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I've done the primary searching of similar questions, Found some, tried the solutions and they didn't work for me. So asking again.

All the keys of my Fn combination works except the brightness ones. And there are no options to control brightness in Monitor or Power Management settings.

Update: If I boot without those two acpi_osi and acpi_brightness parameters, and run acpi_listen on terminal and press those brightness keys, I get these outputs -

video/brightnessdown BRTDN 00000087 00000000
video/brightnessup BRTUP 00000086 00000000

if I run acpi_listen after booting with those two acpi parameters, acpi_listen gives me nothing when I press those keys.

Update: There is no backlight folder under /sys/devices/virtual/.

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