nVidia

https://wiki.gentoo.org/wiki/NVidia/nvidia-drivers
When using the nVidia drivers, it is advisable to disable the nouveau driver in the kernel configuration, as explained in the above article.
However, if desired, the nouveau driver could still be compiled as a module, and blacklist the kernel module to make sure it's not loaded at the same time as the nvidia driver.

https://wiki.archlinux.org/index.php/NVIDIA/Tips_and_tricks

Troubleshooting

Nouveau drivers

https://nouveau.freedesktop.org/wiki/Bugs/
https://nouveau.freedesktop.org/wiki/TroubleShooting/
Help with the development of nouveau:
https://nouveau.freedesktop.org/wiki/HardwareDonations/

nVidia drivers

Reporting bugs to nvidia, see:
https://devtalk.nvidia.com/default/topic/522835/linux/if-you-have-a-prob...
https://devtalk.nvidia.com/default/topic/790452/reporting-graphics-drive...

Xorg

See Xorg.0.log.

(EE) Unknown chipset: NVnnn
http://linux.overshoot.tv/wiki/ee_unknown_chipset_nvnnn