Auto FDT Address Adjustment in U-Boot

Problem

You may see this error in U-Boot when booting a kernel:

Did not find a cmdline Flattened Device Tree Could not find a valid device tree

Root cause: the kernel image ended up larger than expected and its end address overlaps (overwrites) the DTB already placed in memory. U-Boot loads kernel and FDT according to configured addresses (variables like fdt_addr_r). If the kernel’s end address grows and overlaps the FDT location, the kernel never sees a valid device tree.

Quick manual fix

Move the FDT address (fdt_addr_r) further up memory so it lives beyond the kernel end address and won’t be overwritten. That’s fine for one-off cases, but it’s manual and brittle when kernel sizes change.

Auto solution (what I implemented)

I created a general, boot-time auto-adjust mechanism that:

• Detects the actual kernel size / end address at boot.
• If the configured FDT address would be overwritten by the kernel, computes a new safe FDT address (linearly offset based on kernel size) and updates fdt_addr_r before the FDT is used.
• Works for both interactive boot command flows and automated sysboot.
• Is shipped as a bash script you place in the U-Boot root directory and enable (it’s enabled by default in my package).
• Targeted and tested for U-Boot v2023.10 (23.10). Might work on other versions but behavior/paths can vary.

How it behaves (conceptually)

1. At boot (or when boot/sysboot runs), U-Boot normally prepares kernel and FDT using configured addresses.
2. The auto-adjust logic calculates the kernel end address from the loaded kernel size.
3. If fdt_addr_r < kernel_end + padding, the script bumps fdt_addr_r upward to a safe value (kernel_end + padding).
4. U-Boot proceeds with the adjusted FDT address; the FDT is no longer overwritten by the kernel.

Benefits

• No manual edits to device tree address variables after each kernel change.
• Works automatically for scripted and interactive boots.
• Prevents hard-to-debug boot failures caused by image-size growth.

Limitations & notes

• Default behavior in my package is enabled; you can disable if you prefer manual control.
• Make sure your chosen new FDT address does not collide with other memory usages (ramdisk, initrd, etc.). The script uses a simple padding policy; adjust if your board has special constraints.
• This is a runtime mitigation — best practice remains: choose stable, non-overlapping load addresses in your board config when possible.
• Tested on U-Boot 23.10; if you run a different U-Boot version, test carefully in QEMU or on a dev board.

Where to get it

• Script (bash): https://mega.nz/file/46tCmaAY#o57eQxyM_-iFh1MAGwsUMJW264ESE8LG8J2-yXlxuPI
• Documentation for the script: https://mega.nz/file/UjlUSLiS#tnx-g-EC0YlZp1s3e2JP034hNCtvINt37Pr99K4KKl0