I hadn’t seen it suggested yet, so for anyone using KMS/nvidia-drm or otherwise loading the nvidia kernel module early on who can’t get the xorg.conf setting to fix DDC on newer cards, I’ve had to add the following to modprobe.conf or modprobe.d/whatever.conf instead:
options nvidia NVreg_RegistryDwords=RMUseSwI2c=0x01;RMI2cSpeed=100
It took me a while to realise the workaround wasn’t even being applied, for a while I assumed it just didn’t fix things on my system. To confirm that the settings are working:
$ grep RegistryDwords /proc/driver/nvidia/params
RegistryDwords: "RMUseSwI2c=0x01;RMI2cSpeed=100"
This makes DDC over HDMI work with my Pascal card and fixes the “invalid EDID” errors at least, and also doesn’t require starting X to use it, should help with wayland, etc. The second issue reported in this thread still persists however: monitors connected over DisplayPort do not show up nor respond on I2C at all.