### Additional diagnostic results — Dell Precision M3800 I performed further tests to distinguish hardware, desktop environment and kernel driver issues.
**System information** - Laptop: Dell Precision M3800 - BIOS version: A14 (dated 2020-07-02) - Current Ubuntu kernel: `7.0.0-38-generic` (`7.0.0-38.38`) - The issue has also been reproduced on Debian 13 (kernel 6.12), Ubuntu 26.10 (kernel 7.3.0-9) and Fedora 44. **Observed behaviour** The keyboard backlight has three levels: off, dim and bright. The `Fn`+`F10` hardware shortcut cycles through these states correctly. The following sysfs interface is present: `/sys/class/leds/dell::kbd_backlight/brightness` Its `max_brightness` value is `2`. Reading `brightness` correctly reflects changes made using `Fn`+`F10`. However, software control is only partially functional: 1. With the backlight at level `2`, writing `1` leaves the reported brightness at `2` and does not change the physical backlight. 2. Writing `2` also has no effect. 3. Writing `0` successfully turns the backlight off, and the reported brightness becomes `0`. 4. Writing `2` after this does not turn the backlight on; the reported brightness remains `0`. The writes return successfully at the sysfs level, but the requested non-zero brightness is not applied. No relevant Dell keyboard-backlight errors were found in the kernel journal during these tests. **Driver investigation** The relevant driver is `drivers/platform/x86/dell/dell-laptop.c`. The LED callback `kbd_led_level_set()` obtains the current state, calls `kbd_set_level()`, and then attempts to apply the new state through `kbd_set_state_safe()`. The latter sends a Dell firmware request using `dell_send_request()`. The function `kbd_set_level()` also contains an early return when the current level already equals the requested level, before its mode- restoration logic. This is a potential area to investigate. Could a kernel maintainer advise how to trace the firmware request and returned state on this model, or whether the Dell Precision M3800 requires a driver quirk or a different command sequence to restore a non-zero keyboard-backlight level? The hardware shortcut works, and the problem has been reproduced across multiple Linux distributions and kernel versions. -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2150737 Title: M3800 keyboard backlight problem ubuntu 26.04 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2150737/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
