Sorry for the long post... This is an annoying problem I've been researching this for three days and trying different things with no success.

I've been running 8.04 on a Dell C640 for about 3 months. Throughout this period, I would have occasional issues with the LCD backlight not working during bootup. In most cases, if I unplugged the power for a second, the display would be fine.

About 7-10 days ago, when I bootup with the power plugged in, the backlight is off unless I do the quick unplug/plugin trick. During the bootup process, I can see everything, and my keyboard brightness keys work fine - I can dim and brighten. After the Ubuntu load status bar completes and disappears, but before the login screen appears, the backlight goes off. Then the login screen comes up but I can barely see it, but it's useless other than to shutdown via Alt-T > D > Alt-D.

Now, if I unplug the laptop during the bootup before the Ubuntu load status bar completes, the backlight will turn on fine, but it is dimmed per the BIOS settings. I can login and work, but the brightness controls are non-functional. The applet moves, but nothing happens, and Fn DnArrow/UpArrow do nothing. Even if I reconnect the power, the display stays in the dimmed mode. If I go into the BIOS and set the "battery" display brightness higher than halfway, the display may not turn on at all during bootup. I've had to reflash the BIOS from a CD to reset the defaults and allow me to boot up again with a working display.

If I connect an external monitor before booting up, all is fine whether on AC power or battery.

I don't know if this is ACPI related, if a recent update did something, or if it could be hardware (LCD inverter). I've tried adding a line /etc/init.d/gdm to set brightness to 100% at the login screen but it had no effect. I've tried disabling power management in the BIOS, and setting acpi=off during bootup.

I have a Dell C640, running Hardy 8.04, 2.6.24-23-generic, GNOME 2.22.3. Video driver is Radeon Mobility M7 LW (ATI Radeon 7500). My BIOS info is
841-BIOS Information
858: Vendor: Dell Computer Corporation
893- Version: A10
1782- Manufacturer: Dell Computer Corporation
1823: Product Name: Latitude C640
1871- Version: Not Specified
2056- Manufacturer: Dell Computer Corporation
2097: Product Name:
2119- Version: