I (maybe) fixed this non-working trackpad issue after running into it in Dec. 2020 after dual-booting a Lenovo Laptop IdeaPad 5 14IIL05 81YH000NUS with Windows 10 and Ubuntu 20.04 after installing updates both on Windows 10 and Ubuntu.
The kernel (found using uname -sr) is Linux 5.4.0-58-generic which was installed by default when installing 20.04.
The fix that worked for me was:
sudo nano /etc/default/grub
Add pci=nocrs and initcall_blacklist=elants_i2c_driver_init as settings:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash pci=nocrs"
GRUB_CMDLINE_LINUX="initcall_blacklist=elants_i2c_driver_init"
Save the file and run:
sudo update-grub
and restart.
Note: I've also restarted in both Windows and Ubuntu a few times to make sure it worked, since this initially seemed like an intermittent issues.
Update: after another few tests/restarts this didn't seem to work until booting back up in windows, then restarting again and booting to ubuntu. If anyone figures out what the root causes of this are, it would be much appreciated.
New Update: I ended up restoring the laptop to factory settings, re-installing Ubuntu 20.04 and then upgrading the kernel to 5.10.2 which seemed to fix this issue.