I solved this problem by re-install the OS. When I install the OS, I did not check the checkboxes to install updates and third-party software (I checked to install third-party software last time). Therefore the nouveau driver instead of the NVIDIA one will be installed. Then I changed the apt source to a local software mirror, and use Ubuntu Drivers to install the NVIDIA driver. And everything now works fine. I think the problem may caused by incomplete install on the NVIDIA driver. Since I am in mainland China, the network may be the primary reason for this incomplete install.
During the above procedure, I found that the OS will become extremely slow in the following situation. - Using nouveau driver. - The laptop internal display is disabled (lip off or disabled in Display Setting). - The external monitor is the only display. I did not investigate deeper and I quickly installed the NVIDIA driver. Thanks for the help! -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1875892 Title: [nvidia][Lenovo P51] Mini-DP and HDMI ports are missing To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers-440/+bug/1875892/+subscriptions -- ubuntu-bugs mailing list firstname.lastname@example.org https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs