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+# Table of Contents
+
+1.  [Overview](#org2633a51)
+2.  [Anatomy of a toolchain](#orgc440e9e)
+3.  [Bootstrapping a Toolchain](#orgd42b6c3)
+4.  [Defining the Packages](#org55042c5)
+	1.  [Binutils](#org67da1ec)
+	2.  [GCC sans libc](#org82d6f83)
+	3.  [Newlib(-nano)](#orgf6bafbc)
+	4.  [Complete toolchain](#org052f2a2)
+5.  [Integrating with Zephyr Build System](#orgc3f87f4)
+	1.  [Testing](#org9f3c314)
+
+All code is available at [guix-zephyr](https://github.com/paperclip4465/guix-zephyr) channel.
+
+
+<a id="org2633a51"></a>
+
+# Overview
+
+In order to deploy embedded software using Guix we first need to teach Guix
+how to build it. Since Guix bootstraps everything this means we must teach Guix
+how to build our toolchain.
+
+The [Zephyr Project](https://zephyrproject.org) uses its own fork of GCC with custom configs for
+the architectures supported by the project.
+
+
+<a id="orgc440e9e"></a>
+
+# Anatomy of a toolchain
+
+Toolchains are responsible for taking high level descriptions of programs
+and lowering them down to a series of equivalent machine instructions.
+This process involves more than just a compiler. The compiler uses the `binutils`
+to manipulate it's internal representation down to a given architecture.
+It also needs the use of the C standard library as well as a few other libraries
+needed for some compiler optimizations.
+
+The C library provides the interface to the underlying kernel. System calls like `write`
+and `read` are provided by `Glibc` on most Linux distributions.
+
+In embedded systems smaller implementations like `newlib` and `newlib-nano` are used.
+
+
+<a id="orgd42b6c3"></a>
+
+# Bootstrapping a Toolchain
+
+In order to compile GCC we need a C library that's been compiled for
+our target architecture. How can we cross compile our C library if we
+need our C library to build a cross compiler? The solution is to build
+a simpler compiler that doesn't require the C library to function.
+It will not be capable of as many optimizations and it will be very slow,
+however it will be able to build the C libraries as well as the complete version
+of GCC.
+
+In order to build the simpler compiler we need to compile the `binutils` to
+work with our target architecture.
+The `binutils` can be bootstrapped with our host GCC and have no target dependencies.
+
+[For more information read this.](https://crosstool-ng.github.io/docs/toolchain-construction/)
+
+Doesn't sound so bad right? It isn't&#x2026; in theory.
+However internet forums since time immemorial have been
+littered with the laments of those who came before.
+From incorrect versions of `ISL` to the wrong C library being linked
+or the host linker being used, etc.
+The one commonality between all of these issues is the environment.
+Building a cross toolchain is difficult because isolating build
+environments is difficult.
+
+In fact as of `v0.14.2` the zephyr SDK repository took down the build
+instructions and posted a sign that read
+"Building this is too complicated, don't worry about it."<sup><a id="fnr.1" class="footref" href="#fn.1" role="doc-backlink">1</a></sup>
+
+We will neatly side step all of these problems and not
+risk destroying or polluting our host system with garbage
+by using Guix to manage our environments for us.
+
+Our toolchain only requires the first pass compiler because
+newlib(-nano) is statically linked and introduced to the toolchain
+by normal package composition.
+
+
+<a id="org55042c5"></a>
+
+# Defining the Packages
+
+All of the base packages are defined in `zephyr/packages/zephyr.scm`.
+Zephyr modules are defined in `zephyr/packages/zephyr-xyz.scm`, following
+the pattern of other module systems implemented by Guix.
+
+
+<a id="org67da1ec"></a>
+
+## Binutils
+
+First thing we need to build is the `arm-zephyr-eabi` binutils.
+This is very easy in Guix.
