Using RaspberyPi as Time Machine for Mac

I am trying to use my RPi as a Time Machine for my mac. I have RPi running with attached pidrive (formatted to EXT4). I want to use Netatalk that imitates Time Machine but some guides say it will work with EXT4 partition (just allocate a folder for mac's backup) and others say the drive / partition should be formatted to HFS+. I don't want to format my drive to HFS+ just for that and I am wondering if someone could shed some light on it please :)