Linux Kernel Testing: Intro to kernelci.org
baylibre.com
baylibre.com
That's ... a scary amount, and it's easy to see that automated testing might be a good thing, there.
I think the combinatorial explosion happens at least in part because even though there's a limited number of actual ARM cores, the peripherals which Linux needs to support are often vendor-defined and thus different for each system-on-a-chip (or at least different for each device series from a particular manufacturer). I didn't dig through the sources to verify this, but I've heard of the problem before, ARM doesn't define a standard way for the CPU core to learn about its peripherals at run-time.
Imagine having to use a different kernel every time you upgrade your desktop, that's basically how the ARM world works so far.
How many other major projects the size of Linux have as little upstream testing?
You can bet pretty much every hardware vendor is running some kind of kernel CI internally.
Not to pick on hardware companies. Nearly every type of company has a few select areas that they focus on, hire for, and are truly good at. With everything else, they do what it takes to get by. Not because they don't care but because it takes a concerted effort to develop your organization into one that has high competency in any particular area.
Wow, looks like the definition got cleaned up a bit over the decades. Back when I was a kid, it was just every computing device the group could get their hands on, networked. Running simple delayed-echo servers, in our case.