Backlight/brightness not working in Ubuntu 23.10/24.04 on Dell 7700 AIO

Ubuntu 23.10/24.04
Dell Inspiron 7700 AIO
6.5.0-35-generic/6.8.0-31-generic
X11

Some recent 23.10/24.04 update has broken my system. The brightness slider is no longer visible. And the directory /sys/class/backlight/ is empty.

The backlight driver dell_uart_backlight is not loaded on my 24.04 system. It's not even present on the 24.04 distribution.

If I boot to a Ubuntu 23.10.1 flash drive, it all works fine. So the problem has occurred since the 23.10.1 version.

[Update] Turns out that booting to the 23.10.1 flash drive only partially fixes the problem. The brightness slider appears, but it really doesn't update the screen brightness when moved. Booting to a 23.04 flash drive, and everything works as its supposed to.

[Update] Brightness control no longer working after update to Ubuntu 23.10 in computer Dell OptiPlex 7780 AIO

The command gdbus call --session --dest org.gnome.SettingsDaemon.Power --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepUp returns:

GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._gsd_5fpower_5fmanager_5ferror.Code1: No usable backlight could be found!

[Update] Adding the acpi_backlight=vendor kernel parameter in /etc/default/grub makes no difference.

[Update] I am able to set backlight brightness with the following xrandr command:

xrandr --output HDMI-1 --brightness 0.85

[Update] It seems that the dell_uart_backlight driver is missing on my 24.04 system.

ls -alR /lib/modules/6.5.0-35-generic/ | grep dell
-rw-r--r--   1 root root 13689 Apr 26 02:32 dell-smm-hwmon.ko.zst
drwxr-xr-x  3 root root  4096 May 15 19:10 dell
/lib/modules/6.5.0-35-generic/kernel/drivers/platform/x86/dell:
-rw-r--r-- 1 root root 18647 Apr 26 02:32 dell-laptop.ko.zst
-rw-r--r-- 1 root root  6158 Apr 26 02:32 dell-rbtn.ko.zst
-rw-r--r-- 1 root root  8431 Apr 26 02:32 dell_rbu.ko.zst
-rw-r--r-- 1 root root 13963 Apr 26 02:32 dell-smbios.ko.zst
-rw-r--r-- 1 root root  4432 Apr 26 02:32 dell-smo8800.ko.zst
-rw-r--r-- 1 root root  7762 Apr 26 02:32 dell-uart-backlight.ko.zst
-rw-r--r-- 1 root root  3799 Apr 26 02:32 dell-wmi-aio.ko.zst
-rw-r--r-- 1 root root  9209 Apr 26 02:32 dell-wmi-ddv.ko.zst
-rw-r--r-- 1 root root  4532 Apr 26 02:32 dell-wmi-descriptor.ko.zst
-rw-r--r-- 1 root root 11198 Apr 26 02:32 dell-wmi.ko.zst
-rw-r--r-- 1 root root  3115 Apr 26 02:32 dell-wmi-led.ko.zst
drwxr-xr-x 2 root root  4096 May 15 19:10 dell-wmi-sysman
/lib/modules/6.5.0-35-generic/kernel/drivers/platform/x86/dell/dell-wmi-sysman:
-rw-r--r-- 1 root root 18312 Apr 26 02:32 dell-wmi-sysman.ko.zst

ls -alR /lib/modules/6.8.0-31-generic/ | grep dell
-rw-r--r--   1 root root 15140 Apr 19 14:46 dell-smm-hwmon.ko.zst
drwxr-xr-x  3 root root  4096 May 25 10:15 dell
/lib/modules/6.8.0-31-generic/kernel/drivers/platform/x86/dell:
-rw-r--r-- 1 root root 18629 Apr 19 14:46 dell-laptop.ko.zst
-rw-r--r-- 1 root root  6180 Apr 19 14:46 dell-rbtn.ko.zst
-rw-r--r-- 1 root root  8582 Apr 19 14:46 dell_rbu.ko.zst
-rw-r--r-- 1 root root 14726 Apr 19 14:46 dell-smbios.ko.zst
-rw-r--r-- 1 root root  4433 Apr 19 14:46 dell-smo8800.ko.zst
-rw-r--r-- 1 root root  3790 Apr 19 14:46 dell-wmi-aio.ko.zst
-rw-r--r-- 1 root root  9166 Apr 19 14:46 dell-wmi-ddv.ko.zst
-rw-r--r-- 1 root root  4492 Apr 19 14:46 dell-wmi-descriptor.ko.zst
-rw-r--r-- 1 root root 11306 Apr 19 14:46 dell-wmi.ko.zst
-rw-r--r-- 1 root root  3108 Apr 19 14:46 dell-wmi-led.ko.zst
drwxr-xr-x 2 root root  4096 May 25 10:15 dell-wmi-sysman
/lib/modules/6.8.0-31-generic/kernel/drivers/platform/x86/dell/dell-wmi-sysman:
-rw-r--r-- 1 root root 18307 Apr 19 14:46 dell-wmi-sysman.ko.zst

Removing acpi_backlight=vendor from /dev/default/grub makes no difference. However, adding acpi_backlight=video gives me the appropriate files in /sys/class/backlight, and I now get a brightness slider... although changing the slider doesn't actually change the brightness.

[Update] Submitted bug report https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2067498