btrfs with multi devices fails automount in fstab

I'm trying to automount btrfs on 12.04-desktop-amd64.
During bootsplash, fstab failed to mount btrfs and asked skip or manually prepare.

On the rootshell, btrfs device scan and mount -a could automount properly.
It seems "btrfs device scan" was missing from bootstrap.

Where should I add the line to prepare bootstrap?

Add device=/dev/sda1,/dev/sdb1 etc to the mount options (e.g. subvol=@,device=/dev...) depending on what your actual btrfs partitions are to grub.cfg.

Note that update-grub will overwrite this. You can monkeypatch /etc/grub.d/10_linux.

Theoretically initrd should do a btrfs device scan, and I think I've already seen bugs open for this. At any rate once I have a cleaner solution I will double check.