Pi Zero format boards that are supported.

[Phishfry]

I think it helps to use a serial port so you can capture the messages.
There is no indicator in system logs of what DTB's have loaded.

Loading kernel...                                                          
/boot/kernel/kernel text=0x318 text=0xa5b378 text=0x27df90 data=0x1814e0 data=0x0+0x2c5000 0x8+0x1699f8+0x8+0x198602|
Loading configured modules...
/boot/kernel/ds3231.ko text=0x1280 text=0x1168 data=0x3b8 0x8+0x990+0x8+0x644
/etc/hostid size=0x25
/boot/entropy size=0x1000
Using DTB provided by EFI at 0x3ce8c000.
Loading DTB overlays: 'ds3231-i2c3.dtb,i2c4.dtb,pwm8.dtb,pwm9.dtb'
/boot/dtb/overlays/ds3231-i2c3.dtb size=0x186
/boot/dtb/overlays/i2c4.dtb size=0x3cf
/boot/dtb/overlays/pwm8.dtb size=0x27f
/boot/dtb/overlays/pwm9.dtb size=0x30b
Loading splash ok
applying DTB overlay '/boot/dtb/overlays/ds3231-i2c3.dtb'
applying DTB overlay '/boot/dtb/overlays/i2c4.dtb'
applying DTB overlay '/boot/dtb/overlays/pwm8.dtb'
applying DTB overlay '/boot/dtb/overlays/pwm9.dtb'
---<<BOOT>>---
GDB: debug ports: uart
GDB: current port: uart
KDB: debugger backends: ddb gdb
KDB: current backend: ddb
WARNING: Cannot find freebsd,dts-version property, cannot check DTB compliance
Copyright (c) 1992-2024 The FreeBSD Project.

My PWM devices are not showing up in /dev/pwm so this is where I start debugging.

Did the overlay apply cleanly. It did.

 

[Phishfry]

My FreeBSD built Rock Zero 3E Slave Port

mkdir /usr/ports/sysutils/u-boot-radxa-zero-3

Makefile

MASTERDIR=    ${.CURDIR}/../u-boot-master

MODEL=        radxa-zero-3
BOARD_CONFIG=    radxa-zero-3-rk3566_defconfig
FAMILY=        rk356X

.include "${MASTERDIR}/Makefile"

pkg-desc

U-Boot loader and related files for the Radxa Zero 3.

To install this bootloader on an sdcard just do:
dd if=/usr/local/share/u-boot/u-boot-radxa-zero-3/idbloader.img of=/path/to/sdcarddevice seek=64 bs=512 conv=sync
dd if=/usr/local/share/u-boot/u-boot-radxa-zero-3/u-boot.itb of=/path/to/sdcarddevice seek=16384 bs=512 conv=sync

I also jigger with u-boot-master to get newer version.
Also RK356X fails to build on 2024.10-u-boot due to a fragment in u-boot-master. I have deleted it with no problem.

U-Boot 2024.10 (Nov 25 2024 - 18:58:06 -0500)

Model: Radxa ZERO 3E
DRAM:  1 GiB (effective 1022 MiB)
PMIC:  RK817 (on=0x40, off=0x00)
Core:  286 devices, 30 uclasses, devicetree: separate
MMC:   mmc@fe2b0000: 1
Loading Environment from nowhere... OK
In:    serial@fe660000
Out:   serial@fe660000
Err:   serial@fe660000
Model: Radxa ZERO 3E
Net:   eth0: ethernet@fe010000
Hit any key to stop autoboot:  0