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
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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
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