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 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. Best, Anthony Arrascue On Wed, Jul 7, 2021 at 2:15 PM Mauro Salvini <[email protected]> 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/ > > -- > Mauro Salvini | KOAN sas | Bergamo - Italia > embedded software engineering > Phone: +39 035-255235 > http://KoanSoftware.com >
