ADB constantly disconnects, shows device offline
https://www.kernel.org/doc/Documentation/usb/power-management.txt
$ cat /sys/module/usbcore/parameters/autosuspend 2
Edit the /etc/default/grub file and change the GRUB_CMDLINE_LINUX_DEFAULT line to add the usbcore.autosuspend=-1 option:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash usbcore.autosuspend=-1"
Note that quit splash were already present options. So keep other options you have too.
After save the file, update grub:
sudo update-grub
And reboot.
Now check autosuspend value:
cat /sys/module/usbcore/parameters/autosuspend
And it should display -1.