Comment 13 for bug 773710

On a Lenovo 420s, running Natty (2.6.38-8-generic (64bit), xorg-server 2:1.10.1-1ubuntu1.1, NVIDIA dlloader X Driver 270.41.06 Mon Apr 18 14:55:25 PDT 2011), I can switch to a console (Ctrl-Shift-F1) and change the brightness there.

Using Fn+Home/End (brightness up/down) changes the brightness indicator in the top, right corner when running X11. The values of /sys/devices/virtual/backlight/acpi_video0/{brightness,actual_brightness} changes as one would expect. However, the builtin LCD remains at a fixed brightness level when in X.

I noticed these error messages in /var/log/Xorg.0.log which might or might not be related:

[ 5.073] (WW) Open ACPI failed (/var/run/acpid.socket) (No such file or directory)

[ 6.423] (II) NVIDIA(0): ACPI: failed to connect to the ACPI event daemon; the daemon
[ 6.423] (II) NVIDIA(0): may not be running or the "AcpidSocketPath" X
[ 6.423] (II) NVIDIA(0): configuration option may not be set correctly. When the
[ 6.423] (II) NVIDIA(0): ACPI event daemon is available, the NVIDIA X driver will
[ 6.423] (II) NVIDIA(0): try to use it to receive ACPI event notifications. For
[ 6.423] (II) NVIDIA(0): details, please see the "ConnectToAcpid" and
[ 6.423] (II) NVIDIA(0): "AcpidSocketPath" X configuration options in Appendix B: X
[ 6.423] (II) NVIDIA(0): Config Options in the README.

However, the acpid.socket file exists and is readable

# file /var/run/acpid.socket
/var/run/acpid.socket: socket

# ls -la /var/run/acpid.socket
srw-rw-rw- 1 root root 0 2011-06-24 23:15 /var/run/acpid.socket