Support builds that completely exclude I2CDev support

.formatter.exs

@@ -1,4 +1,4 @@
 # Used by "mix format"
 [
-  inputs: ["{mix,.formatter,.credo}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+  inputs: ["{mix,.formatter,.credo}.exs", "{config,i2c_dev,lib,test}/**/*.{ex,exs}"]
 ]

Makefile

@@ -14,7 +14,7 @@
 # Variables to override:
 #
 # MIX_APP_PATH  path to the build directory
-# CIRCUITS_I2C_I2CDEV Backend to build - `"normal"`, `"test"`, or `"disabled"` will build a NIF
+# CIRCUITS_I2C_I2CDEV Backend to build - `"normal"` or `"test"`
 #
 # CC            C compiler
 # CROSSCOMPILE	crosscompiler prefix, if any
@@ -30,7 +30,7 @@ BUILD  = $(MIX_APP_PATH)/obj
 
 NIF = $(PREFIX)/i2c_nif.so
 
-SRC = c_src/i2c_nif.c
+SRC = i2c_dev/c_src/i2c_nif.c
 CFLAGS ?= -O2 -Wall -Wextra -Wno-unused-parameter -pedantic
 
 # Check that we're on a supported build platform
@@ -40,7 +40,7 @@ ifeq ($(shell uname -s),Linux)
 CFLAGS += -fPIC
 LDFLAGS += -fPIC -shared
 else
-CFLAGS += -Ic_src/compat
+CFLAGS += -Ii2c_dev/c_src/compat
 LDFLAGS += -undefined dynamic_lookup -dynamiclib
 ifeq ($(CIRCUITS_I2C_I2CDEV),normal)
 $(error Circuits.I2C Linux I2CDev backend is not supported on non-Linux platforms. Review circuits_i2c backend configuration or report an issue if improperly detected.)
@@ -59,17 +59,16 @@ ifeq ($(CIRCUITS_I2C_I2CDEV),test)
 # Stub out ioctls and send back test data
 CFLAGS += -DTEST_BACKEND
 else
-# Don't build NIF
-NIF =
+$(error Invalid CIRCUITS_I2C_I2CDEV value: $(CIRCUITS_I2C_I2CDEV))
 endif
 endif
 
 # Set Erlang-specific compile and linker flags
 ERL_CFLAGS ?= -I"$(ERL_EI_INCLUDE_DIR)"
 ERL_LDFLAGS ?= -L"$(ERL_EI_LIBDIR)" -lei
 
-HEADERS =$(wildcard c_src/*.h)
-OBJ = $(SRC:c_src/%.c=$(BUILD)/%.o)
+HEADERS =$(wildcard i2c_dev/c_src/*.h)
+OBJ = $(SRC:i2c_dev/c_src/%.c=$(BUILD)/%.o)
 
 calling_from_make:
 	mix compile
@@ -80,7 +79,7 @@ install: $(PREFIX) $(BUILD) $(NIF)
 
 $(OBJ): $(HEADERS) Makefile
 
-$(BUILD)/%.o: c_src/%.c
+$(BUILD)/%.o: i2c_dev/c_src/%.c
 	@echo " CC $(notdir $@)"
 	$(CC) -c $(ERL_CFLAGS) $(CFLAGS) -o $@ $<
 

README.md

@@ -48,14 +48,33 @@ may be helpful.
 
 Internally, it uses the [Linux "i2c-dev"
 interface](https://elixir.bootlin.com/linux/latest/source/Documentation/i2c/dev-interface)
-so that it does not require board-dependent code.
+so that it does not require board-dependent code. See the application
+configuration for disabling all things Linux/Nerves specific.
 
 ## Getting started without hardware
 
 If you don't have any real I2C devices, it's possible to work with simulated
 devices. See the [CircuitsSim](https://github.com/elixir-circuits/circuits_sim)
 project for details.
 
