Overview

gokrazy packs your Go application(s) into an SD card image for the Raspberry Pi 3 which — aside from the Linux kernel and proprietary Raspberry Pi bootloader — only contains Go software.

The motivation is that @stapelberg spends way more time on C software and their various issues than he would like. Hence, he is going Go-only where feasible.

Usage

Installation

Install Go if you haven’t already:

sudo apt install golang-go
export GOPATH=~/go
export PATH=$GOPATH/bin:$PATH

Then, use the go tool to download and install gokr-packer:

go get -u github.com/gokrazy/tools/cmd/gokr-packer

Overwriting an SD card for the Raspberry Pi 3

To re-partition and overwrite the SD card /dev/sdb, use:

sudo setcap CAP_SYS_ADMIN,CAP_DAC_OVERRIDE=ep ~/go/bin/gokr-packer
gokr-packer -overwrite=/dev/sdb github.com/gokrazy/hello

Then, put the SD card into your Raspberry Pi 3 and power it up! Once the Raspberry Pi 3 has booted (takes about 10 seconds), you should be able to reach the gokrazy web interface at the URL which gokr-packer printed.

Under the hood, gokr-packer…

1. …packed the latest firmware and kernel binaries into the boot file system.

2. …built the specified Go packages using go install and packed all their binaries into the /user directory of the root file system.

3. …created a minimal gokrazy init program which supervises all binaries (i.e. restarts them when they exit).

Updating your installation

To update gokrazy, including the firmware and kernel binaries, use:

go get -u github.com/gokrazy/tools/cmd/gokr-packer

To update your gokrazy installation (running on a Raspberry Pi 3), use:

gokr-packer -update=http://gokrazy:mysecretpassword@gokrazy/ github.com/gokrazy/hello

SD card contents

gokrazy uses the following partition table:

num	size	purpose	file system
1	100 MB	boot (kernel+firmware)	FAT16B
2	500 MB	root2 (gokrazy+apps)	FAT16B (but see issue #10)
3	500 MB	root3 (gokrazy+apps)	FAT16B (but see issue #10)
4	rest	permanent data	ext4

The two root partitions are used alternatingly (to avoid modifying the currently active file system) when updating.

If you’d like to store permanent data (i.e. data which will not be overwritten on the next update), you’ll need to create an ext4 file system on the last partition. If your SD card is /dev/sdb, use mkfs.ext4 /dev/sdb4.

Customization

Changing program behavior for gokrazy

gokr-packer sets the “gokrazy” build tag for conditional compilation.

You can find an example commit which implements a gokrazy-specific controller that triggers the main program logic every weekday at 10:00 at 6f90ace359

Changing init behavior

gokr-packer \
  -overwrite_init=$(go env GOPATH)/src/github.com/stapelberg/mediaserver/cmd/init/init.go \
  github.com/gokrazy/hello

(Note that the package must result in a binary called “init”.)

Then, edit the github.com/stapelberg/mediaserver package to your liking. When done, pack an image with your own init package:

gokr-packer \
  -init_pkg=github.com/stapelberg/mediaserver \
  -overwrite=/dev/sdb \
  github.com/gokrazy/hello

Repository structure

• github.com/gokrazy/gokrazy: system code, main issue tracker, documentation
• github.com/gokrazy/tools: SD card image creation code, pulling in:
  • github.com/gokrazy/firmware: Raspberry Pi 3 firmware files
  • github.com/gokrazy/kernel: pre-built kernel image and bootloader config