Case study of interweaving zephyr and zig

Motivation

Zephyr is a mature RTOS which is written in C, Zig is a new envolving programming language.

We like to investigate the possibilities of using Zig in the context of Zephyr. This could mean:

• writing the application in Zig
• write Zephyr drivers or modules in Zig
• use Zig as Build System

Could this be achieved in a minimal invasive approache?

One existing example for an aplication written in Zig using Zephyr can be found here: zig-zephyr-hello . What could be improved to get a smoother interaction without forcing us to write wrappers in C to make use of Zephyr from Zig.

• [X] Devicetree is translated to Zig because C Devicetree macros are not usable from Zig.
• [X] Writing a Zephyr application in Zig, using C imported types and functions.
• [X] Compiling Zephyr using zig cc as toolchain (linking zephyr_pre0.elf step does not work).
• [ ] Translate KConfig to Zig
• [ ] Building Zephyr using the Zig build system instead of CMake

• A working Zephyr setup. I set it up as I have described it here. I used the zephyr sdk as non-zig toolchain.
• In order to compile the Zig example the file

  zephyrproject/zephyr/scripts/build/gen_syscalls.py
      

  has to be patched by moving the line

  wrap += "\t" + "compiler_barrier();\n"
      

  just before the line:

  wrap += "#endif\n"
      

  or removing it at all. The reason for this is propably a bug in Zig which refuses to inline the syscall functions if the compiler (memory) barrier is there an results in undefined references.

• An Arm Board for testing. For other boards the file of this repository

  zig-zephyr/cmake/compiler/zig/zig-target.cmake
      

  could be extended.

• If using zig cc as toolchain, then the path of the zephyr sdk has to be adapted to your installation in the file CMakeLists.txt:

  set(ZEPHYR_SDK_INSTALL_DIR $ENV{HOME}/bin/zephyr-sdk-0.15.1)
      

  and the file zephyr-sdk-gcc.

• The file src/c.zig contains a board (in our case for adafruit_feather_nrf52840) specific define:

  @cDefine("NRF52840_XXAA", "");
      

  this should be changed to suits your board.

You have to adapt the commands for your board, I used the adafruit_feather_nrf52840 board. The C application (~src/main.c) is the blinky example taken from the Zephyr samples. The Zig application (~src/main.zig) is the equivalent in zig.

• Compile main.c using Zephyr sdk

  west build -d build -b adafruit_feather_nrf52840 -p
      

• Compile main.c using zig cc

  west build -d build -b adafruit_feather_nrf52840 -p -- -DZIGCC=1
      

• Compile main.zig using Zephyr sdk

  west build -d build -b adafruit_feather_nrf52840 -p -- -DZIGMAIN=1
      

• Compile main.zig using zig cc

  west build -d build -b adafruit_feather_nrf52840 -p -- -DZIGCC=1 -DZIGMAIN=1
      

Is done like usual by flashing the image to the board, for example:

west flash -r blackmagicprobe --gdb-serial /dev/ttyACM0 --skip-rebuild --elf-file build/zephyr/zephyr.elf 

Inspired from Lup Yuen Lee’s Article about Building an App with Zig and Apache NuttX RTOS we will try to translate C Code to Zig using zig translate-c. We have to do this in the context of the Zephyr Build process, all compiler flags have to be the same like for invoking the C compiler. Therefore I started with:

We try to setup a toolchain which uses zig cc as C compiler.

By following the Custom CMake Guide of Zephyr we use this repository as TOOLCHAIN_ROOT with the cmake files under the cmake directory and setting the TOOLCHAIN_VARIANT to zig.

For the generic (host) part of the build we use the zephyr-sdk toolchain. We just direct some files to the correspondig files in ZEPHYR_BASE. Zig specific settings are in cmake/compiler/zig/target.cmake. I just tried it for an arm board so it is written for satisfy this needs.

The critical step was the Unfixed size binary Build Step which builds zephyr_pre0.elf. If we use zigs clang it stops at

[149/159] Linking C executable zephyr/zephyr_pre0.elf

and tells

ld.lld: error: cannot find linker script -Map

The C compiler is called for this step.

Then I tried to use the zephyr-sdk-gcc for this step by writing a conditional clause in zigcc, which is the wrapper for zig cc.

If we do this we have no complaining about the arguments but get an

undefined reference to `__aeabi_memclr8'

This could be solved by adding -lc_nano to the compiler ars which links in the nano libc.

To view the invocation of the compiler do:

cd build
ninja -v

I used zig translate-c with the same compiler arguments we found Zephyr was using to compile main.c. From this I extracted the relevant part for main.zig. In this form all macros were already expanded. With some CMake wizardry I managed it to fit the compiled object file into the Zephyr app application (see zig.cmake) Then I was faced with the compiler_barrier() issue, mentioned in the *Preconditions. I was happy to got it to compile, but the expanded macros were not usable from a programmers point of view. Thats way I

My aim was to have an easy usable perdant to the devicetree_generated.h header file in Zig. Whereas the header file is an artwork of encoding the devicetree data into C Preprocessor Macros my goal was to code the devicetree data in a clean and simple way which could be human viewable and usable.

Therefore Zig’s reach possibilities to create and initialize struct’s were realy useful. We got a tree which looks nearly like the zepyr.dts:

pub const soc = struct {
  const gpio_50000300 = .{
    ._device = @as([*c]const c.struct_device, &c.__device_dts_ord_10),
    .reg = [_]u32{0x50000300, 0x200, 0x50000800, 0x300},
    .port = @as( u32, 1),
    .gpio_controller = true,
    .ngpios = @as( u32, 16),
    .status = "okay",
    .compatible = [_][]const u8{"nordic,nrf-gpio"},
    .wakeup_source = false,
  };
};
pub const leds = .{
  .compatible = [_][]const u8{"gpio-leds"},
  // 16
  .led_0 = .{
    .gpios = .{.ph=&soc.gpio_50000300,.pin=@as( u32, 15),.flags=@as( u32, 0)},
    .label = "Red LED",
  },
};

In this tree the phandles are real references of their targets. Thus refer to gpios of led_0 is as easy as writing:

dt.leds.led_0.gpios

which is the analog of Zephyrs:

 GPIO_DT_SPEC_GET( DT_PATH( leds, led_0), gpios)

Right now not all properties are translated, but to complete it is not too difficult. (Aliases, Labels, Memory Maps for example)