+## Application config
+
+`Circuits.I2C` supports the following application environment keys:
+
+| Key | Description |
+| --- | ----------- |
+| `:include_i2c_dev` | Set to `true` or `false` to indicate whether or not to include the `Circuits.I2C.I2CDev` backend. |
+| `:default_backend` | Default I2C backend to use when unspecified in API calls. The format is either a module name or a `{module_name, options}` tuple. |
+
+`Circuits.I2C` uses a heuristic for the default values of both
+`:include_i2c_dev`. For example, if the `:default_backend` is set to
+`Circuits.I2C.I2CDev`, then `:include_i2c_dev` has to be `true`. If
+`:default_backend` is set to something else, then `:include_i2c_dev` defaults to
+`false`. If neither is set, then `Circuits.I2C.I2CDev` is built. On non-Linux
+platforms, the `Circuits.I2C.I2CDev` NIF will be compiled in test mode which
+minimally supports unit testing. Mocking is generally a better option for most
+users, though.
+
 ## I2C background
 
 An [Inter-Integrated Circuit](https://en.wikipedia.org/wiki/I%C2%B2C) (I2C) bus

REUSE.toml

@@ -28,7 +28,7 @@ SPDX-FileCopyrightText = "2014 Frank Hunleth"
 SPDX-License-Identifier = "CC-BY-4.0"
 
 [[annotations]]
-path = ["c_src/linux/i2c-dev.h"]
+path = ["i2c_dev/c_src/linux/i2c-dev.h"]
 precedence = "aggregate"
 SPDX-FileCopyrightText = "None"
 SPDX-License-Identifier = "GPL-2.0-or-later WITH Linux-syscall-note"

c_src/i2c_nif.c → i2c_dev/c_src/i2c_nif.c

@@ -30,8 +30,10 @@ int test_open(const char *path, int flags)
         return 0x20;
     else if (strcmp(path, "/dev/i2c-flaky") == 0)
         return 0x30;
-    else
+    else {
+        errno = ENOENT;
         return -1;
+    }
 }
 int test_close(int fd)
 {

lib/i2c/nil_backend.ex

@@ -9,6 +9,9 @@ defmodule Circuits.I2C.NilBackend do
   @behaviour Circuits.I2C.Backend
 
   alias Circuits.I2C.Backend
+  alias Circuits.I2C.Bus
+
+  defstruct []
 
   @doc """
   Return the I2C bus names on this system
@@ -35,4 +38,27 @@ defmodule Circuits.I2C.NilBackend do
   def info() do
     %{name: __MODULE__}
   end
+
+  defimpl Bus do
+    @impl Bus
+    def flags(%Circuits.I2C.NilBackend{}), do: []
+
+    @impl Bus
+    def read(%Circuits.I2C.NilBackend{}, _address, _count, _options) do
+      {:error, :unimplemented}
+    end
+
+    @impl Bus
+    def write(%Circuits.I2C.NilBackend{}, _address, _data, _options) do
+      {:error, :unimplemented}
+    end
+
+    @impl Bus
+    def write_read(%Circuits.I2C.NilBackend{}, _address, _write_data, _read_count, _options) do
+      {:error, :unimplemented}
+    end
+
+    @impl Bus
+    def close(%Circuits.I2C.NilBackend{}), do: :ok
+  end
 end

mix.exs

@@ -6,47 +6,54 @@ defmodule Circuits.I2C.MixProject do
   @source_url "https://github.com/elixir-circuits/circuits_i2c"
 
