Hi Joey , 

Per my understood, EC firmware should change brightness but didn't do
that, another
way is touch i915 register in _BCM.

  how do we do this ? you mean change the _BCM implementation ? 

Acer machine provide a broken _BCM implementation and they didn't test
it.

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By ' ACPI interface' , I mean 'acpi_video0' inside the
/sys/class/backlight. I havn't tried the /sys/class/backlight interface
directly . I will try that also.

So writing values into /sys/class/backlight/acpi_video0/brightness does 
nothing?

No change in value when writing
to /sys/class/backlight/acpi_video0/brightness.

Another thing is that when i did boot with acpi_backlight = 'acer_wmi' ,
in new kernel (3.3.0-rc7) , it shows following messages , 

[    8.350825] wmi: Mapper loaded
[   10.363975] acer_wmi: Acer Laptop ACPI-WMI Extras
[   10.396186] acer_wmi: Function bitmap for Communication Device: 0x91
[   10.396385] acer_wmi: Brightness must be controlled by generic video
driver

Also there was no interface inside /sys/class/backlight for acer_wmi. 

Yes, acer_wmi support backlight control with AMW0 interface, your
machine didn't have AMW0 interface.

Normally, backlight should control by standard acpi interface.

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I also tried writing directly to Embedded controller register .But no
change.

The machine has broken _BCM method, because EC should do something after
_BCM changed EC register.

Thanks , 

Pradeep Subrahmanion