by Mitchell Schmeisser <email@example.com> — 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)!
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
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
read are provided by
Glibc on most Linux distributions.
In embedded systems smaller implementations like
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
work with our target architecture.
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
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 modules are defined in
the pattern of other module systems implemented by Guix.
First thing we need to build is the
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"))))
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
We can test our package definition using the
-L flag with
to add our packages.
guix build -L guix-zephyr zephyr-binutils /gnu/store/a947nb4rb2vymz2gaqnafgm1bsq4ipqp-zephyr-binutils-2.38
This directory contains the results of
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.
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
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
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"))
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
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
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
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)))
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
directory with all our build artifacts. The SDK is installed correctly!