   def project do
-    [
+    base = [
       app: :circuits_i2c,
       version: @version,
       elixir: "~> 1.13",
       description: @description,
       package: package(),
       source_url: @source_url,
-      compilers: [:elixir_make | Mix.compilers()],
-      elixirc_paths: elixirc_paths(Mix.env()),
-      make_targets: ["all"],
-      make_clean: ["clean"],
       docs: docs(),
-      aliases: [compile: [&set_make_env/1, "compile"], format: [&format_c/1, "format"]],
       start_permanent: Mix.env() == :prod,
       dialyzer: [
         flags: [:missing_return, :extra_return, :unmatched_returns, :error_handling, :underspecs]
       ],
       deps: deps()
     ]
+
+    if build_i2c_dev?() do
+      additions = [
+        compilers: [:elixir_make | Mix.compilers()],
+        elixirc_paths: ["lib", "i2c_dev/lib"],
+        make_targets: ["all"],
+        make_clean: ["clean"],
+        aliases: [compile: [&set_make_env/1, "compile"], format: [&format_c/1, "format"]]
+      ]
+
+      Keyword.merge(base, additions)
+    else
+      base
+    end
   end
 
   def cli do
     [preferred_envs: %{docs: :docs, "hex.publish": :docs, "hex.build": :docs}]
   end
 
-  defp elixirc_paths(env) when env in [:test, :dev], do: ["lib", "examples"]
-  defp elixirc_paths(_env), do: ["lib"]
-
   def application do
-    # IMPORTANT: This provides a default at runtime and at compile-time when
+    # IMPORTANT: This provides defaults at runtime and at compile-time when
     # circuits_i2c is pulled in as a dependency.
-    [env: [default_backend: default_backend()]]
+    [env: [default_backend: default_backend(), build_i2c_dev: false]]
   end
 
   defp package do
     %{
       files: [
         "CHANGELOG.md",
-        "c_src/*.[ch]",
-        "c_src/linux/*.h",
-        "c_src/compat/linux/*.h",
+        "i2c_dev/c_src/*.[ch]",
+        "i2c_dev/c_src/linux/*.h",
+        "i2c_dev/c_src/compat/linux/*.h",
+        "i2c_dev/lib",
         "lib",
         "LICENSES",
         "Makefile",
@@ -85,24 +92,43 @@ defmodule Circuits.I2C.MixProject do
     ]
   end
 
-  defp default_backend(), do: default_backend(Mix.env(), Mix.target())
-  defp default_backend(:test, _target), do: {Circuits.I2C.I2CDev, test: true}
+  defp build_i2c_dev?() do
+    include_i2c_dev = Application.get_env(:circuits_i2c, :include_i2c_dev)
+
+    if include_i2c_dev != nil do
+      # If the user set :include_i2c_dev, then use it
+      include_i2c_dev
+    else
+      # Otherwise, infer whether to build it based on the default_backend
+      # setting. If default_backend references it, then build it. If it
+      # references something else, then don't build. Default is to build.
+      default_backend = Application.get_env(:circuits_i2c, :default_backend)
 
-  defp default_backend(_env, :host) do
+      default_backend == nil or default_backend == Circuits.I2C.I2CDev or
+        (is_tuple(default_backend) and elem(default_backend, 0) == Circuits.I2C.I2CDev)
+    end
+  end
+
+  defp default_backend(), do: default_backend(Mix.env(), Mix.target(), build_i2c_dev?())
+  defp default_backend(:test, _target, true), do: {Circuits.I2C.I2CDev, test: true}
+
+  defp default_backend(_env, :host, true) do
     case :os.type() do
       {:unix, :linux} -> Circuits.I2C.I2CDev
       _ -> {Circuits.I2C.I2CDev, test: true}
     end
   end
 
+  defp default_backend(_env, _target, false), do: Circuits.I2C.NilBackend
+
   # MIX_TARGET set to something besides host
-  defp default_backend(env, _not_host) do
+  defp default_backend(env, _not_host, true) do
     # If CROSSCOMPILE is set, then the Makefile will use the crosscompiler and
     # assume a Linux/Nerves build If not, then the NIF will be build for the
     # host, so use the default host backend
     case System.fetch_env("CROSSCOMPILE") do
       {:ok, _} -> Circuits.I2C.I2CDev
-      :error -> default_backend(env, :host)
+      :error -> default_backend(env, :host, true)
     end
   end
 