+
+	(define-module (zephyr packages zephyr)
+	  #:use-module (guix packages))
+
+	(define-public arm-zephyr-eabi-binutils
+	  (let ((xbinutils (cross-binutils "arm-zephyr-eabi")))
+		(package
+		  (inherit xbinutils)
+		  (name "arm-zephyr-eabi-binutils")
+		  (version "2.38")
+		  (source
+		   (origin (method git-fetch)
+				   (uri (git-reference
+						 (url "https://github.com/zephyrproject-rtos/binutils-gdb")
+						 (commit "6a1be1a6a571957fea8b130e4ca2dcc65e753469")))
+				   (file-name (git-file-name name version))
+				   (sha256 (base32 "0ylnl48jj5jk3jrmvfx5zf8byvwg7g7my7jwwyqw3a95qcyh0isr"))))
+		  (arguments
+		   `(#:tests? #f
+			 ,@(substitute-keyword-arguments (package-arguments xbinutils)
+				 ((#:configure-flags flags)
+				  `(cons "--program-prefix=arm-zephyr-eabi-" ,flags)))))
+		  (native-inputs
+		   (append
+			(list texinfo
+				  bison
+				  flex
+				  gmp
+				  dejagnu)
+			(package-native-inputs xbinutils)))
+		  (home-page "https://zephyrproject.org")
+		  (synopsis "binutils for zephyr RTOS"))))
+
+The function `cross-binutils` returns a package which has been
+configured for the given gnu triplet.  We simply inherit that package
+and replace the source.
+The zephyr build system expects the binutils to be prefixed with
+`arm-zephyr-eabi-` which is accomplished by adding another flag to the
+`#:configure-flags` argument.
+
+We can test our package definition using the `-L` flag with `guix build`
+to add our packages.
+
+	guix build -L guix-zephyr zephyr-binutils
+
+	/gnu/store/...-zephyr-binutils-2.38
+
+This directory contains the results of `make install`.
+
+
+<a id="org82d6f83"></a>
+
+## GCC sans libc
+
+This one is a bit more involved. Don't be afraid!
+This version of GCC wants ISL version 0.15. It's easy enough
+to make that happen. Inherit the current version of ISL and swap
+out the source and update the version. For most packages the build process doesn't
+change that much between versions.
+
+	(define-public isl-0.15
+	  (package
+		(inherit isl)
+		(version "0.15")
+		(source (origin
+				  (method url-fetch)
+				  (uri (list (string-append "mirror://sourceforge/libisl/isl-"
+											version ".tar.gz")))
+				  (sha256
+				   (base32
+					"11vrpznpdh7w8jp4wm4i8zqhzq2h7nix71xfdddp8xnzhz26gyq2"))))))
+
+Like the binutils, there is a function for creating cross-gcc packages.
+This one accepts keywords specifying which binutils and libc to use.
+If libc isn't given (like here), gcc is configured with many options disabled
+to facilitate being built without libc. Therefore we need to add the extra options
+we want <sup><a id="fnr.2" class="footref" href="#fn.2" role="doc-backlink">2</a></sup>
+
+	(define-public gcc-arm-zephyr-eabi-12
+	  (let ((xgcc (cross-gcc "arm-zephyr-eabi"
+							 #:xbinutils zephyr-binutils)))
+		(package
+		  (inherit xgcc)
+		  (version "12.1.0")
+		  (source (origin (method git-fetch)
+						  (uri (git-reference
+								(url "https://github.com/zephyrproject-rtos/gcc")
+								(commit "0218469df050c33479a1d5be3e5239ac0eb351bf")))
+						  (file-name (git-file-name (package-name xgcc) version))
+						  (sha256
+						   (base32 "1s409qmidlvzaw1ns6jaanigh3azcxisjplzwn7j2n3s33b76zjk"))
+						  (patches
+						   (search-patches "gcc-12-cross-environment-variables.patch"
+										   "gcc-cross-gxx-include-dir.patch"))))
+		  (native-inputs
+		   (modify-inputs (package-native-inputs xgcc)
+			 ;; Get rid of stock ISL
+			 (delete "isl")
+			 ;; Add additional dependencies that xgcc doesn't have
+			 ;; including our special ISL
+			 (prepend flex
+					  perl
+					  python-3
+					  gmp
+					  isl-0.15
+					  texinfo
+					  python
+					  mpc
+					  mpfr
+					  zlib)))
+		  (arguments
+		   (substitute-keyword-arguments (package-arguments xgcc)
+			 ((#:phases phases)
+			  `(modify-phases ,phases
+				 (add-after 'unpack 'fix-genmultilib
+				   (lambda _
+					 (substitute* "gcc/genmultilib"
+					   (("#!/bin/sh") (string-append "#!" (which "sh"))))
+					 #t))
+
+				 (add-after 'set-paths 'augment-CPLUS_INCLUDE_PATH
+				   (lambda* (#:key inputs #:allow-other-keys)
+					 (let ((gcc (assoc-ref inputs  "gcc")))
+					   ;; Remove the default compiler from CPLUS_INCLUDE_PATH to
+					   ;; prevent header conflict with the GCC from native-inputs.
