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extlinux.conf support
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.
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.
- The upstream UEFI bootloader does not implement this; it was tacked on by NVIDIA in their
L4TLauncherEFI application. - The ext4 filesystem implementation that NVIDIA provides in their bootloader may have some bugs/limitations that could prevent it from reading the
extlinux.confor 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.confsyntax supported inL4TLauncheris 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.
In L4T R32.x:
- The TX1/Nano platforms use U-Boot by default, so no changes are required to use
extlinux.conffiles. - The TX2 platform defaults to using U-Boot which supports
extlinux.conf. TX2 builds can be configured to usecbootwithout U-Boot, and the TX2cbootimplementation does not supportextlinux.conf. - The Xavier platforms have a different
cbootcode base which (unlike the TX2 implementation) does have some support forextlinux.conffiles. The rest of this page covers the Xavier implementation.
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.
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).
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.
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.
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.
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).