@@ -123,12 +149,8 @@ defmodule Circuits.I2C.MixProject do
     end
   end
 
-  defp i2c_dev_compile_mode(Circuits.I2C.I2CDev) do
-    "normal"
-  end
-
   defp i2c_dev_compile_mode(_other) do
-    "disabled"
+    "normal"
   end
 
   defp format_c([]) do
@@ -137,7 +159,7 @@ defmodule Circuits.I2C.MixProject do
         Mix.Shell.IO.info("Install astyle to format C code.")
 
       astyle ->
-        System.cmd(astyle, ["-n", "c_src/*.c"], into: IO.stream(:stdio, :line))
+        System.cmd(astyle, ["-n", "i2c_dev/c_src/*.c"], into: IO.stream(:stdio, :line))
     end
   end
 

mix.lock

@@ -2,7 +2,7 @@
   "bunt": {:hex, :bunt, "1.0.0", "081c2c665f086849e6d57900292b3a161727ab40431219529f13c4ddcf3e7a44", [:mix], [], "hexpm", "dc5f86aa08a5f6fa6b8096f0735c4e76d54ae5c9fa2c143e5a1fc7c1cd9bb6b5"},
   "credo": {:hex, :credo, "1.7.12", "9e3c20463de4b5f3f23721527fcaf16722ec815e70ff6c60b86412c695d426c1", [:mix], [{:bunt, "~> 0.2.1 or ~> 1.0", [hex: :bunt, repo: "hexpm", optional: false]}, {:file_system, "~> 0.2 or ~> 1.0", [hex: :file_system, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "8493d45c656c5427d9c729235b99d498bd133421f3e0a683e5c1b561471291e5"},
   "credo_binary_patterns": {:hex, :credo_binary_patterns, "0.2.6", "cfcaca0bc5c6447b96c5a03eff175c28f86c486be8e95d55b360fb90c2dd18bd", [:mix], [{:credo, "~> 1.6", [hex: :credo, repo: "hexpm", optional: false]}], "hexpm", "d36a2b56ad72bdf3183ccc81d7e7821e78c97de7c127bc8dd99a5f05ca702187"},
-  "dialyxir": {:hex, :dialyxir, "1.4.5", "ca1571ac18e0f88d4ab245f0b60fa31ff1b12cbae2b11bd25d207f865e8ae78a", [:mix], [{:erlex, ">= 0.2.7", [hex: :erlex, repo: "hexpm", optional: false]}], "hexpm", "b0fb08bb8107c750db5c0b324fa2df5ceaa0f9307690ee3c1f6ba5b9eb5d35c3"},
+  "dialyxir": {:hex, :dialyxir, "1.4.6", "7cca478334bf8307e968664343cbdb432ee95b4b68a9cba95bdabb0ad5bdfd9a", [:mix], [{:erlex, ">= 0.2.7", [hex: :erlex, repo: "hexpm", optional: false]}], "hexpm", "8cf5615c5cd4c2da6c501faae642839c8405b49f8aa057ad4ae401cb808ef64d"},
   "earmark_parser": {:hex, :earmark_parser, "1.4.44", "f20830dd6b5c77afe2b063777ddbbff09f9759396500cdbe7523efd58d7a339c", [:mix], [], "hexpm", "4778ac752b4701a5599215f7030989c989ffdc4f6df457c5f36938cc2d2a2750"},
   "elixir_make": {:hex, :elixir_make, "0.9.0", "6484b3cd8c0cee58f09f05ecaf1a140a8c97670671a6a0e7ab4dc326c3109726", [:mix], [], "hexpm", "db23d4fd8b757462ad02f8aa73431a426fe6671c80b200d9710caf3d1dd0ffdb"},
   "erlex": {:hex, :erlex, "0.2.7", "810e8725f96ab74d17aac676e748627a07bc87eb950d2b83acd29dc047a30595", [:mix], [], "hexpm", "3ed95f79d1a844c3f6bf0cea61e0d5612a42ce56da9c03f01df538685365efb0"},