With kernel 3.11.0-17-generic, _BCL returns as below:
[ 4792.647432] Brightness[0] = 100
[ 4792.647439] Brightness[1] = 100
[ 4792.647442] Brightness[2] = 0
[ 4792.647445] Brightness[3] = 1
[ 4792.647447] Brightness[4] = 2
[ 4792.647450] Brightness[5] = 3
[ 4792.647452] Brightness[6] = 4
[ 4792.647454] Brightness[7] = 5
[ 4792.647456] Brightness[8] = 6
[ 4792.647459] Brightness[9] = 7
[ 4792.647461] Brightness[10] = 8
[ 4792.647464] Brightness[11] = 9
[ 4792.647466] Brightness[12] = 10
[ 4792.647469] Brightness[13] = 11
[ 4792.647471] Brightness[14] = 12
[ 4792.647474] Brightness[15] = 13
[ 4792.647476] Brightness[16] = 14
[ 4792.647478] Brightness[17] = 15
[ 4792.647481] Brightness[18] = 16
// .... skip the middle
[ 4792.647653] Brightness[92] = 90
[ 4792.647655] Brightness[93] = 91
[ 4792.647657] Brightness[94] = 92
[ 4792.647660] Brightness[95] = 93
[ 4792.647662] Brightness[96] = 94
[ 4792.647664] Brightness[97] = 95
[ 4792.647667] Brightness[98] = 96
[ 4792.647669] Brightness[99] = 97
[ 4792.647671] Brightness[100] = 98
[ 4792.647674] Brightness[101] = 99
[ 4792.647676] Brightness[102] = 100
This is the result of _OSI("Windows 2012") = true and therefore "WIN8" =
1 in BIOS's ASL code. This also means that Linux kernel does not use
_BCL incorrectly (just to clarify #18).
In order to not to break original function in Win7, BIOS can be modified
as below
Method (_BCM, 1, NotSerialized)
{
If (\WIN8) // does not check BRTW for WIN8 or later.
{
Subtract (Local0, 0x02, Local1)
Store (Local1, \BRLV)
\_SB.PCI0.LPC.EC.BRNS (\UCMS (0x16))
}
else
{
Store (Match (\_SB.PCI0.LPC.EC.BRTW, MEQ, Arg0, MTR, 0x00,
0x02), Local0)
If (LNotEqual (Local0, Ones))
{
Subtract (Local0, 0x02, Local1)
Store (Local1, \BRLV)
\_SB.PCI0.LPC.EC.BRNS (\UCMS (0x16))
}
}
}
Alternatively, BIOS can also have WIN8 detection for BRTW, ex. returns
0~100 if "WIN8" = 1 annd _BCM can stay unchanged.
** Changed in: linux-lts-raring (Ubuntu)
Status: Confirmed => Triaged
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https://bugs.launchpad.net/bugs/1270079
Title:
[Lenovo T540p, W540] Backlight interfaces not fully working
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