Works with fully free software, However you need to upgrade its internal firmware once before use. Firmware update from Avid here, also mirrored on ZamAudio here. The firmware update can only be applied from a Mac computer. However, once firmware update is applied, it works with any Linux kernel >= 4.0 For more information on this soundcard with GNU/Linux, see here. To inspect your current firmware version:

sudo lsusb -vv -d 0dba:1000 |grep bcdDevice |awk '{print $2}'

Anything less than 0.22 needs an update to 0.22