12 files changed, 205 insertions, 139 deletions
diff --git a/docs/develop/cmodules.rst b/docs/develop/cmodules.rst
new file mode 100644
index 000000000..c3de90a0f
--- /dev/null
+++ b/docs/develop/cmodules.rst
@@ -0,0 +1,163 @@
+MicroPython external C modules
+==============================
+
+When developing modules for use with MicroPython you may find you run into
+limitations with the Python environment, often due to an inability to access
+certain hardware resources or Python speed limitations.
+
+If your limitations can't be resolved with suggestions in :ref:`speed_python`,
+writing some or all of your module in C is a viable option.
+
+If your module is designed to access or work with commonly available
+hardware or libraries please consider implementing it inside the MicroPython
+source tree alongside similar modules and submitting it as a pull request.
+If however you're targeting obscure or proprietary systems it may make
+more sense to keep this external to the main MicroPython repository.
+
+This chapter describes how to compile such external modules into the
+MicroPython executable or firmware image.
+
+
+Structure of an external C module
+---------------------------------
+
+A MicroPython user C module is a directory with the following files:
+
+* ``*.c`` and/or ``*.h`` source code files for your module.
+
+  These will typically include the low level functionality being implemented and
+  the MicroPython binding functions to expose the functions and module(s).
+
+  Currently the best reference for writing these functions/modules is
+  to find similar modules within the MicroPython tree and use them as examples.
+
+* ``micropython.mk`` contains the Makefile fragment for this module.
+
+  ``$(USERMOD_DIR)`` is available in ``micropython.mk`` as the path to your
+  module directory. As it's redefined for each c module, is should be expanded
+  in your ``micropython.mk`` to a local make variable,
+  eg ``EXAMPLE_MOD_DIR := $(USERMOD_DIR)``
+
+  Your ``micropython.mk`` must add your modules C files relative to your
+  expanded copy of ``$(USERMOD_DIR)`` to ``SRC_USERMOD``, eg
+  ``SRC_USERMOD += $(EXAMPLE_MOD_DIR)/example.c``
+
+  If you have custom ``CFLAGS`` settings or include folders to define, these
+  should be added to ``CFLAGS_USERMOD``.
+
+  See below for full usage example.
+
+
+Basic Example
+-------------
+
+This simple module named ``example`` provides a single function
+``example.add_ints(a, b)`` which adds the two integer args together and returns
+the result.
+
+Directory::
+
+    example/
+    ├── example.c
+    └── micropython.mk
+
+
+``example.c``
+
+.. code-block:: c
+
+    // Include required definitions first.
+    #include "py/obj.h"
+    #include "py/runtime.h"
+    #include "py/builtin.h"
+
+    #define MODULE_EXAMPLE_ENABLED (1)
+
+    // This is the function which will be called from Python as example.add_ints(a, b).
+    STATIC mp_obj_t example_add_ints(mp_obj_t a_obj, mp_obj_tab_obj) {
+        // Extract the ints from the micropython input objects
+        int a = mp_obj_get_int(a_obj);
+        int b = mp_obj_get_int(b_obj);
+
+        // Calculate the addition and convert to MicroPython object.
+        return mp_obj_new_int(a + b);
+    }
+    // Define a Python reference to the function above
+    STATIC MP_DEFINE_CONST_FUN_OBJ_1(example_add_ints_obj, example_add_ints);
+
+    // Define all properties of the example module.
+    // Table entries are key/value pairs of the attribute name (a string)
+    // and the MicroPython object reference.
+    // All identifiers and strings are written as MP_QSTR_xxx and will be
+    // optimized to word-sized integers by the build system (interned strings).
+    STATIC const mp_rom_map_elem_t example_module_globals_table[] = {
