While not enabled by default (except on the Jetsons that use the U-Boot bootloader), you can use the L4T extlinux.conf support in your builds.

For L4T R35.x

In the kirkstone and later branches based on the L4T R35.x series of releases, set UBOOT_EXTLINUX = "1" to configure the build to use an extlinux.conf file. (As of 14 Apr 2024, "1" is now the default setting in the master branch.)

See the comments in l4t-extlinux-config.bbclass for additional configuration settings you can use.

Caveats

• The upstream UEFI bootloader does not implement this; it was tacked on by NVIDIA in their L4TLauncher EFI application.
• The ext4 filesystem implementation that NVIDIA provides in their bootloader may have some bugs/limitations that could prevent it from reading the extlinux.conf or other files in your root filesystem. Using newer ext4 features, or non-ext4 filesystems for your root filesystem, could lead to boot failures.
• The extlinux.conf syntax supported in L4TLauncher is not the same as U-Boot's, and the parsing code isn't the most robust/forgiving, so be careful about any modifications you may want to make, to avoid boot failures.

For L4T R32.x

In L4T R32.x:

• The TX1/Nano platforms use U-Boot by default, so no changes are required to use extlinux.conf files.
• The TX2 platform defaults to using U-Boot which supports extlinux.conf. TX2 builds can be configured to use cboot without U-Boot, and the TX2 cboot implementation does not support extlinux.conf.
• The Xavier platforms have a different cboot code base which (unlike the TX2 implementation) does have some support for extlinux.conf files. The rest of this page covers the Xavier implementation.

Configuring Xavier extlinux.conf support

Add the cboot-extlinux package to your image to enable booting your Xavier device with the kernel loaded from /boot in the rootfs instead from from a separate partition. This is only available in the kirkstone-l4t-r32.7.x branch (as of this writing).

Use with caution. Not recommended for production use.

Notes

The cboot bootloader on the Xavier (t194) platforms has support for loading the kernel, initial ramdisk, and device tree from files in the rootfs, rather than the kernel partition. The stock L4T BSP has supported this for several releases, installing the kernel image and initrd into /boot and a /boot/extlinux/extlinux.conf file that cboot uses to locate the files. This can simplify kernel development by eliminating the need to reflash the device to boot with updated kernels.

To implement this in meta-tegra, the cboot-extlinux recipe has been added. Adding cboot-extlinux to your image will include the necessary files -- kernel, initrd (if not bundled), and optionally the device tree, along with the extlinux.conf file and signatures for the files that are expected to be signed -- in your rootfs.

When extlinux support in cboot is enabled (which it is by default), cboot will first try to mount the rootfs to locate the extlinux.conf file. The rootfs is either marked as such with a partition GUID (see below) or is assumed to be the first partition on the boot medium (SDcard, eMMC, or external device). cboot then tries to open /boot/extlinux/extlinux.conf on that filesystem. If successful, it parses the configuration, then attempts to load the kernel, initrd, and/or device tree based on the path names in the file. For elements that are not configured in that file (or all of them, if the file does not exist), cboot falls back to loading them from partitions on the device (kernel for the kernel+initrd, kernel-dtb for the device tree).

extlinux.conf file format

The format of the configuration file is a subset of the format used in the distro boot feature of U-Boot. The cboot-extlinux-config.bbclass file implements the cboot-specific configuration subset; see the comments in that file for more information.

WARNING Modifying the extlinux.conf file incorrectly will often result in cboot crashes, making your device unbootable. Use caution when making any changes to the file.

Adding the device tree

By default, the cboot-extlinux recipe installs the default kernel image and initrd (if configured to be separate from the kernel), but not the device tree, to align with the default stock L4T setup. Set UBOOT_EXTLINUX_FDT = "/boot/${DTBFILE}" in either a bbappend or in your local.conf to include the device tree.

Incompatible with A/B redundancy

Using cboot-extlinux for loading the kernel is not compatible with the A/B redundancy mechanism - the kernel will always be loaded from the A rootfs partition.

It may be possible to fix this by assigning a unique partition GUID to each of the two rootfs partitions, and creating cboot options files (cbo.dtb files) to configure the rootfs GUIDs - one to be loaded into the CPUBL-CFG partition, and the other into CPUBL-CFG_b. However, that would conflict with the normal bootloader update mechanism, since BUP payloads don't distinguish between the A and B slot for their content. Some extra mechanism would be needed to keep the two CPUBL-CFG partitions synchronized with the corresponding rootfs partition GUIDs.

Filesystem restrictions

This has only been tested with ext4-formatted root filesystems, and bugs found in cboot's ext4 implementation have been patched to make this work. Other filesystem types are unlikely to work. Also, you should use the cboot-t19x recipe that builds cboot from source to get the required patches (this is the default).