Unexpected RPi5 Behavior with Saved U-Boot Environment

[Richard Weinberger <richard@sigma-star.at>]

Hi!

We recently started using U-Boot on various Raspberry Pi 5 boards
with different hardware configurations.

While testing, Simon noticed unusual boot failures, such as the kernel
failing to boot with no output or U-Boot reporting:
"ERROR: Did not find a cmdline Flattened Device Tree."

Today, I spent time investigating these issues. It appears that U-Boot
retrieves several variables from the Raspberry Pi firmware, with fdt_addr
being particularly important.
If the environment is saved, fdt_addr gets stored and takes precedence
over what the firmware provides to U-Boot.

Interestingly, the RPi firmware seems to modify fdt_addr based on connected
hardware.  Our experiments showed that even attaching an RPi camera module
changes the computed fdt_addr and U-Boot reads from the wrong memory location.

As a result, saving the environment, whether via the saveenv command in U-Boot
or fw_setenv in Linux, can lead to boot failures if the hardware configuration changes.

Do you have any suggestions for mitigating this issue?
In my opinion, U-Boot should not override such critical variables.

Thanks,
//richard

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