{SOLVED} Lost LCD Backlight Control

[CentaurHauler]

Mint5 KDE running on an old Dell C640 laptop with a Radeon 7500 graphic card. This thing used to work great and I was able to control the backlight intensity using the keyboard controls, and it would automatically dim to the setting in Power Manager when on batteries. Naturally I decided to mess with the video driver, and now I see that it only controls the brightness to the BIOS setting depending on whether it is plugged in at boot. I'm not really sure when along the way this happened as I had not unplugged it for several days.

I've returned all the drivers to the previous versions - removed any special repositories on Adept, uninstalled and reinstalled the XORG ATI driver, and restored a saved xorg.conf file. But the brightness control is still missing. I'd really like to get that working again!

[CentaurHauler]

When I do a cat /proc/acpi/video/VID/LCD/brightness the response is <not supported>, and the file size is 0. Is there supposed to be something in there?

[CentaurHauler]

I almost wonder if it is related to this bug: https://bugs.launchpad.net/ubuntu/+sour ... bug/406515 but I am not running Karmic. Perhaps some common update to something else broke it.

[CentaurHauler]

I did a clean install of Mint5 on a spare drive, then I did all the Mint updates. Backlight control still worked. Then I did the update to the xserver-xorg-video-ati (1:6.8.0-1ubuntu1) update from the ubuntu repository. That broke it.

So I did another clean install without the bad update and copied all of the files back to my original disk (using a USB adapter). Now it is working fine again.

I suspect this bug exists in all the newer distributions as well, as none of them can run the backlight in this laptop either.