+        { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_example) },
+        { MP_ROM_QSTR(MP_QSTR_add_ints), MP_ROM_PTR(&example_add_ints_obj) },
+    };
+    STATIC MP_DEFINE_CONST_DICT(example_module_globals, example_module_globals_table);
+
+    // Define module object.
+    const mp_obj_module_t example_user_cmodule = {
+        .base = { &mp_type_module },
+        .globals = (mp_obj_dict_t*)&example_module_globals,
+    };
+
+    // Register the module to make it available in Python
+    MP_REGISTER_MODULE(MP_QSTR_example, example_user_cmodule, MODULE_EXAMPLE_ENABLED);
+
+
+``micropython.mk``
+
+.. code-block:: make
+
+    EXAMPLE_MOD_DIR := $(USERMOD_DIR)
+
+    # Add all C files to SRC_USERMOD.
+    SRC_USERMOD += $(EXAMPLE_MOD_DIR)/example.c
+
+    # We can add our module folder to include paths if needed
+    # This is not actually needed in this example.
+    CFLAGS_USERMOD += -I$(EXAMPLE_MOD_DIR)
+
+
+Compiling the cmodule into MicroPython
+--------------------------------------
+
+To build such a module, compile MicroPython (see `getting started
+<https://github.com/micropython/micropython/wiki/Getting-Started>`_) with an
+extra ``make`` flag named ``USER_C_MODULES`` set to the directory containing
+all modules you want included (not to the module itself). For example:
+
+
+Directory::
+
+    my_project/
+    ├── modules/
+    │   └──example/
+    │       ├──example.c
+    │       └──micropython.mk
+    └── micropython/
+        ├──ports/
+       ... ├──stm32/
+          ...
+
+Building for stm32 port:
+
+.. code-block:: bash
+
+    cd my_project/micropython/ports/stm32
+    make USER_C_MODULES=../../../modules all
+
+
+Module usage in MicroPython
+---------------------------
+
+Once built into your copy of MicroPython, the module implemented
+in ``example.c`` above can now be accessed in Python just
+like any other builtin module, eg
+
+.. code-block:: python
+
+    import example
+    print(example.add_ints(1, 3))
+    # should display 4
diff --git a/docs/develop/index.rst b/docs/develop/index.rst
new file mode 100644
index 000000000..6b7b3c391
--- /dev/null
+++ b/docs/develop/index.rst
@@ -0,0 +1,15 @@
+Developing and building MicroPython
+===================================
+
+This chapter describes modules (function and class libraries) which are built
+into MicroPython. There are a few categories of such modules:
+
+This chapter describes some options for extending MicroPython in C. Note
+that it doesn't aim to be a complete guide for developing with MicroPython.
+See the `getting started guide
+<https://github.com/micropython/micropython/wiki/Getting-Started>`_ for further information.
+
+.. toctree::
+   :maxdepth: 1
+
+   cmodules.rst
diff --git a/docs/index.rst b/docs/index.rst
index 235185b6c..c0417c227 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -6,6 +6,7 @@ MicroPython documentation and references
     library/index.rst
     reference/index.rst
     genrst/index.rst
+    develop/index.rst
     license.rst
     pyboard/quickref.rst
     esp8266/quickref.rst
diff --git a/docs/reference/cmodules.rst b/docs/reference/cmodules.rst
deleted file mode 100644
index f361af4a3..000000000
--- a/docs/reference/cmodules.rst
+++ /dev/null
@@ -1,86 +0,0 @@
-Extending MicroPython with C
-============================
-
-Some specialized code would be unacceptably slow or needs to access hardware in
-a way that cannot be done from MicroPython. Therefore, it supports a way of
-extending the language with custom modules written in C. But before you consider
-writing a module in C, please take a look at :ref:`speed_python`.
-
-`Unlike CPython <https://docs.python.org/3/extending/building.html>`_, these
-modules are (currently) embedded directly in the program image instead of being
