How to chain u-boot with something that I don't understand.

[Mario Marietto]

Hello.

I've installed the ubuntu jammy iso image created by "hexdump0815" to my sd
card because I wanted to install a recent ubuntu distro powered by the
mainline kernel. This is the kernel of the iso file :


https://github.com/hexdump0815/imagebuilder/releases/download/220619-01/chromebook_snow-armv7l-jammy.img.gz


So,now,I'm going to explain what's the content of the partitions on the sd
card.



   1.

   /dev/sdh1 = unknown = 16 MB
   2.

   /dev/sdh2 = unknown = 16 MB
   3.

   /dev/sdh3 = ext2 = BOOT



Inside the partition /dev/sdh3 I have :


a) dtb-5.18.1-stb-cbe+ = directory

b) extlinux = directory

c) extra = directory

d) config.5.18.1-stb-cbe+ : file

e) initrd.img-5.18.1-stb-cbe+ : file

f) System.map-5.18.1-stb-cbe+ : file

g) vmlinux.kpart-5.18.1-stb-cbe+ : file

h) zImage-5.18.1-stb.cbe+ : file


Inside the directory "dtb-5.18.1-stb-cbe+" there are the following files :


a) exynos5250-snow.dtb

b) exynos5250-snow-rev5.dtb

c) exynos5250-spring.dtb


Inside the directory "extlinux",there is the file called extlinux.conf that
has the following content :


TIMEOUT 30

DEFAULT linux-snow-rev4

MENU TITLE snow chromebook boot options

# rev4 snow chromebook
LABEL linux-snow-rev4
      MENU LABEL linux rev4 snow
      LINUX ../zImage-5.18.1-stb-cbe+
      INITRD ../initrd.img-5.18.1-stb-cbe+
      FDT ../dtb-5.18.1-stb-cbe+/exynos5250-snow.dtb
      APPEND console=tty1 root=LABEL=rootpart ro rootwait
net.ifnames=0 ipv6.disable=1 fsck.repair=yes noresume

# rev5 snow chromebook
LABEL linux-snow-rev5
      MENU LABEL linux rev5 snow
      LINUX ../zImage-5.18.1-stb-cbe+
      INITRD ../initrd.img-5.18.1-stb-cbe+
      FDT ../dtb-5.18.1-stb-cbe+/exynos5250-snow-rev5.dtb
      APPEND console=tty1 root=LABEL=rootpart ro rootwait
net.ifnames=0 ipv6.disable=1 fsck.repair=yes noresume

# spring chromebook - untested
LABEL linux-spring
      MENU LABEL linux spring
      LINUX ../zImage-5.18.1-stb-cbe+
      INITRD ../initrd.img-5.18.1-stb-cbe+
      # rev4 snow chromebook
      FDT ../dtb-5.18.1-stb-cbe+/exynos5250-spring.dtb
      APPEND console=tty1 root=LABEL=rootpart ro rootwait
net.ifnames=0 ipv6.disable=1 fsck.repair=yes noresume


Inside the extra directory there are the following files :


a) kernel-chromebook_snow-legacy.tar.gz

b) uboot.kpart.cbe-snow

c) uboot.kpart.cbe-snow-alternative

d) uboot.kpart.cbe-spring


*4) /dev/sdh4* : inside here I have copied all the files of the jammy
userland prepared by hexdump0818.


That's all. What I want to know is which method he used to boot Ubuntu like
this ? I don't see grub.cfg and I don't see u-boot. So what method has
hexdump0818 used ? I want to know this because my plan is to enable KVM on
the kernel 5.4 and to do this I need to install a not verified u-boot
bootloader on the first partition of the sd card that I should chain u-boot
with something. And I should understand what this something is.
-- 
Mario.
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