slarm64 u-boot
 

this question may sound stupid, as it probably has been answered about a zillion times before, however I've found no answer:
why is slarm64 using u-boot in the first place, instead of just loading kernel8 ?
any pointers welcome
regards,
-rasp

Because the kernel (and the relative initrd.gz, if any) is usually too big to be loaded in the first place.
AllWinner chips, for example, have only a region of 32K that is automatically loaded in memory from the MMC in the boot process. Also u-boot does not fit in this small region: in fact there is the "SPL" (Secondary Program Loader) that is loaded in the boot process before u-boot and it setups the memory and MMC controller and then it loads and runs u-boot (and ATF - Arm Trusted Firmware - for aarch64).
U-boot also allow a smarter booting (e.g. from network) if something is wrong with a new kernel.

Luca

the booting 'something else' thing sounds good, although probably only applicable via serial console, no ?
nevertheless if the kernel has everything it needs to boot from, say btrfs hdd, it should be possible to have a 'old style' boot partition, loading kernel8.img right ?
(switching kernels back and forth is easy by mounting the boot partition on some other machine and changing it there....)

-rasp

No, you can also use network console. You have to set (from u-boot itself or with u-boot's fw_setenv):
fw_setenv serverip 10.10.10.10
fw_setenv ipaddr 10.10.10.100
fw_setenv if_netconsole 'ping $serverip'
fw_setenv start_netconsole 'setenv ncip $serverip; setenv bootdelay 10; setenv stdin nc; setenv stdout nc; setenv stderr nc; version;'
fw_setenv preboot 'run if_netconsole start_netconsole'

And from the PC with IP 10.10.10.10:
nc -l -u -p 6666

Yes, I usually do it: creating an "old style" boot partition, put kernel (and uinitrd, if needed), and set u-boot to load kernel/initrd from the boot partition. With older armvt5-kirkwoord systems, I usually try before USB and, if it fails, load from SATA-IDE/MMC. Example of u-boot multi-boot possibility (it will try both fat and ext2 filesystem), with additional u-boot command to be placed in "boot.scr" script (for Seagate GoFlexNet NAS-like ARMv5te system):

fw_setenv fdtfile /kirkwood-goflexnet.dtb
fw_setenv zimage /zImage-armv5
fw_setenv uinitrd /uinitrd-armv5
fw_setenv initaddr 0x01100000
fw_setenv fdtaddr 0x00700000
fw_setenv kernaddr 0x00800000
fw_setenv script boot.scr
fw_setenv scriptaddr 0x00780000
fw_setenv bootargs 'console=ttyS0,115200 root=/dev/disk/by-label/slackroot'
fw_setenv bootcmd_script '$fload $bootbus ${bootdev}:${bootpart} $scriptaddr $script; source $scriptaddr'
fw_setenv bootcmdz 'bootz $kernaddr $initaddr $fdtaddr'
fw_setenv bootcmd_slk 'run bootcmd_script; $fload $bootbus ${bootdev}:${bootpart} $initaddr $uinitrd; $fload $bootbus ${bootdev}:${bootpart} $kernaddr $zimage; $fload $bootbus ${bootdev}:${bootpart} $fdtaddr $fdtfile; run bootcmdz'
fw_setenv bootcmd_ide 'setenv bootbus ide;setenv bootdev 0;setenv bootpart 1;setenv fload ext2load;run bootcmd_slk; setenv bootdev 1;run bootcmd_slk'
fw_setenv bootcmd_usb 'setenv bootbus usb;setenv bootdev 0;setenv bootpart 1;setenv fload ext2load;run bootcmd_slk; setenv fload fatload;run bootcmd_slk'
fw_setenv bootcmd 'usb start; run bootcmd_usb; ide reset; run bootcmd_ide; usb reset; run bootcmd_usb; reset'

Luca

only raspberry pi has a veiled spi bootloader, other boards may/may not have spi with a bootloader. At the moment, all the boards I have come across (except for the raspberry pi) use u-boot as a hardware initialization system with the subsequent transfer of control to the kernel, so the same approach is used in images_build_kit. But you can customize your boards without using u-boot.

"I love it, when a plan comes together" :-)
 

thank you all for your effort!.
to solve my own question, this is what I did now:
booted slarm64 from:slarm64-current-aarch64-xfce-raspberry_pi_4-5.14.0-build-20210902.img (using u-boot)
'stole' the boot partition contents from raspberrypios64.
put the boot stuff on a 64g usb-stick (part1/vfat)
rebuild the kernel as https://www.raspberrypi.com/document...ux_kernel.html
activated some 'extras' btrfs[*] and .config support.
gzipped 22M kernel to 8M.
copied ext4 slarm to 2nd partition usb btrfs.
changed cmdline.txt to PARTLABEL for rootfs
and hey it boots off usb/btrfs without u-boot....
-rasp