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 so
I tried to use Ubuntu Drivers to switch my GPU driver from NVIDIA to
nouveau driver. But nothing happens and the ports are still missing.
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https://bugs.launchpad.net/bugs/1875892
Title:
Public bug reported:
My laptop (Lenovo ThinkPad P51) has 2 display interfaces. One mini-dp
and one HDMI. But none of them is shown in xrandr, and my external
display connected through mini-dp is not detected also.
When I used LiveCD, the external monitor works normally. The above
situation happen