OEM BIOS engineer has given the following feedback on Mar. 18:
"
//
It should not be removed, since it is the key process to convert OS brightness 
percentage (0-100) to the actual LCD PWM brightness duty value (0-255) along 
with the brightness curves which are defined for each LCD panels. If you remove 
it, the brightness function will break on Windows 7.
The available brightness percentage values are defined in ACPI _BCL. Linux 
driver should only use the values defined in _BCL. Please see "Appendix: Video 
Extensions" chapter in ACPI spec.
 
B.5.3 _BCM (Set the Brightness Level)
This method allows OSPM to set the brightness level of a built-in display 
output device.
The OS will only set levels that were reported via the _BCL method. This method 
is required if _BCL is implemented.
 
Best Regards,
 
Norihito Ishida
//
"

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https://bugs.launchpad.net/bugs/1270079

Title:
  [Lenovo T540p, W540] Backlight interfaces not fully working

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