Without adding "acpi_backlight=vendor" in the boot parameters:
ls /sys/class/backlight
acpi_video0 intel_backlight
Adding "acpi_backlight=vendor" in the boot parameters:
ls /sys/class/backlight/
intel_backlight
So Fn keys start working with "acpi_backlight=vendor" boot parameter because the keys stop changing /sys/class/backlight/acpi_video0/brightness instead of /sys/class/backlight/intel_backlight/brightness.
Without adding "acpi_backlight =vendor" in the boot parameters:
ls /sys/class/ backlight
acpi_video0 intel_backlight
Adding "acpi_backlight =vendor" in the boot parameters:
ls /sys/class/ backlight/
intel_backlight
So Fn keys start working with "acpi_backlight =vendor" boot parameter because the keys stop changing /sys/class/ backlight/ acpi_video0/ brightness instead of /sys/class/ backlight/ intel_backlight /brightness.