Confusion about which device tree is used - does not appear to be A_kernel-dtb nor B_kernel-dtb on scarthgap

[tbottomparallel]

I am in the process of updating to scarthgap and am confused about what device tree is being used. We're now using A/B roofs and after flashing the B_kernel-dtb partition and rebooting to the B slot, the expected changes to the device tree were not present:

"dd" "status=progress" "bs=8M" "iflag=fullblock" "oflag=direct" "conv=sparse" "if=kernel_tegra234-p3737-0000+p3701-0000-nv.dtb" "of=/dev/disk/by-partlabel/B_kernel-dtb"

My initial suspicion was that the A slot device tree was still being used. In an attempt to demonstrate this I created a modified device tree for the A and B slots that would identify which slot was in use:

$ dtc -I dtb -O dts A_kernel_modified.dtb 2>&1 | grep model
        model = "NVIDIA Jetson AGX Orin Developer Kit A";

$ dtc -I dtb -O dts B_kernel_modified.dtb 2>&1 | grep model
        model = "NVIDIA Jetson AGX Orin Developer Kit B";

I flashed the modified device trees to the A_kernel-dtb and B_kernel-dtb slots respectively:

$ dd if=A_kernel_modified.dtb of=/dev/disk/by-partlabel/A_kernel-dtb
532+1 records in
532+1 records out
272497 bytes (272 kB, 266 KiB) copied, 0.0317612 s, 8.6 MB/s

$ dd if=B_kernel_modified.dtb of=/dev/disk/by-partlabel/B_kernel-dtb
531+1 records in
531+1 records out
272233 bytes (272 kB, 266 KiB) copied, 0.0302397 s, 9.0 MB/s

rebooted and observed that neither modified device tree appeared to be in use:

$ cat /sys/firmware/devicetree/base/model
NVIDIA Jetson AGX Orin Developer Kit

I read the device trees back and verified that the changes were present on disk:

$ dd if=/dev/disk/by-partlabel/A_kernel-dtb of=A_kernel-readback.dtb
1536+0 records in
1536+0 records out
786432 bytes (786 kB, 768 KiB) copied, 0.0104452 s, 75.3 MB/s

$ dtc -I dtb -O dts A_kernel-readback.dtb 2>&1 | grep model
        model = "NVIDIA Jetson AGX Orin Developer Kit A";

$ dd if=/dev/disk/by-partlabel/B_kernel-dtb of=B_kernel-readback.dtb
1536+0 records in
1536+0 records out
786432 bytes (786 kB, 768 KiB) copied, 0.0113336 s, 69.4 MB/s

$ dtc -I dtb -O dts B_kernel-readback.dtb 2>&1 | grep model
        model = "NVIDIA Jetson AGX Orin Developer Kit B";

Out of concern that a driver might be truncating or modifying the model field behind my back, I also tried adding a new node to the B_kernel-dtb, which likewise does not appear in the device tree:

$ dtc -I dtb -O dts B_kernel_modified.dtb 2>&1 | grep -A5 new-test-node
        new-test-node {
                compatible = "no-driver-exists";
                status = "okay";
        };

$ nvbootctrl dump-slots-info
Current version: 36.3.0
Capsule update status: 1
Current bootloader slot: B
Active bootloader slot: B
num_slots: 2
slot: 0,             status: normal
slot: 1,             status: normal

$ ls /sys/firmware/devicetree/base/new-test-node
ls: cannot access '/sys/firmware/devicetree/base/new-test-node': No such file or directory

The device tree does not change regardless of what I write to these partitions. Prior to scarthgap we always just wrote the device tree to A_kernel-dtb and this worked without issue.

So my questions are:

Where is the device tree stored if not inA_kernel-dtb nor B_kernel-dtb?

How can I configure it to use A_kernel-dtb and B_kernel-dtb?

[dwalkes]

See d4dfb31, https://github.com/OE4T/meta-tegra/wiki/extlinux.conf-support and the logic in https://github.com/OE4T/meta-tegra/blob/scarthgap/recipes-bsp/uefi/l4t-launcher-extlinux.bb - the dtb partitions are only used for backup I believe as long as UBOOT_EXTLINUX, which is now the default on master and scarthgap, is used.

[tbottomparallel]

thanks for the quick reply!

All I needed was UBOOT_EXTLINUX_FDT = "/boot/${DTBFILE}", now my devicetree gets updated along with the rootfs.