Load modules from /etc/modules

Auto-load modules listed in the `/etc/modules` file. This makes it
possible for systems to specify (and users to override) the load of
kernel modules that may not be detectable via `uevent` messages.

Previously these modules were compiled into the kernel, but this does
not support cases where they could be optional. For example, it would
be nice to have a few USB gadget driver options without hardcoding them
into the kernel. This lets systems be made where the default case loads
a Ethernet/serial gadget driver (for example), but that can be disabled
by overriding `/etc/modules` with a different set of drivers.

Author: fhunleth

README.md

@@ -21,10 +21,10 @@ information, see the [hex docs](https://hexdocs.pm/nerves_runtime).
 ## 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:
+initialize the system on boot. 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`
@@ -40,9 +40,9 @@ assume that the system is already initialized before they start. To set up
 
 ### 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:
+`nerves_runtime` loads kernel modules from the `/etc/modules` file and on
+demand as devices are discovered by `uevent` messages. You can disable this
+feature by configuring `autoload: false` in your application configuration:
 
 ```elixir
 config :nerves_runtime, :kernel,

lib/nerves_runtime/application.ex

@@ -1,7 +1,10 @@
 defmodule Nerves.Runtime.Application do
   @moduledoc false
 
+  @module_list "/etc/modules"
+
   use Application
+  require Logger
 
   alias Nerves.Runtime.{
     Init,
@@ -12,13 +15,6 @@ defmodule Nerves.Runtime.Application do
   alias Nerves.Runtime.Log.{KmsgTailer, SyslogTailer}
 
   def start(_type, _args) do
-    # On systems with hardware random number generation, it is important that
-    # "rngd" gets started as soon as possible to start adding entropy to the
-    # system. So much code directly or indirectly uses random numbers that it's
-    # very easy to block on the random number generator or get low entropy
-    # numbers.
-    try_rngd()
-
     target = Nerves.Runtime.target()
 
     children =
@@ -37,6 +33,9 @@ defmodule Nerves.Runtime.Application do
   defp target_children(_target) do
     kernel_opts = Application.get_env(:nerves_runtime, :kernel, [])
 
+    # Kick off startup tasks asynchronously
+    spawn(fn -> run_startup_tasks(kernel_opts) end)
+
     [
       KmsgTailer,
       SyslogTailer,
@@ -45,6 +44,20 @@ defmodule Nerves.Runtime.Application do
     ]
   end
 
+  defp run_startup_tasks(opts) do
+    # Auto-load hardcoded modules
+    if Keyword.get(opts, :autoload_modules, true) do
+      load_kernel_modules()
+    end
+
+    # On systems with hardware random number generation, it is important that
+    # "rngd" gets started as soon as possible to start adding entropy to the
+    # system. So much code directly or indirectly uses random numbers that it's
+    # very easy to block on the random number generator or get low entropy
+    # numbers.
+    try_rngd()
+  end
+
   defp try_rngd() do
     rngd_path = "/usr/sbin/rngd"
 
@@ -53,4 +66,23 @@ defmodule Nerves.Runtime.Application do
       System.cmd(rngd_path, [])
     end
   end
+
+  defp load_kernel_modules() do
+    with {:ok, contents} <- File.read(@module_list) do
+      contents
+      |> String.split("\n")
+      |> Enum.map(&String.trim/1)
+      |> Enum.each(&process_modules_line/1)
+    end
+  end
+
+  defp process_modules_line(""), do: :ok
+  defp process_modules_line("#" <> _comment), do: :ok
+
+  defp process_modules_line(module_name) do
+    case System.cmd("/sbin/modprobe", [module_name]) do
+      {_, 0} -> :ok
+      _other -> Logger.warn("Error loading module #{module_name}. See #{@module_list}.")
+    end
+  end
 end