Differences in loading device-tree with u-boot/ without u-boot

[Clemens Kilgus]

Hello,

I observe a strange behavior while loading a device-tree. Short description:
- I merged a .dtb with an .dtbo with the dtmerge command (raspbian) for my
rpi zero w.
- If I start the system the traditional way (without u-boot) everything
seems fine. The new device (/dev/ttySC0) appears.
- If i start the system with u-boot (bootz) the device (/dev/ttySC0) isn't
there... if I inspect the device-tree with dtc -I fs /proc/device-tree it
seems the device-tree was loaded correctly. But some different things are
going on while using u-boot and not using it...

Can someone explain me what difference this is, or what exactly happens to
the device-tree when u-boot is loading it instead the raspberry pi
bootloader is loading it?

I'm using Alpine Linux 3.12, U-boot v2020.04 with standard rpi_0_w_defconfig

Thank you!

Clemens