Recently I got "my" Turris Omnia and it didn't take long to replace the original firmware with Debian.

If you want to reproduce, here is what you have to do:

Open the case of the Turris Omnia, connect the hacker pack (or an RS232-to-TTL adapter) to access the U-Boot prompt (see Turris Omnia: How to use the "Hacker pack"). Then download the installer and device tree:

# cd /srv/tftp
# wget https://d-i.debian.org/daily-images/armhf/daily/netboot/vmlinuz
# wget https://d-i.debian.org/daily-images/armhf/daily/netboot/initrd.gz
# wget https://www.kleine-koenig.org/tmp/armada-385-turris-omnia.dtb

(The latter is not included yet in Debian, but I'm working on that.)

and after connecting the Turris Omnia's WAN to a dhcp managed network start it in U-Boot:

dhcp
setenv serverip 192.168.1.17
tftpboot 0x01000000 vmlinuz
tftpboot 0x02000000 armada-385-turris-omnia.dtb
tftpboot 0x03000000 initrd.gz
bootz 0x01000000 0x03000000:$filesize 0x02000000

With 192.168.1.17 being the IPv4 of the machine you have the tftp server running.

I suggest to use btrfs as rootfs because that works well with U-Boot. Before finishing the installation put the dtb in the rootfs as /boot/dtb.

To then boot into Debian do in U-Boot:

setenv mmcboot=btrload mmc 0 0x01000000 boot/vmlinuz\; btrload mmc 0 0x02000000 boot/dtb\; btrload mmc 0 0x03000000 boot/initrd.img\; bootz 0x01000000 0x03000000:$filesize 0x02000000
setenv bootargs console=ttyS0,115200 rootfstype=btrfs rootdelay=2 root=/dev/mmcblk0p1 rootflags=commit=5 rw
saveenv
boot

Known issues:

• rtc doesn't work (workaround: mw 0xf10184a0 0xfd4d4cfa in U-Boot)
• SFP and switch don't work, MAC addresses are random
• wifi fails to probe

If you have problems, don't hesitate to contact me.

Also check the Debian Wiki for further details.