Lenovo BIOS engineer commented on Mar. 19 as:
"
//
I understand the situation. Since ThinkPad BIOS assumes that Windows 8
or later OS always uses the OS-driver initiated brightness control
method, so the brightness logic won't work if an OS declares it is Win8
but it uses OS-BIOS initiated brightness control method.
To workaround this, it would be better to change ASL code as below,
because BRNS() is only designed for 16-step brightness.
Please note that it is just a quick W/A.
If Linux Intel IGD driver has an option to change its behavior to OS-
driver initiated brightness control method as well as real Win8, it
would be easier.
Windows Intel IGD driver can change it to OS-driver mode with Bit8 of
FeatureTestControl = 0 in INF file.
----
External( \_SB.PCI0.LPC.EC.BRCD, MethodObj )
Method ( _BCM, 1 ) {
If ( \WIN8 ) {
\_SB.PCI0.VID.AINT( 1, \_SB.PCI0.LPC.EC.BRCD( Arg0 ) )
} Else {
Store( Match( \_SB.PCI0.LPC.EC.BRTW, MEQ, Arg0, MTR, 0, 2 ),
Local0 )
If( LNotEqual( Local0, Ones ) ){ // OS
requested value is found and we got index
Subtract (Local0, 2, Local1)
Store( Local1 , \BRLV )
\_SB.PCI0.LPC.EC.BRNS( )
\UCMS( TOGGLE_BIT_AND_UPDATE_CMOS )
}
}
"
** Attachment added: "code-changes.PNG"
https://bugs.launchpad.net/ubuntu/+source/linux-lts-raring/+bug/1270079/+attachment/4033205/+files/code-changes.PNG
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https://bugs.launchpad.net/bugs/1270079
Title:
[Lenovo T540p, W540] Backlight interfaces not fully working
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