Special Nvidia Driver Settings
When using Nvidia's proprietary driver, I2C communication fails on some cards. This has been the topic of some discussion on Nvidia's web site. See, for example.
While not a universal solution, setting undocumented Nvidia options sometimes enables I2C communication to work. There are several ways to set these options. The simplest, outlined in a post by user arcnmx, puts the settings in /etc/modprobe.d. Add a file whose name ends in ".conf" to that directory, e.g. /etc/modprobe.d/nvidia_i2c.conf. The file should contain the following:
options nvidia NVreg_RegistryDwords=RMUseSwI2c=0x01;RMI2cSpeed=100
After rebooting, confirm that the settings have been applied:
$ grep RegistryDwords /proc/driver/nvidia/params
An earlier version of this page discussed how to set the options using a file in /etc/X11/xorg.conf.d. It has been retained for reference as it may be useful if the /etc/modprobe.d method is not suitable for your setup.