FwupDelta

A library for generating fwup delta updates from Elixir. Fwup itself does not provide a way to generate delta updates.

This can be used as a library in Elixir code for your OTA platform, a mix task during your Nerves development or even for scripting.

This was extracted from NervesHub which had the only reference implementation of this procedure.

Installation

Most easy to install using igniter in your project:

mix archive.install hex igniter_new
mix igniter.install fwup_delta

Or the package can be installed by adding fwup_delta to your list of dependencies in mix.exs:

def deps do
  [
    {:fwup_delta, "~> 0.1.0"}
  ]
end

Mix task

After installing the package you run:

mix fwup.delta firmware_1.fw firmware_2.fw

Scripting

Assuming you have Elixir installed:

Mix.install([:fwup_delta])

{:ok, %{filepath: delta_fw}} = FwupDelta.generate({:local, "firmware-v1.0.fw"}, {:local, "firmware-v2.0.fw"})

Better delta performance

The Nerves documentation has a section on good settings for optimizing your firmware for better deltas. This is is the relevant config:

# Customize non-Elixir parts of the firmware. See
# https://hexdocs.pm/nerves/advanced-configuration.html for details.

config :nerves, :firmware,
  rootfs_overlay: "rootfs_overlay",
  mksquashfs_flags: ["-noI", "-noId", "-noD", "-noF", "-noX"]

# Set the SOURCE_DATE_EPOCH date for reproducible builds.
# See https://reproducible-builds.org/docs/source-date-epoch/ for more information

config :nerves, source_date_epoch: "1596027629"

Optimizations

fwup_delta performs at least one non-obvious optimization.

If a file resource is included in the archive but not reference in any task with a name that starts with upgrade we will remove the file resource. Some files are only used for the initial complete flash. This could only be a negative if you used file-resource to smuggle additional files that you want to extract from the archive in some custom way.

Checking capabilities of the current fwup install

If you need to determine if a fwup_delta delta will with the version of fwup installed on a device that is best done with the confuse library which offers Confuse.get_features_usage/1 for this. The confuse library has various utilities for interpreting fwup.conf files and the meta.conf included in .fw files made by fwup. This is the mechanism used in NervesHub to determine if a delta is possible for a particular fwup config.