Ubuntu seems unable to control the backlight brightness on my Dell Precision 7550. I'm using Ubuntu 20.04, and selecting nvidia with prime-select. The keyboard brightness keys will make a brightness indicator appear, but it doesn't affect brightness. I tried sudo find /sys -type f -name brightness and I get:
/sys/devices/platform/i8042/serio0/input/input3/input3::capslock/brightness
/sys/devices/platform/i8042/serio0/input/input3/input3::scrolllock/brightness
/sys/devices/platform/dell-laptop/leds/dell::kbd_backlight/brightness
/sys/devices/platform/dell-laptop/leds/platform::micmute/brightness
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/nvidia_0/brightness
/sys/devices/pci0000:00/0000:00:14.3/leds/phy0-led/brightness
Most of them look like led's; only the nvidia_0 or kbd_backlight lines look like they might refer to the screen backlight (though I suspect kbd means 'keyboard'). I tried echoing values to either of these, and while the values update, they have no effect on the backlight brightness.
I also tried changing the acpi_backlight value in /etc/default/grub to either 'vendor' or 'none' (or simply omitting it; the above values are for 'none'), and while this changes the names of some things in /sys/devices, it doesn't result in any files that let me control the screen backlight. I've also tried using --brightness in xrandr, but it only adjusts screen contrast rather than backlight brightness. Are there any other things I can try to access brightness settings? Is there some missing Dell driver that will let me control the backlight?
