# Building Toolchains with Guix

### by Mitchell Schmeisser <mitchellschmeisser@librem.one> — February 23, 2023

Today's post is a guest post from my new internet friend Mitchell. We met on the #guix irc channel, and I offered to post a few of his blog posts on this blog. Without further ado, here is Michell's first blog post (it's pretty fantastic)!

# 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 uses its own fork of GCC with custom configs for the architectures supported by the project.

# 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 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.

# 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.

Doesn’t sound so bad right? It isn’t… 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 GCC is difficult because isolating build environments is hard.

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.” (I’m paraphrasing, but not by much.)

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.

# 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.

## 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/a947nb4rb2vymz2gaqnafgm1bsq4ipqp-zephyr-binutils-2.38

This directory contains the results of `make install`.

## 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 (I got them from the SDK configuration scripts on the sdk github as well as the commits to use for each of the tools. ).

    (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/qmp8bzmwwimw0r6fh165hgfhkxkxilpj-gcc-cross-sans-libc-arm-zephyr-eabi-12.1.0-lib
    /gnu/store/38rli0rbn7ksmym3wq99cr4p2cjdz4a7-gcc-cross-sans-libc-arm-zephyr-eabi-12.1.0

Great! We now have our stage-1 compiler.

## Newlib(-nano)

The newlib 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/4lx37gga1jv3ckykrxsfgwy9slaamln4-zephyr-newlib-3.3

## Complete toolchain

Note that the toolchain is 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.

Anyway, 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 must be done for every combination of binutils/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/9jnanr27v6na5qq3dlgljraysn8r1sad-arm-zephyr-eabi-toolchain-0.15.0

# 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 webpage, the variable `ZEPHYR_SDK_INSTALL_DIR` needs to point to our custom spot.

We also need to grab the cmake files from the repository 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.

      (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
          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)))

## 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 (For now. Eventually we will need to create a package for `zephyr-base` that our guix zephyr-build-system can use).

    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 (These will also need to become guix packages to allow the build system to compose modules).

    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!