-dynamically loaded. This requires a `custom build of MicroPython
-<https://github.com/micropython/micropython/wiki/Getting-Started>`_.
-
-
-Writing a module
-----------------
-
-A module is a directory with the following files:
-
-  * ``micropython.mk``, which contains the Makefile fragment for this module.
-  * All C files you would like included.
-
-Put the required build commands in ``micropython.mk``. For a simple module, you
-will only have to add the file paths to ``SRC_MOD``, which will include these C
-files in the build:
-
-.. highlight:: make
-.. code::
-
-    # Add all C files to SRC_MOD.
-    SRC_MOD += $(USER_C_MODULES)/example/example.c
-
-This is a very bare bones module named ``example`` that provides
-``example.double(x)``. Note that the name of the module must be equal to the
-directory name and is also used in the name of the ``mp_obj_module_t`` object at
-the bottom.
-
-.. highlight:: c
-.. code::
-
-    // Include required definitions first.
-    #include "py/obj.h"
-    #include "py/runtime.h"
-
-    // This is the function you will call using example.double(n).
-    STATIC mp_obj_t example_double(mp_obj_t x_obj) {
-        // Check input value and convert it to a C type.
-        if (!MP_OBJ_IS_SMALL_INT(x_obj)) {
-            mp_raise_ValueError("x is not a small int");
-        }
-        int x = mp_obj_int_get_truncated(x_obj);
-
-        // Calculate the double, and convert back to MicroPython object.
-        return mp_obj_new_int(x + x);
-    }
-    STATIC MP_DEFINE_CONST_FUN_OBJ_1(example_double_obj, example_double);
-
-    // Define all properties of the example module, which currently are the name (a
-    // string) and a function.
-    // All identifiers and strings are written as MP_QSTR_xxx and will be
-    // optimized to word-sized integers by the build system (interned strings).
-    STATIC const mp_rom_map_elem_t example_module_globals_table[] = {
-        { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_example) },
-        { MP_ROM_QSTR(MP_QSTR_double), MP_ROM_PTR(&example_double_obj) },
-    };
-    STATIC MP_DEFINE_CONST_DICT(example_module_globals, example_module_globals_table);
-
-    // Define module object.
-    const mp_obj_module_t example_user_cmodule = {
-        .base = { &mp_type_module },
-        .globals = (mp_obj_dict_t*)&example_module_globals,
-    };
-
-
-Using a module
---------------
-
-To build such a module, compile MicroPython (see `getting started
-<https://github.com/micropython/micropython/wiki/Getting-Started>`_) with an
-extra ``make`` flag named ``USER_C_MODULES`` set to the directory containing
-all modules you want included (not to the module itself!). For example:
-
-.. highlight:: shell
-.. code::
-
-    $ make USER_C_MODULES=path-to-modules-folder all
diff --git a/docs/reference/index.rst b/docs/reference/index.rst
index e2e08a7f7..d0c7f69de 100644
--- a/docs/reference/index.rst
+++ b/docs/reference/index.rst
@@ -26,11 +26,3 @@ implementation and the best practices to use them.
    constrained.rst
    packages.rst
    asm_thumb2_index.rst
-   cmodules.rst
-
-.. only:: port_pyboard
-
-   .. toctree::
-      :maxdepth: 1
-
-      asm_thumb2_index.rst
diff --git a/mpy-cross/Makefile b/mpy-cross/Makefile
index c42c2c5ab..4ff96fc80 100644
--- a/mpy-cross/Makefile
+++ b/mpy-cross/Makefile
@@ -9,6 +9,8 @@ override undefine MICROPY_FORCE_32BIT
 override undefine CROSS_COMPILE
 override undefine FROZEN_DIR
 override undefine FROZEN_MPY_DIR
+override undefine USER_C_MODULES
+override undefine SRC_MOD
 override undefine BUILD
 override undefine PROG
 endif
diff --git a/ports/stm32/Makefile b/ports/stm32/Makefile
index 4fddc6c7d..b4c7a15c1 100644
--- a/ports/stm32/Makefile
+++ b/ports/stm32/Makefile
@@ -494,7 +494,7 @@ $(BUILD)/firmware.hex: $(BUILD)/firmware.elf
 
