I found a similar Fedora bug report, it is a kernel issue, and there is a workaround. See: https://bugzilla.redhat.com/show_bug.cgi?id=1594488
Need to add 'amdgpu.dc=0' to the kernel command line in grub.cfg: GRUB_CMDLINE_LINUX="amdgpu.dc=0" then run grub-update reboot after booting to a version 5 kernel, video on both monitors works, and a check of the proc cmdline shows: $ cat /proc/cmdline BOOT_IMAGE=/boot/vmlinuz-5.4.0-40-generic root=UUID=95b8de7f-9de3-4cea-b947-e135a738948c ro amdgpu.dc=0 ** Bug watch added: Red Hat Bugzilla #1594488 https://bugzilla.redhat.com/show_bug.cgi?id=1594488 ** Also affects: linux (Fedora) via https://bugzilla.redhat.com/show_bug.cgi?id=1594488 Importance: Unknown Status: Unknown -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1882442 Title: Upgrade from 18.04 to 20.04 deactivates internal monitor in display manager To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1882442/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