+					   (setenv "CPLUS_INCLUDE_PATH"
+							   (string-join
+								(delete (string-append gcc "/include/c++")
+										(string-split (getenv "CPLUS_INCLUDE_PATH")
+													  #\:))
+								":"))
+					   (format #t
+							   "environment variable `CPLUS_INCLUDE_PATH' changed to ~a~%"
+							   (getenv "CPLUS_INCLUDE_PATH"))
+					   #t)))))
+
+			 ((#:configure-flags flags)
+			  ;; The configure flags are largely identical to the flags used by the
+			  ;; "GCC ARM embedded" project.
+			  `(append (list "--enable-multilib"
+							 "--with-newlib"
+							 "--with-multilib-list=rmprofile"
+							 "--with-host-libstdcxx=-static-libgcc -Wl,-Bstatic,-lstdc++,-Bdynamic -lm"
+							 "--enable-plugins"
+							 "--disable-decimal-float"
+							 "--disable-libffi"
+							 "--disable-libgomp"
+							 "--disable-libmudflap"
+							 "--disable-libquadmath"
+							 "--disable-libssp"
+							 "--disable-libstdcxx-pch"
+							 "--disable-nls"
+							 "--disable-shared"
+							 "--disable-threads"
+							 "--disable-tls"
+							 "--with-gnu-ld"
+							 "--with-gnu-as"
+							 "--enable-initfini-array")
+					   (delete "--disable-multilib" ,flags)))))
+		  (native-search-paths
+		   (list (search-path-specification
+				  (variable "CROSS_C_INCLUDE_PATH")
+				  (files '("arm-zephyr-eabi/include")))
+				 (search-path-specification
+				  (variable "CROSS_CPLUS_INCLUDE_PATH")
+				  (files '("arm-zephyr-eabi/include"
+						   "arm-zephyr-eabi/c++"
+						   "arm-zephyr-eabi/c++/arm-zephyr-eabi")))
+				 (search-path-specification
+				  (variable "CROSS_LIBRARY_PATH")
+				  (files '("arm-zephyr-eabi/lib")))))
+		  (home-page "https://zephyrproject.org")
+		  (synopsis "GCC for zephyr RTOS"))))
+
+This GCC can be built like so.
+
+	guix build -L guix-zephyr gcc-cross-sans-libc-arm-zephyr-eabi
+
+	/gnu/store/...-gcc-cross-sans-libc-arm-zephyr-eabi-12.1.0-lib
+	/gnu/store/...-gcc-cross-sans-libc-arm-zephyr-eabi-12.1.0
+
+Great! We now have our stage-1 compiler.
+
+
+<a id="orgf6bafbc"></a>
+
+## Newlib(-nano)
+
+The newlib package package is quite straight forward (relatively).
+It is mostly adding in the relevent configuration flags and patching
+the files the `patch-shebangs` phase missed.
