F37: aarch64: uboot: serial input -> boot failure

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Dear forum,

Trying to get F37 working on a Rpi-4 with an attached GPS module. The GPS module sends serial data which can be be seen on /dev/ttyS1 as printable NMEA0183 sequences every 5 seconds or so.

However, if this data is present when I boot the system, the boot fails. Seems like uboot somehow is affected by the serial data. While testing, I can disconnect the module until the boot is completed, but this is of course not a working solution in the long run.

So: how to I teach uboot to disregard the serial input? Any hints, out there?

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Answering myself… The basic solution here: linux - How to disable serial console(non-kernel) in u-boot - Stack Overflow

However, fw_printenv/setenv does not work out of the box in F37, multiple problems. Setting bootdelay in the uboot prompt did the trick, though.

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that is, setting bootdelay to -2.

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fw_printenv and friends works with a new file /etc/fw_env.config which reads
/boot/efi/uboot.env 0x000 0x4000