I recently had some overscan issues via HDMI from the Pine64 to my Dell U3415W

This is how I fixed the overscan and subsequently the resolution from 720p60Hz (720p is what the Pine64/debian base was telling my monitor the video signal should be):

$ cd /boot
$ sudo nano uEnv.txt

You’ll see this:

console=tty0 console=ttyS0,115200n8 no_console_suspend
kernel_filename=pine64/Image
initrd_filename=initrd.img
optargs=disp.screen0_output_mode=720p60
ethaddr=aa:bb:cc:dd:e1:01

Change it to:

console=tty0 console=ttyS0,115200n8 no_console_suspend
kernel_filename=pine64/Image
initrd_filename=initrd.img
optargs=disp.screen0_output_mode=1080p60
ethaddr=aa:bb:cc:dd:e1:01

Pro tip: Change the MAC address while you are in there if you want your network/putty to work correctly if you have more than one pine64 with longsleep’s debian base image burned to it.