I created an SD card with the Tow-Boot disk-image, followed by two more partitions, a 20M firmware partition (empty except for a single file, config.txt), and a bootable system partition containing a nixos system (These two partitions are copied from another SD card with a working system using u-boot). The internal eMMC does not currently have any bootloaders or bootable partitions on it.

If I fit this SD card, power on the system, and leave it alone, it boots just fine.

If I interrupt it by pressing Ctrl+C when prompted, it shows the menu, which is navigable using the keyboard. However, all of the "Boot from *" options fail, even "Boot from SD", despite there being a (clearly) bootable SD card fitted. Will continue to investigate...

A fresh build from the current tip of development or using that first release? It's possible cancelling boot with ^C is cancelling the commands list if you used the first release. DUH... It's a build from this PR... disregard this.

Note that the "firmware partition" you're adding is useless; in the NixOS generic sd image, that "FIRMWARE" partition is the IBF (Initial Boot Firmware, read: U-Boot) exclusively used for the Raspberry Pi family of hardware.

Is that "bootable system partition" partition marked as bootable? Starting from the disk-image.img file it would be GPT. See #25 for some notes about setting a GPT partition as "bootable".

I guess I could test on any of my A64 targets for this. It wouldn't surprise me if the Boot from __ options are failing due to some weirdness, impurities and environmental pollution. I might have seen a similar issue previously while trying to boot some targets.

Is that "bootable system partition" partition marked as bootable?

Yes. I added a legacy bootable flag in fdisk.

Note that the "firmware partition" you're adding is useless.

I believe you're probably right. In theory the config.txt is supposed to be read to set debug=on, in order to enable/disable the headphone jack serial output (It doesn't have a physical switch like the pinebook-pro and pinephone). It's unlikely to be doing anything at the moment, just lying around on a random partition.