The backlight brightness controls on this laptop with NVS 3100m work while running Nouveau or when using a text mode VT, but not while running X and using proprietary NVIDIA drivers. It is possible to force the nvidia driver to control the backlight, but this causes the LED backlight to buzz. This occurs constantly whenever the backlight is set to less than 100%. The noise changes in tone/pitch as the backlight levels are altered, and the noise continues until the backlight is set to maximum intensity. This noise does not occur when using the open source nouveau driver, which has its own method of controlling the backlight, and it does not occur when the brightness is changed in a text mode VT while running the nvidia driver.
The behaviour as been confirmed via thinkwiki.org
. Because this is an LED backlight which does not have an inverter, it is unexpected that this noise should occur. This leads me to believe that the nvidia driver is sending the wrong level of power (voltage?) to the LED array, and is possibly stressing the physical components.
Is there any information I could provide that would help a developer solve this problem?
The hardware brightness buttons on this laptop always affect /sys/devices/virtual/backlight/acpi_video0/brightness, which accepts values up to 15. If the brightness value is raised while running X, /sys/devices/virtual/backlight/acpi_video0/brightness is changed but the actual backlight is not affected. If I then switch to a text mode VT and press the backlight up button, /sys/devices/virtual/backlight/acpi_video0/brightness is increased again and the backlight is set to the appropriate level. This level will persist when I switch back to X.
I have written a bug report for Ubuntu Lucid Lynx at Launchpad