+
+	  (define-public zephyr-newlib
+	  (package
+		(name "zephyr-newlib")
+		(version "3.3")
+		(source (origin
+				  (method git-fetch)
+				  (uri (git-reference
+						(url "https://github.com/zephyrproject-rtos/newlib-cygwin")
+						(commit "4e150303bcc1e44f4d90f3489a4417433980d5ff")))
+				  (sha256
+				   (base32 "08qwjpj5jhpc3p7a5mbl7n6z7rav5yqlydqanm6nny42qpa8kxij"))))
+		(build-system gnu-build-system)
+		(arguments
+		 `(#:out-of-source? #t
+		   #:configure-flags '("--target=arm-zephyr-eabi"
+							   "--enable-newlib-io-long-long"
+							   "--enable-newlib-io-float"
+							   "--enable-newlib-io-c99-formats"
+							   "--enable-newlib-retargetable-locking"
+							   "--enable-newlib-lite-exit"
+							   "--enable-newlib-multithread"
+							   "--enable-newlib-register-fini"
+							   "--enable-newlib-extra-sections"
+							   "--disable-newlib-wide-orient"
+							   "--disable-newlib-fseek-optimization"
+							   "--disable-newlib-supplied-syscalls"
+							   "--disable-newlib-target-optspace"
+							   "--disable-nls")
+		   #:phases
+		   (modify-phases %standard-phases
+			 (add-after 'unpack 'fix-references-to-/bin/sh
+			   (lambda _
+				 (substitute* '("libgloss/arm/cpu-init/Makefile.in"
+								"libgloss/arm/Makefile.in"
+								"libgloss/libnosys/Makefile.in"
+								"libgloss/Makefile.in")
+				   (("/bin/sh") (which "sh")))
+				 #t)))))
+		(native-inputs
+		 `(("xbinutils" ,zephyr-binutils)
+		   ("xgcc" ,gcc-arm-zephyr-eabi-12)
+		   ("texinfo" ,texinfo)))
+		(home-page "https://www.sourceware.org/newlib/")
+		(synopsis "C library for use on embedded systems")
+		(description "Newlib is a C library intended for use on embedded
+	systems.  It is a conglomeration of several library parts that are easily
+	usable on embedded products.")
+		(license (license:non-copyleft
+				  "https://www.sourceware.org/newlib/COPYING.NEWLIB"))))
+
+And the build.
+
+	guix build -L guix-zephyr zephyr-newlib
+
+	/gnu/store/...-zephyr-newlib-3.3
+
+
+<a id="org052f2a2"></a>
+
+## Complete toolchain<sup><a id="fnr.3" class="footref" href="#fn.3" role="doc-backlink">3</a></sup>
+
+Now that we've got the individual tools it's time to create our complete toolchain.
+For this we need to do some package transformations.
+Because these transformations are going to have to be done for every combination of
+gcc/newlib it is best to create a function which we can reuse for every version
+of the SDK.
+
+	(define (arm-zephyr-eabi-toolchain xgcc newlib version)
+	  "Produce a cross-compiler zephyr toolchain package with the compiler XGCC and the C
+	library variant NEWLIB."
+	  (let ((newlib-with-xgcc (package (inherit newlib)
+									   (native-inputs
+										(alist-replace "xgcc" (list xgcc)
+													   (package-native-inputs newlib))))))
+		(package
+		  (name (string-append "arm-zephyr-eabi"
+							   (if (string=? (package-name newlib-with-xgcc)
+											 "newlib-nano")
+								   "-nano" "")
+							   "-toolchain"))
+		  (version version)
+		  (source #f)
+		  (build-system trivial-build-system)
+		  (arguments
+		   '(#:modules ((guix build union)
+						(guix build utils))
+			 #:builder
+			 (begin
+			   (use-modules (ice-9 match)
+							(guix build union)
+							(guix build utils))
+			   (let ((out (assoc-ref %outputs "out")))
+				 (mkdir-p out)
+				 (match %build-inputs
+				   (((names . directories) ...)
