For many L4T/Jetson Linux releases, NVIDIA has provided a mechanism (the jetson-io scripts) for applying device tree overlays (.dtbo files) dynamically at runtime. For OE/Yocto-based builds, device trees are built from sources, so runtime application of DTB overlays is less of an issue. The meta-tegra layer does provide some mechanisms for applying DTB overlays, through some build-time variable settings.

This mechanism is supported in the branches based on L4T R32.6.x through R35.x only. Overlays are applied to the device tree during the kernel build, directly modifying your kernel DTB. (For L4T R36 and later, the NVIDIA device trees are no longer provided in the kernel source tree.)

The exact list of overlays supplied by NVIDIA varies by target platform. You can find them by building the kernel recipe (virtual/kernel or linux-tegra) and examining its output under ${BUILDDIR}/work/tmp/${MACHINE}/linux-tegra.

Set the KERNEL_DEVICETREE_APPLY_OVERLAYS variable to a blank-separate list of .dtbo file names to have those overlays applied during the kernel build. You can do this in your machine configuration file, or add it, for example, to the local.conf file in your build workspace.

For example, to configure a Jetson Xavier NX development kit for IMX477 and IMX219 cameras, you would add the following line to your $BUILDDIR/conf/local.conf file:

    KERNEL_DEVICETREE_APPLY_OVERLAYS:jetson-xavier-nx-devkit = "tegra194-p3668-all-p3509-0000-camera-imx477-imx219.dtbo"

For U-Boot-based Jetsons (only supported on a subset of Jetson modules with L4T R32.x), the .dtbo files will get populated into the /boot directory in the rootfs, and you could modify the /boot/extlinux/extlinux.conf file to add an FDTOVERLAY line to have one or more overlays applied at boot time. Unfortunately, OE-Core's support for generating extlinux.conf content does not include support for FDTOVERLAY lines, so to make such a change you would have to work out a way to rewrite that file in a bbappend.

For L4T R36.x, the nvidia-kernel-oot recipe is the default device tree provider for the Jetson platforms. You can also set the PREFERRED_PROVIDER_virtual/dtb variable to point to a recipe for providing your own customized device tree. To apply overlays to these device trees, add fdtoverlay invocations to the compilation step via a bbappend (for nvidia-kernel-oot) or in your custom recipe.

See the tegra-demo-distro example at meta-tegrademo/recipes-bsp/tegrademo-devicetree which shows how to modify a base devicetree from nvidia-kernel-oot to one specific to your hardware platform. This simple example just adds a single "compatible" line to your base devicetree. To use this example:

1. Determine which devicetree is currently in use. One way to do this is with bitbake -e <your image> and look at the value of KERNEL_DEVICETREE.
2. Determine whether there's an existing devicetree in the meta-tegrademo/recipes-bsp/tegrademo-devicetree which uses your existing devictree as a base. Current examples are:

• tegra234-p3768-0000+p3767-0005-oe4t.dts: jetson-orin-nano-devkit or jetson-orin-nano-devkit-nvme builds on a p3768 (Orin Nano Devboard) carrier
• tegra234-p3768-0000+p3767-0000-oe4t.dts: Nvidia Jetson Orin NX 16GB in a p3768 (Orin Nano Devboard) carrier
• tegra234-p3737-0000+p3701-0000-oe4t.dts: jetson-agx-orin-devkit

3. If there's not an existing devicetree built from your base KERNEL_DEVICETREE, follow the examples to add one to SRC_URI and to the repo.
4. Modify your MACHINE conf or local conf to specify your dtb provider and KERNEL_DEVICETREE using something like this:

PREFERRED_PROVIDER_virtual/dtb = "tegrademo-devicetree"
KERNEL_DEVICETREE:jetson-orin-nano-devkit-nvme = "tegra234-p3768-0000+p3767-0005-oe4t.dtb"
KERNEL_DEVICETREE:jetson-orin-nano-devkit = "tegra234-p3768-0000+p3767-0005-oe4t.dtb"

Where KERNEL_DEVICETREE overrides the setting for your MACHINE, referencing the devicetree filename with *.dtb in the place of *.dts. 5. Build, flash, and boot the board, and cat /sys/firmware/devicetree/base/compatible to see the compatible string printed as configured in the devicetree. You should see a string which starts with "oe4t", as shown here for the orin nano

root@jetson-orin-nano-devkit-nvme:~# cat /sys/firmware/devicetree/base/compatible
oe4t,p3768-0000+p3767-0005+tegrademonvidia,p3768-0000+p3767-0005-supernvidia,p3767-0005nvidia,tegra234

This mechanism is supported in branches based on L4T R35.x and later. Overlays are appended to the kernel DTB by the NVIDIA flashing/signing tools, and are applied by the UEFI bootloader at runtime.

The exact list of overlays supplied by NVIDIA varies by target platform. You can find them on R35.x-based branches by building the kernel recipe (virtual/kernel or linux-tegra) and examining its output under ${BUILDDIR}/work/tmp/${MACHINE}/linux-tegra. For R36.x-based branches, device trees are built as part of the nvidia-kernel-oot recipe.

Append your additional overlays to the TEGRA_PLUGIN_MANAGER_OVERLAYS variable, which consists of a blank-separate list of .dtbo file names. You can do this in your machine configuration file, or add it, for example, to the local.conf file in your build workspace. That variable is set by the layer to include overlays that NVIDIA requires for its platforms, so be sure to append to it, rather than overwriting it.

For example, to configure the pins on the 40-pin expansion header of the Jetson Orin Nano development kit, you would add the following line to your $BUILDDIR/conf/local.conf file:

    TEGRA_PLUGIN_MANAGER_OVERLAYS:append:jetson-orin-nano-devkit = " tegra234-p3767-0000+p3509-a02-hdr40.dtbo"