CircleCI Hex version

nerves_runtime is a core component of Nerves. It contains applications and libraries that are expected to be useful on all Nerves devices.

Here are its features:

  • Generic system and filesystem initialization (suitable for use with shoehorn)
  • Introspection of Nerves system, firmware, and deployment metadata
  • Device reboot and shutdown
  • A small Linux kernel uevent application for capturing hardware change events and more

The following sections describe the features in more detail. For more information, see the hex docs.

System Initialization

nerves_runtime provides an OTP application (nerves_runtime) that can initialize the system when it is started. For this to be useful, nerves_runtime must be started before other OTP applications, since most will assume that the system is already initialized before they start. To set up nerves_runtime to work with shoehorn, you will need to do the following:

  1. Include shoehorn in mix.exs
  2. Include shoehorn in your rel/config.exs
  3. Ensure that :nerves_runtime is at the beginning of the init: list in your config/config.exs:
config :shoehorn,
  overlay_path: "",
  init: [:nerves_runtime, :other_app1, :other_app2],
  app: :your_app

Kernel Modules

nerves_runtime will attempt to auto-load kernel modules by calling modprobe using the modalias supplied by the device's uevent message. You can disable this feature by configuring autoload: false in your application configuration:

config :nerves_runtime, :kernel,
  autoload_modules: false

nerves_runtime can optionally report device insertions and removals through SystemRegistry. This is currently the default, but you can disable it via configuration:

config :nerves_runtime, :kernel,
  use_system_registry: false

Filesystem Initialization

Nerves systems generally ship with one or more application filesystem partitions. These are used for persisting data that is expected to live between firmware updates. The root filesystem cannot be used since it is mounted as read-only by default.

nerves_runtime takes an unforgiving approach to managing the application partition: if it can't be mounted as read-write, it gets re-formatted. While filesystem corruption should be a rare event, even with unexpected loss of power, Nerves devices may not always be accessible for manual recovery. This default behavior provides a basic recoverability guarantee.

To verify that this recovery works, Nerves systems usually leave the application filesystems uninitialized so that the format operation happens on the first boot. This means that the first boot takes slightly longer than subsequent boots.

Note that a common implementation of "reset to factory defaults" is to purposely corrupt the application partition and reboot.

nerves_runtime uses firmware metadata to determine how to mount and initialize the application partition. The following variables are important:

  • [partition].nerves_fw_application_part0_devpath - the path to the application partition (e.g. /dev/mmcblk0p3)
  • [partition].nerves_fw_application_part0_fstype - the type of filesystem (e.g. ext4)
  • [partition].nerves_fw_application_part0_target - where the partition should be mounted (e.g. /root or /mnt/appdata)

Nerves System and Firmware Metadata

All official Nerves systems maintain a list of key-value pairs for tracking various information about the system. This information is not intended to be written frequently. To get this information, you can call one of the following:

Global Nerves metadata includes the following:

KeyBuild Environment VariableExample ValueDescription
nerves_fw_activeN/A"a"This key holds the prefix that identifies the active firmware metadata. In this example, all keys starting with "a." hold information about the running firmware.
nerves_fw_devpathNERVES_FW_DEVPATH"/dev/mmcblk0"This is the primary storage device for the firmware.
nerves_serial_numberN/A"12345abc"This is a text serial number. See Serial numbers for details.
nerves_fw_validatedN/A0Set to "1" to indicate that the currently running firmware is valid. (Only supported on some platforms)
nerves_fw_autovalidateN/A1Set to "1" to indicate that firmware updates are valid without any additional checks. (Only supported on some platforms)

Firmware-specific Nerves metadata includes the following:

KeyExample ValueDescription
nerves_fw_application_part0_devpath"/dev/mmcblkp3"The block device that contains the application partition
nerves_fw_application_part0_fstype"ext4"The application partition's filesystem type
nerves_fw_application_part0_target"/root"Where to mount the application partition
nerves_fw_architecture"arm"The processor architecture (Not currently used)
nerves_fw_author"John Doe"The person or company that created this firmware
nerves_fw_description"Stuff"A description of the project
nerves_fw_platform"rpi3"A name to identify the board that this runs on. It can be checked in the fwup.conf before performing an upgrade.
nerves_fw_product"My Product"A product name that may show up in a firmware selection list, for example
nerves_fw_version"1.0.0"The project's version
nerves_fw_vcs_identifier"bdeead38..."A git SHA or other identifier (optional)
nerves_fw_misc"anything..."Any application info that doesn't fit in another field (optional)

Note that the keys are stored in the environment block prefixed by the firmware slot for which they pertain. For example, a.nerves_fw_description is the description for the firmware in the "A" slot.

