is the same issue — I filed a separate report before finding this one, marking mine as a duplicate. Adding my data points here in case they help narrow down the root cause:
Hardware: Dell Vostro 15 3510, GPU: Intel Tiger Lake-LP GT2 (UHD Graphics G4, device ID 9a78) — a third distinct machine/panel combination affected, alongside the Latitude 5420 (Tiger Lake) and Latitude 7490 (Kaby Lake-R) already reported. This suggests the regression is not tied to a specific panel/vendor but to the i915 backlight write path itself. Kernel transition: Same 7.0.0-27-generic (working) → 7.0.0-28-generic (broken) as reported by others. i915.enable_dpcd_backlight testing: The original report used enable_dpcd_backlight=0. I tested with it effectively enabled instead (=1 and =3 — note this parameter appears to be boolean per modinfo i915, so both likely evaluate to "true"), combined with acpi_backlight=native: This fixed the boot-time initial brightness (previously stuck at 100%, correctly restored to the saved value by systemd-backlight at boot). It did not fix runtime/live updates — slider and manual echo > brightness writes still update the sysfs value but never reach the physical panel after boot. acpi_backlight= testing: Also tried acpi_backlight=vendor on 7.0.0-28 — this causes the intel_backlight sysfs interface to disappear entirely (i915: [drm] Skipping intel_backlight registration in dmesg), so no backlight device is exposed at all on this hardware with that setting. Happy to provide further logs (dmesg, journalctl) or test additional kernel parameters if useful — same regression pattern as above, just wanted to add another confirmed hardware combination and the additional parameter test results to the pool of data. -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2161148 Title: [SRU] intel_backlight (i915/eDP AUX-DPCD) stops responding to live brightness writes in 7.0.0-28-generic (works in 6.17.0-40-generic) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2161148/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
