According to https://u-boot.readthedocs.io/en/stable/board/st/stm32mp1.html , the SD card requires 2 copies of the fsbl. Why are 2 copies required?

[Rc.1]

I filled the second fsbl partition with all zeroes, u-boot still runs.

[Kevin HUBER · Accepted solution]

Hello @Rc.1​ ,

the fsbl2 is a backup of the FSBL. You can find more information on the wiki page of the ROM Code: https://wiki.st.com/stm32mpu/wiki/STM32MP15_ROM_code_overview#Flash_memory_boot

For instance with a NOR flash, the NOR Flash contains two copies of FSBL. The ROM code tries to load and launch the first copy. In case of failure, it then tries to load the second copy.

This is a security for always have a board that is able to boot, even if an error occurred during the update of the first partition.

That's why your board can still run U-boot if there is nothing in the second fsbl partition.

Regards,

Kevin

[Kevin HUBER]

Hi @Rc.1​ ,

In order to give better visibility on the answered topics, please click on 'Select as Best' on the reply which solved your issue or answered your question. See also 'Best Answers'

Regards,

Kevin

[Rc.1]

Thanks Kevin, your post explains why the system still boots with only 1 FSBL.