[LEDE-DEV] Support for more than a single overlay, which is selected at boot time

[Jurgen Van Ham]

Hi all,

I want to support multiple (currently just two) configurations on a
single device. One of these configurations is chosen at boot time.
Instead of saving and restoring the configuration each time, I came up
with a solution that relies on extending the overlay by splitting it
into two banks. There can be more than two, but I only need two called
'bank_0' and 'bank_1'

This relies on splitting the overlay file system into two (or more)
different banks, which are implemented as directories in the root of
the overlay file system. Separate file systems would require a
decision how to split the size of the single overlay into two overlays
that do not always have the same size.

Until now the writable /overlay (jffs2 or other) partition is mounted
as the upperdir over the /rom squashfs partition as its lowerdir.
Instead, I consider to replace this /overlay mount by the subdirs
/overlay/bank_1 or /overlay_bank_2 by modifying the code of mount_root
from the fstools package.

The mount_root program read the environment variable 'OVERLAY_BANK'
that is set by modifying two files:  /lib/preinit/80_mount_root' and
'/etc/init.d/done'  base-files/files. Using an environment variable
keeps the selection of the overlay bank in scripts. The alternative
would be require to integrate the selection of the bank at boot time
into the mount_root program, but that ties the implementation to
specific hardware.

At a high level, I modified the mount_root program to use
'/overlay/${OVERLAY_BANK}/' instead of '/overlay/'. When /overlay does
not yet contain the directory ${OVERLAY_BANK}, it creates this
directory.

I can imagine that this can also be useful to test new firmware and
make it possible to revert the upgrade by moving back to the previous
overlay bank.

Before polishing my experimental implementation and sending it as a
patch to the mailinglist, I'd like to know whether someone else had to
deal with such requirement, or sees different ways to create a device
that can boot with one out of several configurations.

Regards,

Jurgen