nerves_runtime v0.6.4 Nerves.Runtime.KV

Key Value Storage for firmware vairables provided by fwup

KV provides access to metadata variables set by fwup. It can be used to obtain information such as the active firmware slot, where the application data partition is located, etc.

Values are stored in two ways.

  • Values that do not pertain to a specific firmware slot For example: "nerves_fw_active" => "a"

  • Values that pertain to a specific firmware slot For Example: "a.nerves_fw_author" => "The Nerves Team"

You can find values for just the active firmware slot by using get_active and get_all_active. The result of these functions will trim the firmware slot ("a." or "b.") from the leading characters of the keys returned.

Technical Information

Nerves.Runtime.KV uses a non-replicated U-Boot environment block for storing firmware and provisioning information. It has the following format:

  • CRC32 of bytes 4 through to the end
  • "<key>=<value>" for each key/value pair
  • "" an empty key/value pair to terminate the list. This looks like “” when you’re viewing the file in a hex editor.
  • Filler bytes to the end of the environment block. These are usually 0xff.

The U-Boot environment configuration is loaded from /etc/fw_env.config. If you are using OTP >= 21, the contents of the U-Boot environment will be read directly from the device. This addresses an issue with parsing multi-line values from a call to fw_printenv.

Link to this section Summary

Functions

Returns a specification to start this module under a supervisor

Get the key regardless of firmware slot

Get the key for only the active firmware slot

Get all keys regardless of firmware slot

Get all key value pairs for only the active firmware slot

Invoked when the server is started. start_link/3 or start/3 will block until it returns

Start the KV store server

Link to this section Functions

Link to this function child_spec(arg)

Returns a specification to start this module under a supervisor.

See Supervisor.

Get the key regardless of firmware slot

Link to this function get_active(key)

Get the key for only the active firmware slot

Get all keys regardless of firmware slot

Link to this function get_all_active()

Get all key value pairs for only the active firmware slot

Invoked when the server is started. start_link/3 or start/3 will block until it returns.

args is the argument term (second argument) passed to start_link/3.

Returning {:ok, state} will cause start_link/3 to return {:ok, pid} and the process to enter its loop.

Returning {:ok, state, timeout} is similar to {:ok, state} except handle_info(:timeout, state) will be called after timeout milliseconds if no messages are received within the timeout.

Returning {:ok, state, :hibernate} is similar to {:ok, state} except the process is hibernated before entering the loop. See c:handle_call/3 for more information on hibernation.

Returning :ignore will cause start_link/3 to return :ignore and the process will exit normally without entering the loop or calling c:terminate/2. If used when part of a supervision tree the parent supervisor will not fail to start nor immediately try to restart the GenServer. The remainder of the supervision tree will be (re)started and so the GenServer should not be required by other processes. It can be started later with Supervisor.restart_child/2 as the child specification is saved in the parent supervisor. The main use cases for this are:

  • The GenServer is disabled by configuration but might be enabled later.
  • An error occurred and it will be handled by a different mechanism than the Supervisor. Likely this approach involves calling Supervisor.restart_child/2 after a delay to attempt a restart.

Returning {:stop, reason} will cause start_link/3 to return {:error, reason} and the process to exit with reason reason without entering the loop or calling c:terminate/2.

Callback implementation for GenServer.init/1.

Link to this function load_kv(dev_name, dev_offset, env_size)
Link to this function parse_config(config)
Link to this function read_config(file)
Link to this function start_link(kv \\ "")

Start the KV store server