Kernel Customization¶

You can manually rebuild the kernel used for the NVIDIA^® Jetson™ Linux. You must have Internet access for this.

Obtaining the Kernel Sources with Git¶

Prerequisites¶

You have installed Git. You can install with the following command:
```
$ sudo apt install git-core
```
Your system has the default Git port 9418 open for outbound connections.

To sync the kernel sources¶

Get the kernel source by running source_sync.sh:
```
$ ./source_sync.sh
```

Enter a tag name when the script prompts you. The proper tag name is specified in the Release Notes. This tag name syncs the sources to the source revision from which the release binary was built.

You can sync the sources to any other Linux tag if you need to do so. To see a list of the available release tags, enter this command in the kernel source repository:

$ git tag -l tegra-l4t\*

Manually Downloading and Expanding Kernel Sources¶

As an alternative to syncing the kernel sources, you can download the kernel source files and then manually extract them. If possible, NVIDIA recommends that you sync with Git instead.

To manually download and expand the kernel sources¶

In your browser, open the page https://developer.nvidia.com/embedded/downloads.
Locate and download the Jetson Linux source files for your release.
Extract the .tbz2 file:
```
$ tar -xjf public_sources.tbz2
```
Extract the kernel source file:
```
$ cd Linux_for_Tegra/source/public
$ tar –xjf kernel_src.tbz2
```
This extracts the kernel source to the kernel/ subdirectory.

Building the Kernel¶

Before you build the kernel you must install the Jetson Linux build utilities. Enter the command:

$ sudo apt install build-essential bc

To build the kernel¶

Sync or download and expand the kernel sources, and install the kernel build utilities, as described above.
If you are cross-compiling on a host system (i.e. anything other than the Jetson device you are building for), export the following environment variables:
```
$ export CROSS_COMPILE_AARCH64_PATH=<toolchain-path>/aarch64--glibc--stable-2020.08-1
```
See The Jetson Linux Toolchain for information on how to download and build the reference toolchains.
Enter the command:
```
$ mkdir kernel_out
```
Where kernel_out is the directory where the compiled kernel is to be written.
Build the kernel:
```
$ ./nvbuild.sh -o $PWD/kernel_out
```
Replace Linux_for_Tegra/rootfs/usr/lib/modules/$(uname -r)/kernel/drivers/gpu/nvgpu/nvgpu.ko with a copy of this file:
```
$kernel_out/drivers/gpu/nvgpu/nvgpu.ko
```
Replace Linux_for_Tegra/kernel/Image with a copy of this file:
```
$kernel_out/arch/arm64/boot/Image
```
Optionally, archive the installed kernel modules:
```
$ cd <modules_install_path>
$ tar --owner root --group root -cjf kernel_supplements.tbz2 lib/modules
```
You can use the installed modules to provide the contents of /lib/modules/<kernel_version>/ on the target system. Before you run apply_binaries.sh, make a copy of this archive to replace the one at this location on the target device:
```
Linux_for_Tegra/kernel/kernel_supplements.tbz2
```

To sign and encrypt kernel, kernel-dtb, and initrd binary files¶

To support Secure Boot, the kernel, kernel-dtb, and initrd binary files must be signed and encrypted with keys to generate encrypted binary files and signature files.

See the topic Secure Boot.

Preparing to Build External Kernel Modules¶

These procedures describe how to prepare a set of kernel headers, or a kernel source tree to allow building an out-of-tree kernel module.

To prepare the Jetson Linux kernel headers on a Jetson system¶

On a Jetson system you can use the files in this directory to build out-of-tree modules without further action:

/usr/src/linux-headers-$(uname -r)-ubuntu20.04_aarch64

or:

/lib/modules/$(uname -r)/build/

To prepare the Jetson Linux kernel headers on a non-Jetson system¶

Extract the archive and place it in a local directory with the commands:
```
$ cd <local_src_dir>
$ tar -xjf <top>/Linux_for_Tegra/kernel/kernel_headers.tbz2
```
Where <local_src_dir> is a local directory of your choice.

