NervesUEvent

NervesUEvent listens for events from the Linux kernel, automatically loads device drivers, and forwards them to your Elixir programs.

NervesUEvent supports a small subset of Linux's udevd. This subset is sufficient for many Nerves use cases and you can add listeners that respond similar to what udev's rules files do. Supported features:

• Load kernel modules on demand
• Categorize input devices for programs using libinput
• Support a PropertyTable-style interface detecting device insertions and removals in Elixir

Warning {: .warning}

Almost all Nerves systems require some kernel modules to be automatically loaded or critical functionality won't work. WiFi device drivers, for example, are almost always kernel modules. If you're not using NervesUEvent or udevd or some other library that automatically loads kernel modules, you'll need to add calls to modprobe to your application.

Nerves projects generally depend on either :nerves_uevent or a version of :nerves_runtime that included similar functionality. It's an advanced use case to modify this dependency.

Configuration

NervesUEvent automatically starts on boot. Configuration is supplied via application config. The following options are available:

• :autoload_modules - defaults to true to automatically run modprobe when needed
• :manage_udev - set this to true or false to force NervesUEvent to manage the /run/udev directory. If unset, NervesUEvent manages it if udevd isn't running. Currently, NervesUEvent only maintains input device status for libinput.
• :input_rules - a list of {match, actions} tuples applied to each input device. match is a map of field => value pairs with string keys, compared as a subset against the inputN uevent (e.g. "name", "phys", "uniq") — every key in match must equal the kvmap's value. actions is a keyword list of actions to apply on match. Currently the only supported action is :env, a map of property name to value that gets appended as E:KEY=VALUE lines in /run/udev/data/c<major>:<minor>. Multiple matching rules merge their env; later rules win on conflicting keys.

Here's a config.exs example:

# Rotate a touchscreen by setting libinput's calibration matrix. Matrix
# values: 0° = "1 0 0 0 1 0", 90° CW = "0 -1 1 1 0 0",
# 180° = "-1 0 1 0 -1 1", 270° CW = "0 1 0 -1 0 1"
config :nerves_uevent,
  input_rules: [
    {%{"name" => "TSTP MTouch"},
     env: %{"LIBINPUT_CALIBRATION_MATRIX" => "0 1 0 -1 0 1"}}
  ]

Usage

NervesUEvent is currently very low level in what it reports and reflects the Linux representation. For example, say that you're interested in an MMC device and you've found out that Linux exposes it in the /sys/devices/platform/soc/2100000.bus/2194000.mmc directory. Linux also sends UEvent messages for all devices in /sys/device, so NervesUEvent will know about this too. To query NervesUEvent for device information, drop /sys off the path and convert to a list of strings like this:

iex> NervesUEvent.get(["devices", "platform", "soc", "2100000.bus", "2190000.mmc"])
%{
  "driver" => "sdhci-esdhc-imx",
  "modalias" => "of:NmmcT(null)Cfsl,imx6ull-usdhcCfsl,imx6sx-usdhc",
  "of_alias_0" => "mmc1",
  "of_compatible_0" => "fsl,imx6ull-usdhc",
  "of_compatible_1" => "fsl,imx6sx-usdhc",
  "of_compatible_n" => "2",
  "of_fullname" => "/soc/bus@2100000/mmc@2190000",
  "of_name" => "mmc",
  "subsystem" => "platform"
}

Some devices have more useful information than others. This particular one mostly shows information found in the device tree file for this device. Of note is the "modalias" key. When NervesUEvent sees this, it will try to load the appropriate Linux kernel driver for this device if one exists.

More usefully, you can subscribe for events. For example, if you'd like to be notified when a MicroSD card has been inserted, you can subscribe to all events on that device:

iex> NervesUEvent.subscribe(["devices", "platform", "soc", "2100000.bus", "2190000.mmc"])

If you're not sure what to subscribe to, subscribe to all events to see what happens:

iex> NervesUEvent.subscribe([])

Now if you physically insert a MicroSD card, NervesUEvent will send messages to your process mailbox. Here's one of the events:

iex> flush
%PropertyTable.Event{
  table: NervesUEvent,
  timestamp: 2558213871126,
  property: ["devices", "platform", "soc", "2100000.bus", "2190000.mmc", "mmc_host", "mmc0", "mmc0:1234"],
  value: %{
    "mmc_name" => "SA04G",
    "mmc_type" => "SD",
    "modalias" => "mmc:block",
    "subsystem" => "mmc"
  },
  previous_timestamp: nil,
  previous_value: nil
}

The primary fields of interest are :table, :timestamp, :property, and :value. NervesUEvent uses the PropertyTable library for storing everything and publishing changes. The timestamps are from System.monotonic_time/0.

License

All original source code in this project is licensed under Apache-2.0.

Additionally, this project follows the REUSE recommendations and labels so that licensing and copyright are clear at the file level.

Exceptions to Apache-2.0 licensing are:

• Configuration and data files are licensed under CC0-1.0
• Documentation is CC-BY-4.0