>From BIOS AML, the brightness function (ex. _BCM) checks a table before
making actual brightness adjustment. The table is located in DSDT and
attached below:
Name (BRTW, Package (0x12)
{
0x64,
0x64,
0x01,
0x03,
0x05,
0x08,
0x0C,
0x11,
0x16,
0x21,
0x28,
0x2E,
0x36,
0x3E,
0x47,
0x50,
0x5A,
0x64
})
This looks like a BIOS design. We need a BIOS update to remove the
comparison to the table. Kernel cannot work around this bug at all
except removing the uses of ACPI backlight control.
There is no harm to remove the comparison for Windows 8 as it does not
calls _BCM for brightness.
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https://bugs.launchpad.net/bugs/1270079
Title:
[Lenovo T540p, W540] Backlight interfaces not fully working
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