You can use the files in this directory to build out-of-tree modules:
```
<local_src_dir>/linux-headers-$(uname -r)-linux_x86_64/
```
To build out-of-tree modules, specify the kernel directory as:
```
<local src dir>/linux-headers-$(uname -r)-linux_x86_64/
```

To use a manually built kernel source tree¶

If you built the kernel from source, use the same source tree to build out-of-tree kernel modules. When building out-of-tree modules, use $TEGRA_KERNEL_OUT as the kernel directory.

To prepare a kernel source tree that is not built¶

You can build out-of-tree modules that are compatible with a Linux kernel source tree without building the entire kernel image.

To extract the kernel source, set any required shell or environment variables, and create the .config file, follow the instructions provided in these topics:

Enter this command to prepare the kernel source tree for building out-of-tree kernel modules:

   $ make ARCH=arm64 O=$TEGRA_KERNEL_OUT -j<n> modules_prepare

Where ``<n>`` is the number of parallel processes to be used. NVIDIA recommends using the number of CPUs in your system.

When building out-of-tree modules, point $TEGRA_KERNEL_OUT to the kernel directory.

To build external kernel modules¶

If you are cross-compiling the kernel, ensure that the CROSS_COMPILE environment variable is set as described in preceding sections.

Enter these commands to build an out-of-tree kernel module:

$ cd <path_to_module_source>
$ make ARCH=arm64 –C <kernel_directory> M=$(pwd)

Enter these commands to strip unneeded symbols from the kernel module:

$ <tool_chain_path>/aarch64-linux-gnu-strip -–strip-unneeded <path-of-kernel-module.ko>

Using the Jetson Linux Real-Time Kernel Package¶

Not Yet Supported in This Release This section does not apply for this release. real-time kernel is still work in progress.

NVIDIA^® Jetson AGX Xavier™ series processors offer the Early Access Release quality of the Jetson Linux Real-Time kernel support for r32 releases, starting with r32.5. This section explains how to install and remove Jetson Linux real-time kernel packages on your system through the NVIDIA repository.

Prerequisites¶

Upgrade all Jetson Linux packages to the same version of the point release as the Real-Time kernel.

To install the real-time kernel package¶

Open the apt source configuration file as a superuser with a text editor. The file is at:
```
/etc/apt/sources.list.d/nvidia-l4t-apt-source.list
```
Add a new entry for the rt-kernel repository:
```
deb https://repo.download.nvidia.com/jetson/rt-kernel r<rel> main
```
Where <rel> is the current release number, e.g 32.6.1.
Enter this command:
```
$ sudo apt update
```

Install the real-time kernel packages:

$ sudo apt install nvidia-l4t-rt-kernel nvidia-l4t-rt-kernel-headers

Reboot.
RT Latency Tuning Please apply these settings for Jetson AGX Xavier after RT kernel boot to get the best latency value. echo 100 > /sys/kernel/debug/tegra_mce/rt_window_us echo 20 > /sys/kernel/debug/tegra_mce/rt_fwd_progress_us echo 0x7f > /sys/kernel/debug/tegra_mce/rt_safe_mask

To remove the real-time kernel package¶

Enter this command:

$ sudo apt remove nvidia-l4t-rt-kernel nvidia-l4t-rt-kernel-headers

Reboot.

Switching to a Different Kernel¶

Once you have installed the real-time kernel on a Jetson device, you may want to switch between the real-time kernel image and the original generic kernel image without re-installing or removing kernel packages.

You can do this by editing /boot/extlinux/extlinux.conf and setting the DEFAULT property to specify the kernel you want to boot. Set DEFAULT to real-time for the real-time kernel, or to primary for the generic kernel:

TIMEOUT 30
DEFAULT real-time