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I have a Thinkpad X220 and am running Debian 9 stable with kernel

Linux version 4.9.0-8-amd64 (debian-kernel@lists.debian.org) (gcc version 6.3.0 20170516 (Debian 6.3.0-18+deb9u1) ) #1 SMP Debian 4.9.135-1 (2018-11-11)

using XFCE 4.12, and have encountered a rather strange bug.

Basically, if I initially start from the max brightness setting, and want to reduce it to near 0 (5 seems to be the dimmest setting using the power manager scrollbar), if I use the Fn key, it is unable to go below the value 383 (as reported by cat /sys/class/backlight/intel_backlight/brightness) unless I manually use the scrollbar in the power manager. The actual value (383) does seem to actually depend on which brightness value I start with, and I can always reach the max level using the other Fn key, but the minimum level just gets stuck on a very high value, making it annoying to adjust in low-light settings. Is there a way to fix this behaviour so I can properly reduce the brightness below this (seemingly arbitrary) threshold using the Fn key?

EDIT: unchecking "Handle display brightness keys" from the xfce4-power-manager makes the lowest brightness go down to a hard bound of 73 (as reported by cat /sys/class/backlight/intel_backlight/brightness), which seems to correspond to 0 (as reported by cat /sys/class/backlight/acpi_video0/brightness), so I'm thinking this has to do with the ACPI. I tried passing acpi_backlight=vendor thinkpad-acpi.brightness_enable=1 as a kernel boot parameter, but this seems to disable the brightness hotkeys altogether, while passing acpi.brightness_switch_enabled=0 doesn't fix the issue.

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