 $(BUILD)/firmware.elf: $(OBJ)
 	$(ECHO) "LINK $@"
-	$(Q)$(LD) $(LDFLAGS) -o $@ $^ $(LIBS)
+	$(Q)$(LD) $(LDFLAGS) -o $@ $^ $(LDFLAGS_MOD) $(LIBS)
 	$(Q)$(SIZE) $@
 
 PLLVALUES = boards/pllvalues.py
diff --git a/py/mkenv.mk b/py/mkenv.mk
index 5f4b11b6b..87e92ec6f 100644
--- a/py/mkenv.mk
+++ b/py/mkenv.mk
@@ -61,7 +61,6 @@ endif
 MAKE_FROZEN = $(PYTHON) $(TOP)/tools/make-frozen.py
 MPY_CROSS = $(TOP)/mpy-cross/mpy-cross
 MPY_TOOL = $(PYTHON) $(TOP)/tools/mpy-tool.py
-GEN_CMODULES = $(PYTHON) $(TOP)/tools/gen-cmodules.py
 
 all:
 .PHONY: all
diff --git a/py/mkrules.mk b/py/mkrules.mk
index 65d86834e..caa9527c7 100644
--- a/py/mkrules.mk
+++ b/py/mkrules.mk
@@ -20,12 +20,12 @@ endif
 # can be located. By following this scheme, it allows a single build rule
 # to be used to compile all .c files.
 
-vpath %.S . $(TOP)
+vpath %.S . $(TOP) $(USER_C_MODULES)
 $(BUILD)/%.o: %.S
 	$(ECHO) "CC $<"
 	$(Q)$(CC) $(CFLAGS) -c -o $@ $<
 
-vpath %.s . $(TOP)
+vpath %.s . $(TOP) $(USER_C_MODULES)
 $(BUILD)/%.o: %.s
 	$(ECHO) "AS $<"
 	$(Q)$(AS) -o $@ $<
@@ -42,14 +42,14 @@ $(Q)$(CC) $(CFLAGS) -c -MD -o $@ $<
   $(RM) -f $(@:.o=.d)
 endef
 
-vpath %.c . $(TOP)
+vpath %.c . $(TOP) $(USER_C_MODULES)
 $(BUILD)/%.o: %.c
 	$(call compile_c)
 
 QSTR_GEN_EXTRA_CFLAGS += -DNO_QSTR
 QSTR_GEN_EXTRA_CFLAGS += -I$(BUILD)/tmp
 
-vpath %.c . $(TOP)
+vpath %.c . $(TOP) $(USER_C_MODULES)
 
 $(BUILD)/%.pp: %.c
 	$(ECHO) "PreProcess $<"
@@ -105,7 +105,7 @@ endif
 ifneq ($(FROZEN_MPY_DIR),)
 # to build the MicroPython cross compiler
 $(TOP)/mpy-cross/mpy-cross: $(TOP)/py/*.[ch] $(TOP)/mpy-cross/*.[ch] $(TOP)/ports/windows/fmode.c
-	$(Q)$(MAKE) -C $(TOP)/mpy-cross USER_C_MODULES=
+	$(Q)$(MAKE) -C $(TOP)/mpy-cross
 
 # make a list of all the .py files that need compiling and freezing
 FROZEN_MPY_PY_FILES := $(shell find -L $(FROZEN_MPY_DIR) -type f -name '*.py' | $(SED) -e 's=^$(FROZEN_MPY_DIR)/==')
@@ -123,13 +123,6 @@ $(BUILD)/frozen_mpy.c: $(FROZEN_MPY_MPY_FILES) $(BUILD)/genhdr/qstrdefs.generate
 	$(Q)$(MPY_TOOL) -f -q $(BUILD)/genhdr/qstrdefs.preprocessed.h $(FROZEN_MPY_MPY_FILES) > $@
 endif
 
