Support builds that completely exclude I2CDev support

This removes both the Elixir module and the NIF. There's no longer any
unintended NIF builds or loads or anything extra.

To enable this, either set the `:default_backend` application
environment key or force it by setting `:include_i2c_dev`. Most users
probably want:

```elixir
config :circuits_i2c, default_backend: MyI2CBackend
```

Author: fhunleth

.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

@@ -53,14 +53,33 @@ 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

@@ -17,7 +17,7 @@ SPDX-FileCopyrightText = "None"
 SPDX-License-Identifier = "CC0-1.0"
 
 [[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/compat/linux/types.h renamed to i2c_dev/c_src/compat/linux/types.h

@@ -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