+					(union-build (string-append out "/arm-zephyr-eabi")
+								 directories)
+					#t))))))
+		  (inputs
+		   `(("binutils" ,zephyr-binutils)
+			 ("gcc" ,xgcc)
+			 ("newlib" ,newlib-with-xgcc)))
+		  (synopsis "Complete GCC tool chain for ARM zephyrRTOS development")
+		  (description "This package provides a complete GCC tool chain for ARM
+	bare metal development with zephyr rtos.  This includes the GCC arm-zephyr-eabi cross compiler
+	and newlib (or newlib-nano) as the C library.  The supported programming
+	language is C.")
+		  (home-page (package-home-page xgcc))
+		  (license (package-license xgcc)))))
+
+This function creates a special package which consists of the toolchain in a special directory hierarchy, i.e `arm-zephyr-eabi/`.
+Our complete toolchain definition looks like this.
+
+	(define-public arm-zephyr-eabi-toolchain-0.15.0
+	  (arm-zephyr-eabi-toolchain
+	   gcc-arm-zephyr-eabi-12
+	   zephyr-newlib
+	   "0.15.0"))
+
+To build:
+
+	guix build -L guix-zephyr arm-zephyr-eabi-toolchain
+
+	/gnu/store/...-arm-zephyr-eabi-toolchain-0.15.0
+
+
+<a id="orgc3f87f4"></a>
+
+# Integrating with Zephyr Build System
+
+Zephyr uses CMake as it's build system. It contains numerous CMake
+files in both the so-called `ZEPHYR_BASE`, the zephyr source code
+repository, as well as a handful in the SDK
+which help select the correct toolchain for a given board.
+
+There are standard locations the build system will look for the
+SDK. We are not using any of them.  Our SDK lives in the store,
+immutable forever. According to [this](https://docs.zephyrproject.org/latest/develop/west/without-west.html) the variable
+`ZEPHYR_SDK_INSTALL_DIR` needs to point to our custom spot.
+
+We also need to grab the cmake files from the [repository](https://github.com/zephyrproject-rtos/sdk-ng) and create a
+file `sdk_version` which contains the version string `ZEPHYR_BASE`
+uses to find a compatible SDK.
+
+Along with the SDK proper we need to include a number of python
+packages required by the build system<sup><a id="fnr.4" class="footref" href="#fn.4" role="doc-backlink">4</a></sup>.
+
+	(define-public zephyr-sdk
+	  (package
+		(name "zephyr-sdk")
+		(version "0.15.0")
+		(home-page "https://zephyrproject.org")
+		(source (origin (method git-fetch)
+						(uri (git-reference
+							  (url "https://github.com/zephyrproject-rtos/sdk-ng")
+							  (commit "v0.15.0")))
+						(file-name (git-file-name name version))
+						(sha256 (base32 "04gsvh20y820dkv5lrwppbj7w3wdqvd8hcanm8hl4wi907lwlmwi"))))
+		(build-system trivial-build-system)
+		(arguments
+		 `(#:modules ((guix build union)
+					  (guix build utils))
+		   #:builder
+		   (begin
+			 (use-modules (guix build union)
+						  (ice-9 match)
+						  (guix build utils))
+			 (let* ((out (assoc-ref %outputs "out"))
+					(cmake-scripts (string-append (assoc-ref %build-inputs "source")
+												  "/cmake"))
+					(sdk-out (string-append out "/zephyr-sdk-0.15.0")))
+			   (mkdir-p out)
+
+			   (match (assoc-remove! %build-inputs "source")
+				 (((names . directories) ...)
