Can I install any Linux distribution?
The Linux distribution installer image has to support your hardware.
Additionally, the distribution has to provide either a U-Boot compatible image, or a UEFI installer.
Using a UEFI installer, and installing as a UEFI system is the preferred method.
So I can install to an SD card that boots in any
Yes and no.
First, the "any
AArch64 devices" has to assume that the operating system can
run on those devices. For this question, we assume the operating system does.
It all depends on whether the systems require dedicated or shared storage for the firmware. If the system requires shared storage, and can only boot the initial firmware from the SD card, the SD card must contain its firmware. This will probably not work well for two different enough platforms.
Though, still assuming the operating system can boot on all the devices you want, if there is dedicated storage for the firmware on all of them, it should work fine. Up to one system with shared storage using the pre-built images.
If you craft your own image, you might be able to put more than one firmware for shared storage, if the location the different SoC uses does not conflict.
Why isn't this upstream?
There are four main reasons it could be. The first is easy: because I've not gotten around to posting a patch for it yet. Please press us to it for these kind of changes!
The second is that the implementation is more experimental than final. It is good enough for Tow-Boot, but the implementation needs more finishing touches and more testing before being sent upstream. Think of these changes as if it was part of a "staging" workflow, first maturing here.
The third is for changes that go against the opinions of upstream. These may be changes to default options, or opinionated changes made in an unacceptable form for upstream. Generally speaking, those changes represent opinionated user-experience changes.
Finally, the main fourth reason is for board enablement. Generally speaking those changes are provided by or on behalf of the vendor, and it is not our job to provide those upstream. We try to avoid board enablement that have not already been sent to upstream.
Why use this instead of my distro's U-Boot build?
In this project's opinion, the distribution shouldn't be managing the firmware used to boot the system.
Confusingly enough, U-Boot is both a firmware (think "BIOS") and a bootloader (think "grub").
The firmware should be a basic constant on the system, with well-defined semantics allowing either a bootloader or an operating system to start.
Should all the distributions have to build and manage all the BIOS for all
x86_64 machines too?
Why the Tow-Boot name?
Because of the pun on towboats, and the upstream pun on U-Boat.