Searching and Finding boot.scr and uboot.env on a Raspberry Pi 4

[Marc Haber]

Hi,

for flexibility reasons I want to use u-boot to boot Debian on a 
Raspberry Pi 4. I want to be able to fix a broken boot from remote over 
the serial console. I have therefore written a boot.scr script that 
looks for kernel and initrd on various partitions on various media in 
various directories, loads kernel, initrd and devicetree and boots.

I am using the uboot binary that comes with Debian's u-boot-rpi:arm64 
2025.01-3 package. That one is configured like that:

CONFIG_ENV_IS_IN_FAT=y
CONFIG_ENV_FAT_INTERFACE="mmc"
CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
CONFIG_ENV_FAT_FILE="uboot.env"

This works as long as I am booting from an MMC card: The Raspi Firmware 
reads config.txt (from MMC), loads uboot.bin (from MMC), which in turn 
reads uboot.env and boot.scr (from MMC), the script runs and the system 
comes up. While this happens, u-boot talks to me on the serial console, 
presumably because uboot.env contains variables like
baudrate=115200
stderr=serial,vidconsole
stdin=serial,usbkbd
stdout=serial,vidconsole

However, when I have everything on USB, things don't work as smoothly. 
The system does not come up and it doesn't talk to me on the serial 
console. What I guess is that the Raspi Firmware reads config.txt (from 
USB), loads uboot.bin (from USB) which then looks on the non-existent 
MMC device for uboot.env and boot.scr. Game Over.

Is it somehow possible to have u-boot look for uboot.env and/or boot.scr 
on the same device it was booted from? Can I compile in some kind of 
search order so that u-boot can find uboot.env and boot.scr on other 
devices than the first partition on mmc? Or do I really need to build my 
own u-boot.bin?

If there is another method to ask user questions about u-boot, please 
let me know.

Greetings
Marc

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