Created attachment 131421 [details]
dsdt.dls

Hello.

On the asus ux51vz laptop screen backlight is not working. It is a win8 laptop and has only one single nvidia gpu.

By default in /sys/class/backlight i have acpi_video0.
Pressing the hotkeys actually result in a change in acpi_video0/actual_brightness.
Also writing manually to acpi_video0/brightness results in a change in acpi_video0/actual_brightness.
Apparently everything works except that the actual brightness of the screen doesnt change.
Looking at the the _BCM method in the DSDT i saw that it calls STBR which does nothing it think. So the ACPI is broken in my case.

Method (_BCM, 1, NotSerialized)  // _BCM: Brightness Control Method
{
    DIAG (0x1A)
    If (LGreaterEqual (MSOS (), OSW8))
    {
         Store (Arg0, LBTN)
    }
    Else
    {
        Store (GCBL (Arg0), Local0)
        Subtract (0x0A, Local0, LBTN)
    }

    ^^^^LPCB.EC0.STBR ()
    DIAG (0x1B)
}

Method (STBR, 0, Serialized)
{
    Return (One)
}

I have tried lots of combinations of acpi_osi and acpi_os parameters, but essentially nothing changes.


acpi_backlight=legacy does not work.
acpi_backlight=vendor results in asus-nb-wmi and vn_backlight in /sys/class/backlight.
Both the GNOME slider and the hotkeys do not work. I think that GNOME prioritises the platform type over the war and so should use the asus-nb-wmi one. I think also that the asus-wmi module somehow changes the keybinding for the hotkeys in order to use WMI, I actually do not know how these things work.
asus-nb-wmi/actual_brightness stays always at 100 regardless of what i do. Writing to asus-nb-wmi/brightness does not change the brightness.
Again looking at the DSDT in the WMNB section explayns perfectly the situation and shows also WMI is broken here.

Writing manually to vn_backlight/brightness works. The problem is that asus-nb-wmi is somehow in the way.
Booting with acpi_backlight=vendor and blacklisting the asus-wmi and asus-nb-wmi results in working brightness, also hotkeys work, even if sometimes they react a little bit slowly.

I also tested video.use_native_backlight=1 but acpi_video0 still got registred, maybe becouse of the backlight_device_registered(BACKLIGHT_RAW) in this check:

if (acpi_osi_is_win8() && use_native_backlight && backlight_device_registered(BACKLIGHT_RAW))

If i understand correctly, since the only raw backlight device going to be registed is nv_backlight, and since nouveau requires the video module and therefore gets loaded afterwards, this condition will never become true.

I tried removing the backlight_device_registered(BACKLIGHT_RAW) and than video.use_native_backlight=1 do what it is supposed to do.

The problem remains in the way asus-wmi and nouveau check if they should register their own control devices. They use acpi_video_backlight_support() I think which is unaffected by the use_native_backlight parameter.

Just for testing I tried removing the check from the nouveau driver and forcing it to register the nv_backlight.
This way, booting with use_native_backlight=1 actually gives a system with working backlight, altough this solution is far from beeing optimal and also far from beeing out of the box.


I hope I could give you at least some useful information to resolve this issue. I don't mind providing you more detaied information if you need.

Thank you in advance.