Hello everyone,
I am learning about u-boot and how this works in the Raspberry Pi system. This is what I understood so far:
- The first stage from the RPI cannot or should be modified. In this stage the BootROM simply loads the bootloader from the flash EEPROM. - In the second stage the EEPROM boot loader finds and loads start.elf, whose task is to load the kernel. It first reads* config.txt* which contains a kernel parameter. This is where u-boot is "injected". *kernel=u-boot.bin*
Then u-boot can then in turn load the actual kernel. For a CM4 this would be "*kernel7l.img*".
I would be super satisfied with this knowledge, but in practice I have (possibly) seen other ways of integrating u-boot. I am here referring, for example, to Yocto recipes for the CM4 (meta-raspberrypi / u-boot). The boot directory of such an image contains: *boot.scr*, *uboot.env*, and *uImage*.
The readable part of boot.scr specifies that uImage will be loaded:
*value bootargs /chosen bootargsfatload mmc 0:1 ${kernel_addr_r} uImageif test ! -e mmc 0:1 uboot.env; then saveenv; fi;bootm ${kernel_addr_r} - ${fdt_addr}*
config.txt does not contain a kernel parameter.
*So here my questions:* 1) start.elf cannot be modified since it is proprietary. How can it load u-boot if there is no kernel parameter in config.txt pointing to a binary u-boot? boot.scr is supposed to run before u-boot.bin. Who reads and executes boot.scr? 2) is uImage just another name for u-boot.bin? 3) In this scheme how does u-boot know it should load "*kernel7l.img*"? 4) Are there other ways of integrating u-boot in a RaspberryPi? Is there any documentation which describes these different integration schemes?
Thank you very much for your help!
Best,
Anthony Arrascue