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

[Quentin Schulz]

Hi Marc,

On 6/9/25 6:40 PM, Marc Haber wrote:
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> 
> 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?
> 

On Theobroma boards (Rockchip-based), we force the environment to be 
coming from the same storage medium as U-Boot proper, c.f. 
board/theobroma-systems/common/common.c:arch_env_get_location()

This is the location of the U-Boot environment in raw, not in a 
filesystem, so this won't apply to your use-case.

But quickly looking at env/fat.c which makes use of the CONFIG_ENV_FAT* 
variables from above, it seems like you should be able to override 
env_fat_get_intf and env_fat_get_dev_part from a board file and 
implement what you want there, provided U-Boot (and/or the RPi firmware) 
provides you with the information from where it was booted.

If you don't want to change the U-Boot binary and somehow you can have a 
minimal environment to run a few commands, you could probably erase the 
whole environment and re-import it manually with `env import`?

Cheers,
Quentin