U-boot integration in the bootloader process of Rasperry Pi CM4

[Mauro Salvini]

On 07/07/21 10:05, Anthony Arrascue wrote:
> Hello everyone,

Hello

> 
> 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}*
> 

I'm not a RPI expert, but this is what know:

> config.txt does not contain a kernel parameter.

It must not contain kernel params: it is used by start.elf, and 
configurations are relative to the things that start.elf does.
Kernel parameters are contained in loaded devicetree, and read from it 
by u-boot in u-boot.scr with

fdt addr \${fdt_addr} && fdt get value bootargs /chosen bootargs

command.

> 
> *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?

start.elf loads the binary named kernel7l.img. You need to rename 
u-boot.bin to kernel7.img, so start.elf loads u-boot instead of the 
kernel. You can read this post [1] from Andrei Gherzan, one of the 
meta-raspberrypi authors.

u.boot scr is executed by u-boot, so the execution order is

start.elf -> u-boot.bin -> u.boot.scr

> 2) is uImage just another name for u-boot.bin?

No, it is the kernel image that u-boot loads throug u-boot.scr:

fatload mmc 0:1 ${kernel_addr_r} uImage

> 3) In this scheme how does u-boot know it should load "*kernel7l.img*"?

See above

> 4) Are there other ways of integrating u-boot in a RaspberryPi? Is there
> any documentation which describes these different integration schemes?

AFAIK only the one described in [1] and done in meta-raspberrypi

> 
> Thank you very much for your help!
> 

[1] https://andrei.gherzan.ro/linux/uboot-on-rpi/

-- 
Mauro Salvini | KOAN sas | Bergamo - Italia
embedded software engineering
Phone: +39 035-255235
http://KoanSoftware.com