2.6.38-rc2: acpi backlight control missing

[Thomas Meyer]

with 2.6.38-rc2 the acpi backlight is gone:

$ ll /sys/devices/virtual/backlight 
ls: cannot access /sys/devices/virtual/backlight: No such file or directory

$ ll /sys/class/backlight/
total 0

with 2.6.37:
$ ll /sys/devices/virtual/backlight
insgesamt 0
drwxr-xr-x. 3 root root 0 22. Jan 2011  acpi_video0

$ ll /sys/class/backlight/
insgesamt 0
lrwxrwxrwx. 1 root root 0 22. Jan 2011  acpi_video0
-> ../../devices/virtual/backlight/acpi_video0

dmesg:

$ grep -i video dmesg-3*.txt
dmesg-37.txt:[    0.216370] pci 0000:00:02.0: Boot video device
dmesg-37.txt:[    1.102662] input: Video Bus as /devices/LNXSYSTM:00/device:00/PNP0A08:00/LNXVIDEO:00/input/input4
dmesg-37.txt:[    1.102670] ACPI: Video Device [OVGA] (multi-head: yes  rom: no  post: no)
dmesg-37.txt:[   18.843946] Linux video capture interface: v2.00
dmesg-37.txt:[   19.189825] uvcvideo: Found UVC 1.00 device WebCam (064e:a102)
dmesg-37.txt:[   19.207062] usbcore: registered new interface driver uvcvideo
dmesg-37.txt:[   19.207065] USB Video Class driver (v1.0.0)
dmesg-38.txt:[    0.215527] pci 0000:00:02.0: Boot video device
dmesg-38.txt:[   19.466882] Linux video capture interface: v2.00
dmesg-38.txt:[   19.503083] uvcvideo: Found UVC 1.00 device WebCam (064e:a102)
dmesg-38.txt:[   19.520292] usbcore: registered new interface driver uvcvideo
dmesg-38.txt:[   19.520295] USB Video Class driver (v1.0.0)

$ grep ACPI_VIDEO /boot/config-2.6.3[78]*
/boot/config-2.6.37:CONFIG_ACPI_VIDEO=y
/boot/config-2.6.38-rc2:CONFIG_ACPI_VIDEO=y

any ideas?

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