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

[Mauro Salvini]

On 07/07/21 14:50, Anthony Arrascue wrote:
> Ciao Mauro,
> 
> Thanks for the resource! That clarifies quite a bit.
> - u-boot.bin is renamed as the *kernel7l.img*. In this way *start.elf* 
> loads u-boot instead of the kernel
> - The actual kernel "*kernel7l.img*" is used to create *uImage *using 
> the tool: *mkimage*.
> 
> Now I have only one doubt. Why does u-boot need to convert the kernel 
> image in order to load it (with *bootm*)?
> Could not *bootz *be used to directly load *kernel7l.img*? as this 
> tutorial describes:
> https://www.vdoo.com/blog/setting-up-u-boot-to-harden-the-boot-process 
> <https://www.vdoo.com/blog/setting-up-u-boot-to-harden-the-boot-process>
> 
> BTW I followed the steps from that tutorial, but I didn't manage to boot 
> the kernel in that way. So I guess the extra-step *mkimg -> bootm* is 
> necessary.
> I am just trying to figure out why.

Yes, this is needed because bootm/bootz/booti u-boot commands needs 
different image formats (image headers, offsets, etc.)

regards

-- 
Mauro

> 
> Best,
> 
> Anthony Arrascue
> 
> 
> On Wed, Jul 7, 2021 at 2:15 PM Mauro Salvini <m.salvini at koansoftware.com 
> <mailto:m.salvini at koansoftware.com>> wrote:
> 
>     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/
>     <https://andrei.gherzan.ro/linux/uboot-on-rpi/>
>