Comment 4 for bug 422220

I was able to confirm this bug. smardimmer -i didn't work for me either.

Hardware: Sony Vaio VGN-FE21H with its Nvidia GeForce Go 7400

Kubuntu: 9.10
NVIDIA driver version: 173.14.20

$ uname -a
Linux laptop 2.6.31-19-generic #56-Ubuntu SMP Thu Jan 28 01:26:53 UTC 2010 i686 GNU/Linux

nvclock: nvclock0.8b4-1ubuntu2

This bug is caused by a simple integer arithmetic issues in nv40.c (and nv50.c).
Changing the step width for the increase and decrease (for completeness ) of the background brightness from 1 to 5 fixed this problem.
The below patch works fine for me. I can't test it for any side-effects with other cards.