U-Boot hangs and only shows a big rainbow square on RPi 4B if UART is not enabled

Hi,

We have tried the latest U-Boot v2021.04 with the firmware files [1] at tag 1.20210303. The firmware files include bcm2711-rpi-4-b.dtb, start4* and fixup4*. Building config is the configs/rpi_4_defconfig

However, U-Boot hangs and only shows a big rainbow square on RPi 4B if UART is not enabled

Here is the config.txt we have:

kernel=u-boot.bin #enable_uart=1 arm_64bit=1

And "dtoverlay=vc4-fkms-v3d" is not in the config.txt.

If we enable the UART in config.txt, U-Boot works and shows the message on the screen and serial console correctly.

Is there any way we can help to debug this issue?

[1] https://github.com/raspberrypi/firmware/tree/1.20210303/boot

Jian-Hong Pan