Rapsberry Pi 0 - U-Boot and (rpi foundation) kernel

Toolchain

Note: if you built your own toolchain using toolchain-ng, you can of course use that. There are however some pre-built toolchains in the raspberry repos:

GCC 4.7

wget -Oraspberrypi-tools.tar.gz  https://github.com/raspberrypi/tools/archive/master.tar.gz
tar xf raspberrypi-tools.tar.gz
# remove the "-x64" for 32bit systems
export PATH=$PWD/tools-master/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin/:$PATH

newer GCC 4.9 (64bit only)

wget -Oraspberrypi-tools.tar.gz  https://github.com/raspberrypi/tools/archive/master.tar.gz
tar xf raspberrypi-tools.tar.gz
export PATH=$PWD/tools-master/arm-bcm2708/arm-rpi-4.9.3-linux-gnueabihf/bin/:$PATH

Firmware tools (bootloader etc):

https://github.com/raspberrypi/firmware

mkdir -p firmware
cd firmware
wget https://github.com/raspberrypi/firmware/raw/master/boot/bootcode.bin
wget https://github.com/raspberrypi/firmware/raw/master/boot/fixup.dat
wget https://github.com/raspberrypi/firmware/raw/master/boot/start.elf
because of enable_uart=1, see below
wget https://github.com/raspberrypi/firmware/raw/master/boot/bcm2708-rpi-zero-w.dtb

# insert sdcard, mount...
cp bootcode.bin fixup.dat start.elf /run/media/$USER/boot/

U-Boot

Note: we don't really need u-boot to boot the kernel on raspberry, the bootcode.bin/start.elf bootloader is capable of loading it, but u-boot gives us more flexibility (loading kernels from usb/network, manually booting with different commandlines without having to edit config.txt etc.)

git clone git://git.denx.de/u-boot.git
cd u-boot

# check out a version that does not need GCC 6
# (you can use master if you compiled your own toolchain with newer GCC)
git checkout -b build v2017.11

# ls configs/*rpi*

export CROSS_COMPILE=arm-linux-gnueabihf-

# rpi_2_defconfig for rpi2
make rpi_defconfig
make menuconfig
time ionice -c3 nice -n10 make  -j5
# i5-2520M  (4core): real   0m22.268s    user    0m58.988s   sys    0m7.912s
# i7-8750H (12core): real    0m6.128s    user    0m48.096s   sys    0m5.251s
# TODO might need apt install libssl-dev bc

# insert sdcard, mount...
# we "pretend" u-boot is kernel ;)
cp u-boot.bin  /run/media/$USER/boot/kernel.img

Kernel

https://github.com/raspberrypi/linux/


wget https://github.com/raspberrypi/linux/archive/rpi-5.3.y.tar.gz
tar xf rpi-5.3.y.tar.gz
cd linux-rpi-5.3.y/

export ARCH=arm
export CROSS_COMPILE=arm-linux-gnueabihf-

#defconfig
make help | grep bcm
# or
# ls arch/arm/configs/bcm*

#device tree
ls arch/arm/boot/dts/*rpi*

# bcm2709_defconfig for rpi2
make bcmrpi_defconfig

#  we will also need modules later: make -j5  zImage dtbs modules
time ionice -c3 nice -n10 make -j5 zImage dtbs

# i5-2520M  (4core): real    11m54.312s    user     43m8.471s    sys     2m48.369s
# i7-8750H (12core): real     2m48.009s    user    13m53.905s    sys      1m1.443s

# insert sdcard, mount...
cp arch/arm/boot/zImage /run/media/$USER/boot/
cp arch/arm/boot/dts/*rpi*.dtb /run/media/$USER/boot

Booting

TODO Set up serial uart correctly on wifi boards

echo 'dtoverlay=miniuart-bt' >>/run/media/$USER/boot/config.txt

Boot the RPi with the sd card, you should see u-boot comming up on the serial console.

In u-boot:

#load kernel
load mmc 0:1 $loadaddr zImage


#load device tree
load mmc 0:1 $fdt_addr_r $fdtfile

# run kernel (finally)
setenv bootargs 8250.nr_uarts=1 earlyprintk console=ttyS0,115200
bootz $loadaddr - $fdt_addr_r

Let's automate this a bit

setenv loadmmc 'load mmc 0:1 $loadaddr zImage ; load mmc 0:1 $fdt_addr_r $fdtfile'
setenv bootcmd 'run loadmmc ; bootz $loadaddr - $fdt_addr_r'
setenv bootargs 8250.nr_uarts=1 earlyprintk console=ttyS0,115200
saveenv

The saveenv command will save the current environment to a file named uboot.env on the sd card and will automatically load it on next boot.

Unless interupted at boot, u-boot will then execute the contents of the bootcmd env variable. Note that we also split the file loading part to a separate variable, so if we want to tweak some things, we can interupt the automatic boot process and just type run loadmmc to have the kernel and device tree loaded.