[Bug 2161948] [NEW] Panel brightness capped well below actual maximum on Dell Latitude 7390 2-in-1 (AU Optronics panel) with kernel 7.0.0-28-generic; works correctly on 6.17.0-35-generic

Public bug reported: Description: After the HWE kernel update to 7.0.0-28-generic on Ubuntu 24.04 (Zorin OS), the internal display's actual physical brightness is capped at a dim level regardless of the backlight control value reported/set via sysfs. Hardware: Dell Latitude 7390 2-in-1 Panel: AU Optronics (vendor 0x462d) GPU: Intel integrated (i915) Symptoms: Screen appears dim starting from the OS boot stage (post-BIOS/Dell logo), before any settings are applied /sys/class/backlight/intel_backlight/brightness and max_brightness both report 1023 (i.e., at 100%) Writing lower values to intel_backlight/brightness correctly dims the screen further, confirming the control path works Writing back to 1023 does not restore full brightness — the panel's actual maximum output appears capped below its true capability Confirmed NOT caused by: PSR (tested with i915.enable_psr=0, and confirmed disabled via /sys/kernel/debug/dri/.../i915_edp_psr_status), DPCD backlight mode, GNOME Night Light, GNOME idle-dim, ICC/color profile (removed ~/.local/share/icc/edid-*.icc), or user-profile settings (reproduced under a freshly created user account) Steps to reproduce: Boot Dell Latitude 7390 2-in-1 with kernel 7.0.0-28-generic Observe screen brightness is visibly dim from boot onward Confirm intel_backlight reports 100% (brightness == max_brightness) Reboot into 6.17.0-35-generic instead — brightness is correct/normal Expected behavior: Panel should reach full physical brightness when intel_backlight/brightness is at max_brightness, matching behavior on 6.17.0-35-generic. Workaround: Pinning/booting 6.17.0-35-generic instead of 7.0.0-28-generic resolves the issue completely. ** Affects: linux (Ubuntu) Importance: Undecided Status: New ** Tags: compatibility dell display kernel -- You received this bug notification because you are subscribed to linux in Ubuntu. Matching subscriptions: Bgg, Bmail, Nb https://bugs.launchpad.net/bugs/2161948 Title: Panel brightness capped well below actual maximum on Dell Latitude 7390 2-in-1 (AU Optronics panel) with kernel 7.0.0-28-generic; works correctly on 6.17.0-35-generic Status in linux package in Ubuntu: New Bug description: Description: After the HWE kernel update to 7.0.0-28-generic on Ubuntu 24.04 (Zorin OS), the internal display's actual physical brightness is capped at a dim level regardless of the backlight control value reported/set via sysfs. Hardware: Dell Latitude 7390 2-in-1 Panel: AU Optronics (vendor 0x462d) GPU: Intel integrated (i915) Symptoms: Screen appears dim starting from the OS boot stage (post-BIOS/Dell logo), before any settings are applied /sys/class/backlight/intel_backlight/brightness and max_brightness both report 1023 (i.e., at 100%) Writing lower values to intel_backlight/brightness correctly dims the screen further, confirming the control path works Writing back to 1023 does not restore full brightness — the panel's actual maximum output appears capped below its true capability Confirmed NOT caused by: PSR (tested with i915.enable_psr=0, and confirmed disabled via /sys/kernel/debug/dri/.../i915_edp_psr_status), DPCD backlight mode, GNOME Night Light, GNOME idle-dim, ICC/color profile (removed ~/.local/share/icc/edid-*.icc), or user-profile settings (reproduced under a freshly created user account) Steps to reproduce: Boot Dell Latitude 7390 2-in-1 with kernel 7.0.0-28-generic Observe screen brightness is visibly dim from boot onward Confirm intel_backlight reports 100% (brightness == max_brightness) Reboot into 6.17.0-35-generic instead — brightness is correct/normal Expected behavior: Panel should reach full physical brightness when intel_backlight/brightness is at max_brightness, matching behavior on 6.17.0-35-generic. Workaround: Pinning/booting 6.17.0-35-generic instead of 7.0.0-28-generic resolves the issue completely. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2161948/+subscriptions