4. Kernel

Get Kernel Sources

The recommended way to get the kernel source is calling sudo ./source_sync.sh in your L4T folder, e.g.,

export L4T=$HOME/nvidia/nvidia_sdk/JetPack_4.4_Linux_JETSON_XAVIER_NX_DEVKIT/Linux_for_Tegra
cd $L4T
# for JetPack 4.4
sudo ./source_sync.sh -k tegra-l4t-r32.4.3

Then you will download around 2G sources for further kernel and device tree compile.

You may check the right tag corresponding to the JetPack you are using.tegra-l4t-r32.4.3 is the tag for the L4T version of JetPack 4.4.

Get Cross-Compile Tools

Jetson is an ARM based computer. In order to compile a kernel image on Jetson, i.e., ARM, we have to download a cross-compile toolchain for it. You can download the toolchain (around 900M) by the following commands.

mkdir -p $HOME/l4t-gcc
cd $HOME/l4t-gcc
wget 
http://releases.linaro.org/components/toolchain/binaries/7.3-2018.05/aarch64-linux-gnu/gcc-linaro-7.3.1-2018.05-x86_64_aarch64-linux-gnu.tar.xz

tar xf gcc-linaro-7.3.1-2018.05-x86_64_aarch64-linux-gnu.tar.xz

Compile Kernel

There are 3 types of compile situation.

1. First Time Compile

For the 1st time compilation, you have to generate a default config file. The reference commands are as the follows:

export L4T=$HOME/nvidia/nvidia_sdk/JetPack_4.4_Linux_JETSON_XAVIER_NX_DEVKIT/Linux_for_Tegra
export KERNEL_SOURCES=$L4T/sources/
export TEGRA_KERNEL_OUT=$L4T/build
export KERNEL_MODULES_OUT=$L4T/modules
export TOOLCHAIN_PREFIX=$HOME/l4t-gcc/gcc-linaro-7.3.1-2018.05-x86_64_aarch64-linux-gnu/bin/aarch64-linux-gnu-
export ARCH=arm64

mkdir -p $TEGRA_KERNEL_OUT

cd $KERNEL_SOURCES

## clean up
make -C kernel/kernel-4.9/ O=$TEGRA_KERNEL_OUT LOCALVERSION=-tegra mrproper -j$(nproc)

## get defualt config
make -C kernel/kernel-4.9/ ARCH=arm64 O=$TEGRA_KERNEL_OUT LOCALVERSION=-tegra CROSS_COMPILE=${TOOLCHAIN_PREFIX} tegra_defconfig

## change kernel configs
make -C kernel/kernel-4.9/ ARCH=arm64 O=$TEGRA_KERNEL_OUT LOCALVERSION=-tegra CROSS_COMPILE=${TOOLCHAIN_PREFIX} menuconfig

## make
make -C kernel/kernel-4.9/ ARCH=arm64 O=$TEGRA_KERNEL_OUT LOCALVERSION=-tegra CROSS_COMPILE=${TOOLCHAIN_PREFIX} -j$(nproc) Image

2. Compile after Minor Changes

If you just change a few kernel sources, you can use the below commands as reference. The compile time is supposed to be shorter compared with 1st time compilation.

export L4T=$HOME/nvidia/nvidia_sdk/JetPack_4.4_Linux_JETSON_XAVIER_NX_DEVKIT/Linux_for_Tegra
export KERNEL_SOURCES=$L4T/sources/
export TEGRA_KERNEL_OUT=$L4T/build
export KERNEL_MODULES_OUT=$L4T/modules
export TOOLCHAIN_PREFIX=$HOME/l4t-gcc/gcc-linaro-7.3.1-2018.05-x86_64_aarch64-linux-gnu/bin/aarch64-linux-gnu-
export ARCH=arm64

mkdir -p $TEGRA_KERNEL_OUT

cd $KERNEL_SOURCES

## make
make -C kernel/kernel-4.9/ ARCH=arm64 O=$TEGRA_KERNEL_OUT LOCALVERSION=-tegra CROSS_COMPILE=${TOOLCHAIN_PREFIX} -j$(nproc) Image

3. If Compile Failed

You may try to clean up the build environment by the following commands.

Please notice, although you will still have your modified kernel sources, but you will loose your previous config file.

export L4T=$HOME/nvidia/nvidia_sdk/JetPack_4.4_Linux_JETSON_XAVIER_NX_DEVKIT/Linux_for_Tegra
export KERNEL_SOURCES=$L4T/sources/
export TEGRA_KERNEL_OUT=$L4T/build
export KERNEL_MODULES_OUT=$L4T/modules
export TOOLCHAIN_PREFIX=$HOME/l4t-gcc/gcc-linaro-7.3.1-2018.05-x86_64_aarch64-linux-gnu/bin/aarch64-linux-gnu-
export ARCH=arm64

mkdir -p $TEGRA_KERNEL_OUT

cd $KERNEL_SOURCES

## clean up
make -C kernel/kernel-4.9/ O=$TEGRA_KERNEL_OUT LOCALVERSION=-tegra mrproper -j$(nproc)

## get defualt config
make -C kernel/kernel-4.9/ ARCH=arm64 O=$TEGRA_KERNEL_OUT LOCALVERSION=-tegra CROSS_COMPILE=${TOOLCHAIN_PREFIX} tegra_defconfig

## change kernel configs
make -C kernel/kernel-4.9/ ARCH=arm64 O=$TEGRA_KERNEL_OUT LOCALVERSION=-tegra CROSS_COMPILE=${TOOLCHAIN_PREFIX} menuconfig

## make
make -C kernel/kernel-4.9/ ARCH=arm64 O=$TEGRA_KERNEL_OUT LOCALVERSION=-tegra CROSS_COMPILE=${TOOLCHAIN_PREFIX} -j$(nproc) ImageReplace the Kernel Image

Replace the Kernel Image on NRU Series

You can update your kernel sources by moving the compiled Image to the /boot folder on the NRU series.

Please notice that a mismatched or error kernel image, might make your NRU won't be boot into OS again.

I usually update the kernel image via a local network. The reference commands are as the below.

export L4T=$HOME/nvidia/nvidia_sdk/JetPack_4.4_Linux_JETSON_XAVIER_NX_DEVKIT/Linux_for_Tegra
export TEGRA_KERNEL_OUT=$L4T/build
export NRU_IP=192.168.55.1

cd $L4T 
scp $TEGRA_KERNEL_OUT/arch/arm64/boot/Image nvidia@${NRU_IP}:/tmp
ssh nvidia@${NRU_IP} -t 'sudo mv /tmp/Image /boot/'