LCD brightness not reflected at brightness file

[J. Bakshi]

Hello list,

I have compiled linux kernel 3.0.1. The LCD current brightness
is show by /sys/class/backlight/acpi_video0/actual_brightness

I can also control brightness by <Fn> key and correctly read by
/sys/class/backlight/acpi_video0/actual_brightness

BUT 

echo 0 > /sys/class/backlight/acpi_video0/brightness 

echo 8 > /sys/class/backlight/acpi_video0/brightness

etc.... has no effect !!!

Could anyone give me any clue please ?

TIA
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