+				  (union-build sdk-out directories)))
+
+			   (copy-recursively cmake-scripts
+								 (string-append sdk-out "/cmake"))
+
+			   (with-directory-excursion sdk-out
+				 (call-with-output-file "sdk_version"
+				   (lambda (p)
+					 (format p "0.15.0")))
+				 #t)))))
+		(propagated-inputs
+		 (list
+		  arm-zephyr-eabi-toolchain-0.15.0
+		  zephyr-binutils
+		  python-3
+		  python-pyelftools
+		  python-pykwalify
+		  python-pyyaml
+		  python-packaging
+		  dtc))
+		(native-search-paths
+		 (list (search-path-specification
+				(variable "ZEPHYR_SDK_INSTALL_DIR")
+				(files '("")))))
+		(synopsis "SDK for zephyrRTOS")
+		(description "zephyr-sdk contains bundles a complete gcc toolchain as well
+		as host tools like dtc, openocd, qemu, and required python packages.")
+		(license license:apsl2)))
+
+
+<a id="org9f3c314"></a>
+
+## Testing
+
+In order to test we will need an environment with the SDK installed.
+We can take advantage of `guix shell` to avoid installing test
+packages into our home environment. This way if it causes problems we
+can just exit the shell and try again.
+
+	guix shell -L guix-zephyr zephyr-sdk cmake ninja git
+
+`ZEPHYR_BASE` can be cloned into a temporary workspace to test our
+toolchain functionality.<sup><a id="fnr.5" class="footref" href="#fn.5" role="doc-backlink">5</a></sup>
+
+	mkdir /tmp/zephyr-project
+	cd /tmp/zephyr-project
+	git clone https://github.com/zephyrproject-rtos/zephyr
+	export ZEPHYR_BASE=/tmp/zephyr-project/zephyr
+
+In order to build for the test board (k64f in this case) we need to
+get a hold of the vendor Hardware Abstraction Layers and CMSIS.<sup><a id="fnr.6" class="footref" href="#fn.6" role="doc-backlink">6</a></sup>
+
+	git clone https://github.com/zephyrproject-rtos/hal_nxp &&
+	git clone https://github.com/zephyrproject-rtos/cmsis
+
+To inform the build system about this module we pass it in with
+`-DZEPHYR_MODULES=` which is a semicolon separated list of paths
+containing a module.yml file.
+
+To build the hello world sample we use the following incantation.
+
+	cmake -Bbuild $ZEPHYR_BASE/samples/hello_world \
+		  -GNinja \
+		  -DBOARD=frdm_k64f \
+		  -DBUILD_VERSION=3.1.0 \
+		  -DZEPHYR_MODULES="/tmp/zephyr-project/hal_nxp;/tmp/zephyr-project/cmsis" \
+		&& ninja -Cbuild
+
+If everything is set up correctly we will end up with a `./build`
+directory with all our build artifacts. The SDK is installed correctly!
+
+
+# Footnotes
+
+<sup><a id="fn.1" href="#fnr.1">1</a></sup> I'm paraphrasing, but [not by much](https://github.com/zephyrproject-rtos/sdk-ng/tree/v0.14.2#build-process).
+
+<sup><a id="fn.2" href="#fnr.2">2</a></sup> I got them from the SDK configuration scripts on the [sdk github](https://github.com/zephyrproject-rtos/sdk-ng) as well as the
+commits to use for each of the tools.
+
+<sup><a id="fn.3" href="#fnr.3">3</a></sup> *Mostly* complete. libstdc++ does not build because
+\`arm-zephyr-eabi\` is not \`arm-none-eabi\` so a dynamic link check is
+performed/failed. I cannot figure out how crosstool-ng handles this.
+
+<sup><a id="fn.4" href="#fnr.4">4</a></sup> This is no longer the case. Now python packages are provided by the zephyr package
+which was not available when this was being developed.
+
+<sup><a id="fn.5" href="#fnr.5">5</a></sup> For now. Eventually we will need to create a package for `zephyr-base` that
+our guix zephyr-build-system can use.
+
+<sup><a id="fn.6" href="#fnr.6">6</a></sup> These will also need to become guix packages to allow the build system to compose modules.