Several of the keys can be set in the mix.exs file of your main Nerves project. This is the preferred way to set them because it requires the least amount of effort.

Assuming that your fwup.conf respects the fwup variable names listed in the table, the keys can also be overridden by setting environment variables at build time. Depending on your project, you may prefer to set them using a customized fwup.conf configuration file instead.

The fwup -m value shows the key that you'll see if you run fwup -m -i project.fw to extract the firmware metadata from the .fw file.

Key in Nerves.RuntimeKey in mix.exsBuild Environment VariableKey in fwup -m

Device Reboot and Shutdown

Rebooting, powering-off, and halting a device work by signaling to erlinit an intention to shutdown and then exiting the Erlang VM by calling :init.stop/0. The Nerves.Runtime.reboot/0 and related utilities are helper methods for this. Once they return, the Erlang VM will likely only be available momentarily before shutdown. If the OTP applications cannot be stopped within a timeout as specified in the erlinit.config, erlinit will ungracefully terminate the Erlang VM.

Reverting firmware

If you'd like to go back to the previous version of firmware running on a device, you can do that if the Nerves system supports it. At the IEx prompt, run:

iex> Nerves.Runtime.revert

Running this command manually is useful in development. Production use requires more work to protect against faulty upgrades.

Assisted firmware validation and automatic revert

Nerves firmware updates protect against update corruption and powerloss midway into the update procedure. However, what happens if the firmware update contains bad code that hangs the device or breaks something important like networking? Some Nerves systems support tentative runs of new firmware and if something goes wrong, they'll revert back.

At a high level, this involves some additional code from the developer that knows what constitutes "working". This could be "is it possible to connect to the firmware update server within 5 minutes of boot?"

Here's the process:

  1. New firmware is installed in the normal manner. The Nerves.Runtime.KV variable, nerves_fw_validated is set to 0.
  2. The system reboots like normal.
  3. The device starts a five minute reboot timer (your code needs to do this)
  4. The application attempts to make a connection to the firmware update server.
  5. On a good connection, the application sets nerves_fw_validated to 1 and cancels the reboot timer.
  6. On error, the reboot timer failing, or a hardware watchdog timeout, the system reboots. The bootloader reverts to the previous firmware.

To use this feature, the nerves_fw_autovalidate variable must be set to 0. This can be done at device provisioning time (like when the serial number is set) or inside a custom fwup.conf.

Best effort automatic revert

Unfortunately, the bootloader for platforms like the Raspberry Pi makes it difficult to implement the above mechanism. The following strategy cannot protect against kernel and early boot issues, but it can still provide value:

  1. Upgrade firmware the normal way. Record that the next boot will be the first one in the application data partition.
  2. On the reboot, if this is the first one, record that the boot happened and revert the firmware with reboot: false. If this is not the first boot, carry on.
  3. When you're happy with the new firmware, revert the firmware again with reboot: false. I.e., revert the revert. It is critical that revert is only called once.

To make this handle hangs, you'll want to enable a hardware watchdog.

Operating system log collection

Operating system-level messages from /dev/log and /proc/kmsg, forwarding them to Logger with an appropriate level to match the syslog priority parsed out of the message.

Serial numbers

Nerves systems support several methods for assigning serial numbers to devices. By default serial numbers are derived using board-specific identifiers. Currently, no one place exists that can be queried for the serial number. However, many people are using the nerves_serial_number key in the U-Boot environment block to store a serial number for their device. This location is not "secure" against a determined person who wants to clone a device. However, it is good enough for many use cases and is available on all platforms supported by Nerves.

See embedded-elixir for how to assign serial numbers to devices.

Using nerves_runtime in tests

Applications that depend on nerves_runtime for accessing provisioning information from the Nerves.Runtime.KV can mock the contents with the included Nerves.Runtime.KV.Mock module through the Application config:

config :nerves_runtime, Nerves.Runtime.KV.Mock, %{"key" => "value"}

You can also create your own module based on the Nerves.Runtime.KV behavior and set it to be used in the Application config. In most situations, the provided Nerves.Runtime.KV.Mock should be sufficient, though this would be helpful in cases where you might need to generate the inital state at runtime instead:

defmodule MyApp.KV.Mock do
  @behaviour Nerves.Runtime.KV

  @impl true
  def init(_opts) do
    # initial state
      "howdy" => "partner",
      "dynamic" => some_runtime_calc_function()

  @impl true
  def put(_map), do: :ok

# Then in config.exs
config :nerves_runtime, :modules, [
  {Nerves.Runtime.KV, MyApp.KV.Mock}