-# to build a list of modules for py/objmodule.c.
-ifneq ($(USER_C_MODULES),)
-$(BUILD)/genhdr/cmodules.h: | $(HEADER_BUILD)/mpversion.h
-	@$(ECHO) "GEN $@"
-	$(Q)$(GEN_CMODULES) $(USER_C_MODULES) > $@
-endif
-
 ifneq ($(PROG),)
 # Build a standalone executable (unix does this)
 
diff --git a/py/objmodule.c b/py/objmodule.c
index 04d210260..9191c73ec 100644
--- a/py/objmodule.c
+++ b/py/objmodule.c
@@ -31,6 +31,8 @@
 #include "py/runtime.h"
 #include "py/builtin.h"
 
+#include "genhdr/moduledefs.h"
+
 STATIC void module_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t kind) {
     (void)kind;
     mp_obj_module_t *self = MP_OBJ_TO_PTR(self_in);
diff --git a/py/py.mk b/py/py.mk
index ad7d12204..0fbc9f14b 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -131,9 +131,20 @@ endif
 
 # External modules written in C.
 ifneq ($(USER_C_MODULES),)
-CFLAGS_MOD += -DMICROPY_CMODULES_INCLUDE_H='"genhdr/cmodules.h"'
-include $(USER_C_MODULES)/*/micropython.mk
-SRC_QSTR += $(BUILD)/genhdr/cmodules.h
+# pre-define USERMOD variables as expanded so that variables are immediate 
+# expanded as they're added to them
+SRC_USERMOD := 
+CFLAGS_USERMOD :=
+LDFLAGS_USERMOD :=
+$(foreach module, $(wildcard $(USER_C_MODULES)/*/micropython.mk), \
+    $(eval USERMOD_DIR = $(patsubst %/,%,$(dir $(module))))\
+    $(info Including User C Module from $(USERMOD_DIR))\
+	$(eval include $(module))\
+)
+
+SRC_MOD += $(patsubst $(USER_C_MODULES)/%.c,%.c,$(SRC_USERMOD))
+CFLAGS_MOD += $(CFLAGS_USERMOD)
+LDFLAGS_MOD += $(LDFLAGS_USERMOD)
 endif
 
 # py object files
@@ -335,6 +346,8 @@ $(HEADER_BUILD)/moduledefs.h: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) | $(HEADER
 	@$(ECHO) "GEN $@"
 	$(Q)$(PYTHON) $(PY_SRC)/makemoduledefs.py --vpath="., $(TOP), $(USER_C_MODULES)" $(SRC_QSTR) > $@
 
+SRC_QSTR += $(HEADER_BUILD)/moduledefs.h
+
 # Force nlr code to always be compiled with space-saving optimisation so
 # that the function preludes are of a minimal and predictable form.
 $(PY_BUILD)/nlr%.o: CFLAGS += -Os
diff --git a/tools/gen-cmodules.py b/tools/gen-cmodules.py
deleted file mode 100755
index 524e3c03d..000000000
--- a/tools/gen-cmodules.py
+++ /dev/null
@@ -1,28 +0,0 @@
-#!/usr/bin/env python
-
-# Generate genhdr/cmodules.h for inclusion in py/objmodule.c.
-
-from __future__ import print_function
-
-import sys
-import os
-from glob import glob
-
-def update_modules(path):
-    modules = []
-    for module in sorted(os.listdir(path)):
-        if not os.path.isfile('%s/%s/micropython.mk' % (path, module)):
-            continue # not a module
-        modules.append(module)
-
-    # Print header file for all external modules.
-    print('// Automatically generated by genmodules.py.\n')
-    for module in modules:
-        print('extern const struct _mp_obj_module_t %s_user_cmodule;' % module)
-    print('\n#define MICROPY_EXTRA_BUILTIN_MODULES \\')
-    for module in modules:
-        print('    { MP_ROM_QSTR(MP_QSTR_%s), MP_ROM_PTR(&%s_user_cmodule) }, \\' % (module, module))
-    print()
-
-if __name__ == '__main__':
-    update_modules(sys.argv[1])